Roach C D.tugboat Design.1954.TRANS

Tugboat Desigff BY C. D . ROACH, z ASSOCIATE MEMBER

The paper discusses the three general types of tugs opera ted in the United States; namely, (a) seagoing and salvage tugs, (b) ha rbor tugs, and (c) utility tugs. Char- acteristics of general dimensions, horsepower, heel drag, displacement, and rudder area are given for tugs of modern design. The U. S. Coast Guard cr i ter ion of GM based on wind heel is quest ioned as being not directly applicable to tugs and a new cr i ter ion based on hawser pull and rudder heel is proposed. The results of self- propel led model tests for 10 tugs are analyzed and presented in a g roup of tug- resistance contours. Several main-propulsion systems are discussed with advantages and disadvantages of each. The paper concludes that the most marked gains in tug design are to be reached by matching of the main propuls ion system to p rope r size prope l le r and by provid ing a hull with lines aft that will ease the flow into the prope l le r while prevent ing the propel ler f rom drawing air f rom the surface.

Tugs of the United States may be divided into three general classes; namely (a) seagoing and salvage tugs; (b) harbor tugs; (c) utility tugs.

Salvage tugs are used for long-distance ocean towing, rescue, and salvage operations. They are usually over 125 ft in length. They have more freeboard than harbor tugs and are provided with towing engines. The harbor tug is about 65 to 125 ft in length and is primarily concerned with barge and lighter movement and docking of

Paper presented at the January, 1954, meeting of the New England Section of The Society of Naval Architects and Marine Engineers. Received President 's Award for 1954.

t Chief, I~Iarine Transport Division, Transportat ion Board, Army Transportat ion Corps, Fort Eustis, Va.

593

larger vessels. Utility tugs are designed to fulfill a multi tude of tasks requiring a moderate horsepower from handling barges to transporting personnel from job to job.

In blocking out the design of a tug of normal shape (i.e., not tunnel stern or of other radical design) the length, breadth and draft must be considered first. In Fig. 1 are given some general guides for starting a design. I t is important ' to note. that there is a definite relationship between the gross dimensions of length, breadth, draft, and the horsepower. Many a good hull form and economically capable tug has been ruined by installing a new main engine having more power than the hull could accommodate and requiring by all rights a wheel of diameter in excess of the propeller aperture.

FACTORS AFFECTING DESIGN

Certain operating characteristics will affect the design. A tug used for docking cannot be much more than 115 to 125 ft in over-all length without being too awkward for efficient use in crowded slips. Tugs to operate on canals must be limited in height by the minimum bridge clearance. These conditions will determine limits to external structure of the tug.

The beam and the freeboard are, of course, intimately co-related to stability. Here the U. S. Coast Guard has set up a criterion to insure adequate stability

c A h G M -

where C -~

f B = A = h =

A

I t is

0.005 oceangoing 0.0033 coastwise 0.0025 harbor freeboard (least), ft beam, ft profile area above WL, sq ft distance from half-draft at amidships to

center of gravity of (A) displacement, tons

considered by the author tha t the U. S.

594 T U G B O A T D E S I G N

35

30

25

-15

o X

g ,5 r~

l0 /

20

FIG. 1

/

/

,4 40 GO 80 I00 17_0 140 IGO

LencjJrh BeCweQn Perpendiculors

AVERAGE CHARACTERISTICS OF STEEL D I E S E L T U G S

/

1400

1200

IO00

800 ~_

o 212

600 ~)

m

400

- ' 200

0 180

Coast Guard criterion of GM is not adequate to insure safe tugs.

The Coast Guard formula is based upon the overturning effect of wind and weather upon the above water area of the tug. I t is the author 's conjecture based upon numerous casualty reports of all types of tugs that they are much more likely to be capsized by the towing hawser than by effects of wind and weather.

No account is taken in the Coast Guard formula of horsepower and the necessity for additional stability commensurate with the increased ability to exert tow-rope pull.

Tests conducted by the Transportation Corps indicate that, while towing at low speeds normal to a tug, the rudder is capable of exerting approximately 50 per cent of the tug's hawser pull at 90 deg to the tug's centerline.

A well-designed propeller will develop about 30 lb of hawser pull per bhp. Therefore, the side pull on the bitts will be about 15 lb per bhp. This force will act on a lever of length from the center of effort of the rudder to the center of the towing hawser. The center of the towing hawser may vary in height so the top of the towing bitts has been taken as the upper limit of the hawser position.

Upon these assumptions the author proposes the following modification of the Coast Guard formula :

BHP X 15h G M -

4 where

BHP = brake horsepower h = vertical distance in feet from center of

effort of rudder to top of towing bitts

A = displacement of tug, lb f = minimum freeboard, ft B = maximum beam to outside of shell

plating but not including fender guards, ft

I t is realized that the proposed formula makes no distinction between harbor, coastwise, and oceangoing service. The importance of this distinction, however, may be somewhat over- rated since weather should have no serious effect on a tug with adequate stability while the effect of hawser pull is about the same in the harbor as at sea.

Considering the many unusual duties a tug falls heir to additional GM often may be war- ranted. Thought must be given to a tug's GM which, like a person's life expectancy, grows smaller with the years. This has been particularly noticed in military and naval craft where there a re many tendencies to add topside weight.

Initial stability is of importance and is a function of the vertical center of gravity and the beam

TUGBOAT DESIGN 595

5

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.~ 4

L.

E

"E

I

0 0

. I

I

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FIG, 2

40 60 80 100 120 140 IGO 180 Lengfh Bet-ween Perpendiculars

AVERAGE MINIMUM FREEBOARDS FOR TUGS IN LOADED CONDITION

200

The range of stability is perhaps of even greater importance. This is largely a function of the freeboard. For safety the tug should have a range of positive stability up to at least 70 deg. To realize safely this amount of roll, the deck openings and doors to the house must be watertight. Doors opening on the main deck, particularly engine-room doors, should be Dutch doors so that when the engineer is taking a breather, the tug will not be surprised by sudden ingress of water.

In discussing freeboard for tugs, the particular duty of the tug must be determined. In general, the oceangoing tug will need relatively more freeboard than the harbor or coastwise tug where lines must be handled continually over the bulwarks. Fig. 2 gives what might be considered normal freeboard for the various classes of tugs. These minimum freeboards are for fully loaded condition. The sheer line of the harbor tug will be disposed to give a low working platform aft and then rising forward in a lively spring to give a bow high enough to allow adequate reserve buoyancy forward. The bulwark line will be low so that lines may be worked without the crew having to lift excessively. For harbor tugs the bulwark height should be about 22 to 24 in. from amidships aft. Forward the bulwark must be higher to give added protection. In tugs of 100 ft and longer, the bulwark height forward may be so high that lines cannot easily be handled; then it is customary to fit a grating on the deck that may be used as a working platform.

On seagoing tugs the use of a raised and sometimes a to'gallant forecastle is customary and necessary unless the tug is of such large dimensions as to allow sufficient reserve buoyancy without it.

At this point stress should be placed on tumble home of the bulwarks and the necessity of keeping all the superstructure well inboard. The duties of a tug require that it often come in contact with ship hulls, barge sides, and piers. These contacts are often forcible and damage to projecting superstructure will occur. In Europe the bulwark carries tumble home completely around the vessel. In the United States the tumble home dies out at the stem in order to allow greater deck working space forward. Judging from the number of damaged forward bulwarks existing on our tugs, it would appear that there is much merit in the European scheme.

While the deckhouses and pilothouse should be placed well inboard, care must be taken, at least in utility and harbor tugs, that the pilot is able to see all bitts and cleats from the pilothouse. This precaution will result in less damage to tug and tow as well as accelerate the handling time of making up a tow.

KEEL LINE AND DRAG

The underwater profile is probably more important in tug design than in any other type of ship. Here, the lateral area of the tug to resist action of wind and wave becomes of vital importance since the towing speed is low and the tug is restrained in movement by the towing hawser. In a normal vessel the center of pivot appears to

596 T U G B O A T

I " B / ' ' -

- - ~ a w s e r ~, ~ c'~ " - P i v o +

Tug wifh Normal Drag and Usual Bi÷f Locakiom

.•------.....Bifts I\\ ..~ "~\~'/ / Cenfer of Plvof

Tucj w~'h Hare Drag and Biffs Moved Forward

FIG, 3 STEERING EFFORT DIAGRAM FOR TUG UNDER WAY

DESIGN

be moved aft, the total rudder force may be reduced considerably for the same steering effort. This is achieved by working drag into the keel line. When the tug is not under way but is straining at the hawser, the center of pivot becomes the bitts and the steering effort is propor- tionate to the distance from the center of effort of the rudder to the center of the bitts. Mr. Dwight Simpson (2) gives the desired drag as from 0.04 L to 0.05 L and recommends the use of a bar keel if possible. This amount of drag appears excessive by comparison to the normal tug in this country. Fig. 4 is a plot of the keel drag/- length BP for a number of typical tugs. I t was noticed that the tugs represented by dots above the curve had reputations for being very ma- neuverable while several below the curve were reputed to be a bit slow in responding to the helm. I t is, therefore, concluded that slightly more drag to the keel would improve tugs of this country.

Cutting away of the forefoot has much the same effect as drag to the keel; here, though, caution must be observed not to cut away too severely as a tug needs a certain amount of leading edge to

0.0G

0.05

0.04

.E ~" ~'1~ 0,03 L O~

n r -

~ .2 0.02

0.01

\

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0 0 20 40

FIG. 4

O \ O

O

G0 80 100 120 Lengfh Befween Perpendiculars

K E E L DRAG FOR STEEL D I E S E L T U G S

I¢0 IG0 180

be in the forward one-third length of the ship (1). 3 In order to reduce the rudder force and to make the tug more responsive to steering, it is desirable to alter the lateral plane to shift the center of pivot as far aft as possible. I t can be seen from Fig. 3 that if the center of pivot can

3 Numbers in parentheses refer to the Bib l iography at the end of the paper.

turn on. On one tug of experimental design,

both the forefoot and the skeg were cut away almost completely for reasons other than steering. The result of this was that while the tug would hold a straight course for long periods and would answer the helm well, when once put in a hardover turn, she would proceed to keep~on turning until

T U G B O A T D E S I G N 597

700

600

5O0

J " I 40Q

• -~ 300

E ~ 7.00

tm

100

0 ZO

J 4O

/ /

/ /

FIG. 5

/

I00 IZ0 I40 200 LencJ+h Be+ween Perpendiculars

~o BO 160 IBO

DISPLACEMENT IN LONG TONS FOR STEEL D I E S E L T U G S

the rudder was given hardover in the opposite direction.

In setting the amount of drag it is fortunate that the deep draft aft favors large propellers, a most necessary adjunct to a tug. The designer should work out his trim so tllat, as fuel is burned, the ship keeps a nearly constant draft aft and therefore a constant wheel immersion. This scheme will necessitate the installation of the fuel tanks about amidships or forward.

DISPLACEMENT AND WEIGHTS

Tugs are relatively among the heaviest vessels the naval architect is called upon to design. As will be noticed, the displacement-length ratio varies from about 300 to 450 for all three types of tugs. To complicate the problem further, the tug will operate at speed-length ratios of 1.0 to 1.2 (in the case of smaller tugs) or even higher. This, coupled with the ever-increasing requirement for more tow-line pull, makes the modern tug an overly heavy, overpowered, and over- driven craft.

Now starting with these predetermined handi- caps, the design must produce a money-making seaworthy craft that, in addition to an engine room too fully packed with power, will have commodious accommodations for a discriminating crew.

Fig. 5 is a curve of average light-ship displacement of typical tugs recently built in this country.

This curve indicates only the relative magnitude of displacements and should not be used for esti- mating preliminary weights. The variation in weight of superstructure between a 95-ft tug for open harbor work and another 95-ft tug for canal work can vary by as much as 25 per cent. The difference in weight between a directly connected Diesel and a Diesel-electric installation can vary by 30 per cent or more depending upon the speed of the Diesels and whether the Diesel- electric is single engine or multiengined. The designer therefore should make every effort to secure confirmed, certified weights from the engine and equipment manufacturers. From personal experience, it is suggested that these weights be confirmed for every new tug since some manufacturers are not hesitant radically to alter weights between publications of their sales data and delivery of the engine or equipment. In this regard, and again from personal experience and from that of a number of naval architects doing work for the Transportation Corps, the center of gravity of an engine is not always over the center of the crankshaft. Ttiis little detai l is seldom noted on the installation drawings fur- nished by manufacturers, often assumed by the designer, and as often leads to embarrassing transverse ballasting to correct an unaccountable list.

There appears to be little reason to worry about the over-all displacement as the tug is seldom concerned with free running, and to produce the

598 TUGBOAT DESIGN

TABLE 1 ~Y~ODEL DIMENSIONS AND HULL FORM DATA FOR TEN TUGS

L , F T .

BX t FT.

H I F T .

D t ~ FW.

~ , FT. 3

S I FT. z

A x , FT. =

Aw, FT. z

L / B x •

ex/H.

I I

TXI TX2 T X 3 TX4 TX5 T X 7 T X 8 T X 9 = T X I O TXI I

11.08 I1.11 I1.11 I1,11 I1.11 I1.11 I1.11 10.90 I1.11 I1.11 i i i i i i i 1

3.24"; 3.40 3.62 3.065 2.990 3.042 2.920 2.733 ~ 2.620,2.485 ! i i i i i

1.057i 1.18 1.13 1.216 1.278 1.253 1.097 1,000,1,072 ~ .917

I ' 1093 1297 1338 1175 1441 1382 1039 852 : 1048 i 880

17.52'20.75 21.45 18.83 23.08 22.15 16.63 13.67 16.80 14.10

40.55 46.50 44.966~39.512 44,5(45.071 37.64 53.909 57.226 ]5,394

2.685 3.21 3205 2.875 3.3353.375 2,543 2D67 2,460 1~71

25.50 28.13 29.50 125.25,25.75, 24.08 , 23.25,21.97 22.05, 21.42 i

3,365 3.270 3.070j3.6~ 37,20 3~55 3.805 3,992 4.245 4.470

3.120 2.880 5.200 2.517 2.34.(; 2,465 2,665 2.733 12.442 2.710

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Ce.. p

CwF.

C W A •

Cevr-

CevA.

C p E .

Cpe.

C w ( .

CwR.

CpvE.

Cevn-

4.270 4.04 3.995 4.18 3.90~ = 3.955 4.55~ 4.555 4.335 4.600 , i i = i = i =

6.01 6.14 5.82 5.61 5.485 5.715 5.775 5.915 5.677 6.050 =

L .330 ,377 .364 i.3201 .3605 .365 .3052,2,854 .3015 .2867 ' I J ' ' ; '

17,21 18.13 17,221 15.22 16.42 16,98 16.39 16.45 16.12 14.65 I I I I I I l I

.458 .467 .472,1 .453 .544 .531 .465 ,459 .538 ,556 i - | i i i I |

,595 .583 .602 .589 .623 .592 .586 ,607L ,615 .677 ! I u o u I I

.770 .801 ,784 .770 .873 .898 .7~94 .757 ,876 ,821 I I J 1 I I I I

.757 .745 .733 .742 ,776 .745 ,717 ,738 .758 ,776 , , i , , , I '

.606 .626 .644 ,611 .701 .713 .648 .622 ! ,711 .717

.542 .586 ,558 .550 I "598 ,5 50 ,570 .599 ,602 .654 I I ~ I I I , I I

J .637 .580 .646 .629 .649 ,633 ,601 ~15 .628 .701 | ~ I J J I I

• 655 .698 .663 .681 .699 .644 ,667 .690 .674 .720 I ! • I I I I I I

.860 .793 .803 .803 .852 .845 ,766 .785 .842 .831 ! I I I I ! ! !

• 708 ,702 .692 ,670 ,836 .820 ,709 .695 .800 .745 J l ! I J I I I

.604 .558 .606 .578 .650 .6:51 .597 i .562 .645 .691 I I I I I } I I

.641 .586 .598 .550 .634 .590 .570 I .614 .602 .684 I I i i i i I I

.550, .580 ,606, , 629 , .6 i0 , .593 , .601 , .600, .628i .669

.711 .698 .692 .681 ,725 .672 .667 . 7 0 1 . 6 7 4 .745 I | I I I | I I I

.8041 .793, .776, . 8 0 3 , . 8 2 7 , . 8 1 8 , .766 ,0 "776 , '842 , .808

.694 1 .702 .676 .670 .763 .781 .709 .701 .800 .754

.527 .558 .612 .578 .6431 .645 .597 .558 .645 .680

/X'/(.OIL) ~. 368.0 432.3 446.5 582.5 481.5 461.5 347.¢ 301.3! 350.0 294.0


maximum thrust a deep propeller immersion is necessary. Then too, tug skippers have a pen- chant for ballasting anyway. Consideration should be given weights topside as few tugs have adequate stability. Here weight saved will pay off both in safety and better operation.

I t is sincerely believed that a better tug can be produced if ballasting is not resorted to. The designer may find, however, that beam and form may be restricted by considerations other than purely hydrodynamic and this is so often the case that a more realistic recommendation might be that approximately 10 per cent of the displacement be made available for ballast. If ballast is figured, the freeboard after ballasting should be the controlling factor.

One very able tug operator of long experience stated that no tug would work well unless there was an inch or two of salt water over the after deck. The author does not concur with this opinion but should this philosophy be encountered, just make sure there is ample transverse moment of inertia in the deck erections.

THE LIN~S

While the effective horsepower of a tug is not considerable at normal towing speeds of say 5 to 6 knots, it is an obligation of the designer to obtain as low a resistance as possible in order to increase the tow-rope pull. As a result of tank tests made by the Transportation Corps on 10 tug forms and subsequently added to by inclusion of two more tugs, it appeared that considerable variation of hull form could exist without affecting greatly the effective or shaft horsepower at 6 knots. When all of these tug forms were expanded to 100 ft in length, the SHP varied from 30 to 4'5. At 12 knots these same tugs varied from 520 to 910 shp, a truly significant difference at these higher speeds.

In comparing the table of characteristics of these tugs, Table 1, with the self-propelled re- suits, Figs. 6 to 15, some tentative conclusions may be reached. Mr. Robert Taggart, then a member of the Transportation Research and Development Command, compiled the data .on these unrelated tugs into a set of resistance contours based upon Cr with volumetric coefficients varying from 0.007 through 0.107. The liberty has been taken of converting-the volumetric coefficient to the more familiar displacement-length ratio and presenting these as Figs. 16 through 2l.

I t will be noticed that up to speed length ratios of around 1.25 the low prismatics gave lower resistance. Above V/x/L of 1.25 the penalty paid for low prismatic is prohibitive. Most tugs are designed for towing at speeds in the

region of 5 to 6 knots and to cruise at speeds not greatly exceeding V/x/L = 1.00 to 1.10. There- fore, it appears that selection of prismatic coefficient for ordinary tugs should be based upon the towing speed and take the penalty at top speed, and in the case of rescue-type tugs, design strictly for the top speed. I t will be of interest to know that two of the tugs having a very low shp were small tugs that were expanded to 100 ft, while the tug having 910 shp for 12 knots was a tug 149 ft in length that was reduced to 100 ft for comparison and should by all reason have less resistance.

Since the l0 models tested were not related, and in no way could be considered a series, certain observations are tentatively drawn for the guid- ance of designers of future tugs of so-called normal molded lines.

The extended Taylor series published by the Experimental Towing Tank, Report No. 279 (3), as well as the results of these tests, seem to indicate a Cp of 0.,58 to 0.60 will give the lowest resistance in the top operating range of about V/v /L = 1.00. This choice of prismatic coefficient will give reasonably low resistance through the entire tug range.

The midship coefficient does not appear to have a great influence upon the resistance except as it influences the prismatic coefficient. In general it can be stated that the midship area should be as large as necessary to allow proper machinery arrangement. If the C~ be carried to the ex- t r e m e - s a y over 0.90--some difficulty may be experienced in working the hard midship bilges into fair forward and after lines in the short distance allowed in this type craft. Average values for tugs run about 0.80 to 0.85.

At this point a word of caution should be given with respect to the prismatic and the development of the hull lines, as they affect stability at high speeds. Since tugs a r e overpowered and have extremely high displacement-length ratios, the hull is likely to be very fine at the ends and have an apple-round middle body. When this type configuration is driven at high speeds, the reduction in pressure amidship can be pronounced. The water carries down so far on the hull that stability may be jeopardized. The designer would do well to consider this eventuality when determining the length of the hull for a given horsepower. Choose a length that will allow a speed-length ratio of not nmch more than 1.20 (when this phenomenon begins to reach a serious proportion). Operators would do well to limit the speed of their tugs by placing the tug at a point somewhat below this critical speed.

The phenomenon is much more pronounced in

600

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100

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T U G B O A T D E S I G N

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1500

1400

1300

1200

100

Z . 3 4 5 6 -/ 8 9 l0 II 12 13 14 15 Veloci ty in K n o t s

FIG. 6 SHAFT HORSEPOWER, R P M , AND DERIVED CURVES FOR 100-FT T X 1 ( M o d e l 4 0 8 4 - - P r o p e l l e r 3056 ; d i sp lacemen t , 368.6 t ons ; we t t ed surface , 3304 sq f t . )

16 17

000

900

# 800

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600

500

400

300

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100

shallow waters where the stream constriction still further reduces the pressure, and a tug that may possess ample stability in deep water may find itself unmanageable or in peril. Openings for sea chests should be so pl~ieed that they are not uncovered by this condition.

In view of the rather limited investigation, no definite conclusions could be gained as to the effect of the other characteristics and the best advice that can be offered is that the form be developed to give the fairest sectional area curve possible. Slight hollows in the sectional area curve both at the bow and stern seem to be characteristic of hulls having low resistance and good performance.

The shape of the waterline curve is a problem that must be solved on the drawing board. Here, too, the curve must be fair and sweet forward. Aft, the waterline should be as full as possible to provide cover for the wheel.

I t is believed, by the author, that the frequently annoying and often destructive vibrations that occur in tugs operating under high power at low velocities and during reversing may be attributed to air being sucked down into the wheel. This has been demonstrated in a number of instances to the author by tug skippers who, when ballasting down, can prove a quieter vessel. I t is possible that a hard knuckle worked in the hull just below the waterline in the style of the old

TUGBOAT DESIGN 601

F I G . 6 ( a )

M O D E L R E S I S T A N C E DATA

SHIP_ 4 2 1 ' x l Z ~ L A B O R A T O R Y _ _ I . M L _ _ T E M P E R A T U R E _ 6 4 " F

HARROR T t l A cJ .5 K _ BASIN _ D E ~ . . . . . WATER GOND. FITII I FRFRH

MODEL NO. 4 0 8 4 _ BASIN SIZE 2 7 7 5 ' x 51' x 2 2 ' M O D E L M A T E R I A L w o o f l _ _ _

A P P E N D A G E S _ _ R U D D r ~ _ _ _ _ MODEL LENGTH ~ _ _ M O D E L F i N i S H ~ I I ~ ' [ . . . . .

T E S T ~ D A T E _ _ 5 ~ - ~ 9 _ T U R B U L E N C E _ _ INf lUCFD

R E M A R K S _ T A . / R B U L E N C . , E ~ D U ~ _ S A N [ } RTRIP.~ NN BOW

MAX. WLB/B x_l 0_07._ MAX. IMB/B x 1.00.~_ HS/B g ,003.9_ DR IB x ~ I Z ~ _ BR/B X _ , [ g L BKW/B X

l ° ° l I I i ~ - - - I - ~ - T ~ V ' ~ i I ~ ! 1 I ~ . x i I ~L_ J+_ i I ' ~ I I ~ i . ~ G o ~ ~ ~ ~ - ~ - ~ ~ ~ ~ ~ ,X~AM-~ 0 ou ~ - - ~ - - i ~ ] ~ _ _ ' ~ - T _ - . _ _ I . . ~ _ _L_SEOTION AREAS_: _: _ ~ - - T ' ~ Bx/BM'AJ~I~"

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tJ

~0.50

m Q = t o rque , ] b - f t 15 0.40

4-

T = t h r u s t , lb ~ 0.30

n = R P S 2

v = s p e e d of a d v a n c e , f p s ~_ 0 .20

P = p i t c h , f t

D = d i a m e t e r , f t O.lO

/ 0

-OAO 0

/

e ' i

/ /

/

/

/

Y <

/

\

0.10 0.?_0 0.30 0.40 .0.50 Sl ip R o t i o

0.09

0.08

0.0-/

£)

0 . 0 6 + ~ ' E

U

0.o5~ o

U

0.04

o.o3. ~K

10.02

O.Ol

; 0 0.60


180

IGO L l i i

I 0 0 / i ~ 600

01~ 80 I /-.

60 .--- / / , ~ , / ~ / 500

40 i 450

400 2 l ' EHP

GO I , , l 3 0 0 1-

50 7

Z50

~ 4 0 1 , / , , . 200

30 . . . . . ~ L50

~ . . . .~ ~ ~ ..~r~ ' ' ~

I - 2 0 IZ 5 G -/ 8 9 l0 It

VelociCy In Kno¢s

FIG. 7 SHAFT HORSEPOWER, RPM, AND DERIVED CURVES FOR 100-FT TX2 TUG (Model 4087--Propeller 2225; displacement 433 tons; wetted surface 3766.5 sq ft.)

bustle stern could act to strip off the aerated water and allow the wheel to bite into a more solid flow. Indeed, some of the late river towboats have the bot tom shell plating extending (projecting) an inch or so past the side-shell plating to effect this same result. The efficacy of this system has been demon strated.

MAIN PROPULSION MACHINERY

S/eo;m In choosing the main propulsion machinery the

designer must consider carefully the duty of the vessel. To this important consideration, of

necessity, must be added the ever present economic consideration.

There is little doubt that the most effective power can be obtained by using steam reciprocating machinery. With the low engine revolutions, a wheel of goodly diameter can be used. The pitch/diameter ratio can be set about unity where propeller efficiency is highest. The large wheel is capable of recovering more of the energy previously lost in wake. These are the immediate and obvious advantages of steam power. To these advantages can be added the reliability and simplicity of the steam engine and the ability to

T U G B O A T D E S I G N 6 0 3

F I G . 7(~'t )

SHIP 51'X 15'-6"X 5 ' -5 "

HARBOR TUG'

MODEL NO . . . . 4 _ 0 ~ 7 _ _

A P P E N D A G E S _ _ R U D D E B _ _ _ _

M O D E L R E S I S T A N C E D A T A

S.S. LABORATORY _ T_M B . _ _ TEMPERATURE _ 7 ~ " F

10.5K BASIN._DEE.P_._YL&TE~ . . . . WATER COND. S _ T J L ~ _ _ I t E L

BASIN SIZE 2775' X 51' X 22 ' MODEL MATERIAL w O O D

MODEL LENGTH _ _ l I A r ' _ . _ _ _ MODEL FINISH _~tJI~LT . . . .

TEST ~ DATE_7J_L8/_49 _ TUR BULENCE ~ U C E Q . . . . .

_ _ REMARKS_T_LLRBULEJ~C.,E.~IDUCE~SANO , , ~ 1 ~ . . . .

MAX. W L G / B X J OgO_ MAX. IMB/B X .LOOO_ HS/B X . 0 0 2 7 . _ OR/B x A O I L B R / B x _ . P - B 6 ~ BKW/B X NONE

)oo I [ L [ I I J - - l - - - - - t - ~ J - - ~ F I ~ . I [ ~ I .̂ ~__~__sol- ,1__ I,,_ ~ ~--. ~ : ~ ~ - J - - ~ - _~ ~ 4 : ~ _ _ i _ ~ _ _ ~ _ i ~ : - : ~ _ ~ t '. :

m . - ~ L _ _ L L L .}t ~ . ' ~ _ ~ , ~ < . . . . j ' - . .. 1 '- " ~X/AM'J.O-~ : _ L - : : Z ' , " - ' * , . C T N E " , , B . . . . . . .

°~ V ~ E 2 2 - L - . . : J ~ ' _ V . : . I 12-TSEc-t~,.LLA, E A S . . ...... 2 7 - - > ~ . L . ~ 5 _ : r l , x ' ~ ' ~ ° [ - - - ~ - - " / " / - - - ~ / ' i' . . . . . , I ! , - - I -~_ ! F . . . . . i - , - - ~ ~ - . . . . . ~ , - , - - ~ - = - - - r ~ . - ~ . L

18 ' 16 ~, 14 [ 12 ; I0 [ 8 ~ 6 : 4 i 2 ~ _ ~ ' . . ~ " ," ," . . . . . . ~ = _ ~ . : . _ . " . . . . . . - _. . ~ ~ ' ~ ' i = £ " L ° IB/Bx I o '..,ea..,,~ ~745~.~e!.$9~-L~4~i.9~?~.9B~4~$9~:~9~5!~74~$~.~$~B~694!~5~2'~4~!.2~4: 0

0 . 9 0

0,80

Q = t o r q u e , l b - f t

T = t h r u s t , l b

n = RPS v = s p e e d o f a d v a n c e , f p s

P = p i t c h , f t

D = d i a m e t e r , f t

F I G . 7 (b )

C H A R A C T E R I S T I C C U R V E S PROPELLER NO. 2 2 2 5 ~ 0.70~

D i a m e t e r , in . . . . . . . . . . . . . . . . . . . . . . . . . . 12 . 7 9 2 P i t c h , in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 . 5 0 N o . b l a d e s . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 "~ 0 .60 S p e e d of a d v a n c e . . . . . . . . . . . . . . . . . . . . . . - - ~_ L i n e a r r a t i o , X . . . . . . . . . . . . . . . . . . . . . . . . 9 . 0 P / D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 9 7 8 M W R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - - • 0 .50 P A / D A . . . . . . . . . . . . . . . . . . . . . . . . . . . . - - --'~" B T F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - - c 5

T X 2 ~c 0 .40

c ~ = ~ ' ~ n 2 p a D ~

T ~ 0 . 3 0 _ C r = ~

% T v ~ 0.20

e = ~ : 2Qn

S P n -- v o. I0 ./

-0,10 o

0.09

0.08

0.07

/0.06% +-

/ < o.o5

o) 0.043

o.o3 ~_

0.10 0.20 0,30 Sl ip RQ+;o

0.07_

0.01

0 0.40 0.50 0.60

604

250

240

23O

220

210

200

190

180

I7(?

160

150

n- 140

130

120

I1

I0

90

80

"70

~0

50

40

TUGBOAT DESIGN

EH_~P SHP

• *~--" ~ _ . ~ .

/

J

!/

Y I

r

2 3 4 5 6 7 6 9 10 II 12 13 14 15 IG VelocVr~, in Knots

FIG. 8 SHAFT HORSEPOWER, RPM, AND DERIVED CURVES FOR 100-FT TX3 TUG (Model 4089--Propeller 3056, displacement 447.84 tons; wetted surface 3,640 sq ft.)

1050

IO00

950

900

850

800

75O

JO0

650

6OO

550 o~

500

450

40O

35O

300

250

200

150

~00

5O

0

u t i l i ze l o w - c o s t fuels . Of e q u a l i m p o r t a n c e is t h e a b i l i t y of a s t e a m t u g to h o l d on fo r e x t e n d e d pe r i ods w i t h t h e p r o p e l l e r t u r n i n g o v e r j u s t a f ew r e v o l u t i o n s pe r m i n u t e . T h i s c a n be a n e c e s s i t y w h e n an o c e a n g o i n g t u g is f o r c e d b y h e a v y seas to h e a v e to. T h e f l ex ib i l i t y of t h e

s t e a m e n g i n e a l lows t h e t u g to t a k e t h e s lack o u t of l ines g e n t l y a n d wil l s a v e m u c h m o n e y in t h e cou r se of a y e a r ' s o p e r a t i o n b y r e d u c e d cos t of h a w s e r w e a r a n d tea r .

T h e d i s a d v a n t a g e s are, of course , t h e r ea son w h y we h a v e so f ew s t e a m t u g s o p e r a t i n g in th is

TUGBOAT DESIGN 605

FIG. 8(a )


SHIP , . . ~ J , 3 a . D 2 ~ LABORATORY

, S . ~ / L ~ O I I _ . _ ~ ' L BASIN

MODEL NO. __4_(189~_ BASIN SIZE 2 7 7 5 ' x 5 1 ' x 22' MODEL MATERIAL

A P P E N D A G E S _ R U D D E B _ _ _ _ MODEL LENGTH

TMR TEMPERATURE KR" F

DEEP WATER WATER COND. STILL : FRESH

WOOD

I I I I ' MODEL FINISH PAINT

TEST I ~ 4 D A T E g " 2 T - & 10-4-4OTU R BULENCE IN f l l l ~F f l

R E M A R K S'_T_LLRBU LEN G~- _ IND U C ~ Y _ S A N D ~ S ~ N ~

f

MAX. W L B / B x A . O 0 0 _ MAX IMB/G x L O 0 0 _ H S / B x ,00307 D R / B x _ . I Q O _ _ BR/Gx_ . .181__ BKW/Bx~IDJ~[E_

470 L " ~ .5~0 t •l

~- - ~ " .,,~-_LOO~ 601 / w -. w ~ATERLINE BEAM~ ¢ - - ~ " > " A x / A M ' i Q O ] i EV,O AR?S "'- g,°l' / \ \ \ ~ " ~ - - LGB 0 .518L AFT STA. 0 ~ ' t'~=~.5_ .=°,2ol// /.~ POINTS OF INFLECTION- ~ \ i

~ITATIONSI ' 2"'"~ ~ 18 16 14, 12 I0 8 6 4

A/A x .ozE 121 .298 .458[.605 .732 .838i ¢J20 .975 I,OOO ~993 .954 894 .814 .722 608 .480 340 .197 .O72 O-

B/S x .010 .406,609 .750 850 .917 9611 987 997 10O0 .997 .980 952 .909 .850 ,760 .645 507 .347 .I,71 .005 E30/BDI .OO6.207 .364 .479 587 .668 .742 799 842 .862 .862 .839 797 745 678 599 .508 402 .282 .149 O dA /dL 1.46 3.30 3.47 3.13 Z.Z4 2.33 1.87 1.37 082 0 055 1.00 1.40 75 2.13 2.45 270 285 2.72 2.10 t.lO

1.00 0. I0

o.go

F r o . 8(b) O.BO

C H A R A C T E R I S T I C C U R V E S " ~

PROPELLER NO. 3 0 5 6 " J >,

D i a m e t e r , in . . . . . . . . . . . . . . . . . . . . 12. 105 ~ 0.70 P i t c h , in . . . . . . . . . . . . . . . . . . . . . . 9 . 2 1 0 .~ N o . b l a d e s . . . . . . . . . . . . . . . . 3 ~= S p e e d o f a d v a n c e . . . . . . . . . . . . . 1 . 6 7 - 4 . 4 8 K ~ 0.60 L i n e a r r a t i o , X . . . . . . . . . . . . . . . . 9 . (1 as P / D . . . . . . . . . . . . . . . . . . . . . . . . 0 . 7 6 g M W R . " 0 . 3 1 8 f ~ P A / D A . . . . . . . . . . . . . . . . . . . . 0 . 4 4 8 ~-0.50 B T F . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 049 T X 1 a n d T X 3 -~

O .~ 0.40 Cq n2P3D2 O = t o r q u e , l b - f t

T Cr = nZP2D~ T = t h r u s t , lb u 0.30

n = R P S Tv

e = - - v = s p e e d of a d v a n c e , f p s 2Qn ~- 0.20 P n - v

S P = p i t c h , f t P n D = d i a m e t e r , f t

0.10

0 -OAO

/

/ /

/

/

/ /

-.< /

o.og

0.08

0.07

O

0 . 0 ~

(v

0.05~ c~

0.04 g

0 OAO 02.0 0.30 Slip Ra~io

0.03

0.02

0.01

0.40 0.50 0

0.60

606

800

700

gO0

500

~¢0o

o~

300

200

iO0

CL 30

2 0 _ _ _ _ _ _

I 0 _ _

0

iO 0

TUGBOAT DESIGN

'f I

/ /

~ ..,._,.,___ , lS H P x

X ~ ~ X

/ g .

~ "--- -'---~-~ =e"-- ~ ~

/ - I I 2 3 4 5 6 7 8 9 l0 11 12 13 14 15

Veloci+~, in Kno÷s

FIG. 0 SHAFT HORSEPOWER, R P M , AND DERIVED CURVES FOR 100-FT T X 4 T u o (Model 4093--Propel ler 2452, displacement 393 tons ; wetted surface 3200 sq ft.)

"2.10

7'00

IgO

180

170

160

150

140

130

IZO

I I0 y (3-

100 ~c

90

80

7O

6O

50

40

30

20

I0

0

country. I t will require a larger crew and a s tandby watch must be maintained or a great deal of t ime and money must be spent gett ing up steam. The boilers and condensers take up spaee which is doubly valuable in a tug. The

high specific fuel consumption of the s team plant tends to offset much of the difference in cost of fuels. Notwithstanding these certain and impor tan t disadvantages, the designer, if free to choose the engine, should consider s team for tugs

FIG. 9 ( a )

T U G B O A T D E S I G N


SHiP 74:33' w ~n411' ~ ¢114' LABORATORY TMB TEMPERATURE KS" F

I O K - FIIRQPEAN TYPE TUG BASIN nEEP WATER WATER GOND.

MOOEL NO. 4 0 9 3 BASIN SIZE ' ' ' MODEL MATERIAL WOOD

APPENDAGES RtJ~l~R MODEL LENGTH II lJ' MODEL FINISH PAINT

TEST._IJL2._DATE 5 - 2 3 - 5 0 TURBULENCE INrIIII~..F'~

REMARKS,TIJRRtll ENC~' INI}LICEr~ RY ~;AN(~ STRIP~ (}N ROW

6O7

MAX. WLB/BxJ.Q.QO_ MAX. IMB/Bx I(X}O N S / B x ~ DR/Bx_J_4JS~ BR/Bx_.2~..~__ B K W / B x ~

. 4 9 0 L • k"

60 / / f f --SEGTION - - I I I [

~ 4 0 / , / / LCB 0.521 L AFT STA. 0

ZO L / ¢ 'OIN'rSlsTATIONsjOF INFLECTIONI

18 IS 14 12 I ~0 I s

.510 L

"- ~ Ax/AM- LO00 .~_~, ~.~ ~"-~ BX/~. ~

-.. t-J.4

L / A X 0 2 2 .107 . 2 6 9 A 3 0 .51101,712 . 8 2 Q . 9 0 3 . g 6 2 . 9 9 4 L 0 0 ( . 9 8 3 +933 . 2 5 4 . 7 4 ~ , 6 1 0 A 4 4 . 2 7 0 . 1 2 5 . 0 3 2 0

.011 , 4 2 1 . 6 1 3 .751 8 4 9 9 1 5 . t S a . 9 7 7 . 9 9 2 .g98 I . ( X X l . 9 9 4 . 9 7 8 . 9 4 1 . S I C . ? 9 2 ~ 7 4 . 5 2 4 . 3 4 9 . t 6 3 0

30/IB01.009.23~ 324 ,494,58~ ,665 ,738 ,794 ,831 &59 .25g ,84 | .213 .T62 .69g ,QI5 .~K)9 .393261 . IU 0 ~A/dL 1.22 2.6C 3.28 3.13 2.83 2.41 1.94 1.42 o.gl 0.40 0 0.?2 1.32 1.90 2.50 $.03 3.403.28 2.35 1.42 3.21

FIG. 9(b) 0.70

CHARACTERISTIC CURVES PROPELLER No. 2452

>~ Diameter, in . . . . . . . . . . . . . . . . . . . . . 1 I. 065 ~ 0.60

(v Pitch, in . . . . . . . . . . . . . . . . . . . . . . . . . 12.35 .~ No. blades . . . . . . . . . . . . . . . . . . . . . . . 4 .- Speed of advance . . . . . . . . . . . . . . . . 2 . 2 - 5 . 2 K uJ 0.50 Linear ratio X . . . . . . . . . . . . . . . . . . . . 9.0 P / D . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-.117 = MWR . . . . . . . . . . . . . . . . . . . . . . . . . . 0.237 o ¢--. P A / D A . . . . . . . . . . . . . . . . . . . . . . . . 0.413 ,-0.40 BTF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.047 TX4 -~

Q ._'5 0.30 Cq n~P3D2 Q = torque, lb-ft

T 3 Cr n2p=D2 T = thrust, lb + O.ZO

n = RPS Tv ~-

2Qn 0. l o 2°n -- v

S Pn

v = speed of advance, fps

P = pitch, ft D = diameter, ft

0.80 0.08

/ o ~ o.os~

%// o.os / 0.02

- - ~ 0.01

0 0 -0.10 0 0.10 0.20 0.30 0.40 0.50 0.60

Slip R~i'io

b e i n g b u i l t f o r e x p o r t w h e r e fue l s s u c h as b u n k e r C a n d coal a r e c h e a p a n d g o o d Diese l fue l is dea r . T h e c lass ic r e l i a b i l i t y of s t e a m also m e r i t s con - s i d e r a t i o n w h e r e t h e c r e w s a re l ike ly to b e u n -

sk i l led or r e p a i r p a r t s u n o b t a i n a b l e fo r d e l i c a t e D i e s e l i n j e c t o r s , g o v e r n o r s a n d t h e like. I n o c e a n g o i n g tugs , w h e r e s t e a m m a y b e u s e d fo r t h e t o w i n g en g i n e , h e a t i n g a n d o t h e r a u x i l i a r y


190

180

170

160

150

140

130

12C

llO

a. IO0

90

80

70

60

50

40

30

20

I0

/

®

/

/

EHP -]SHP ...@:I--.. i ~ I - - $-----l~r-" )//

0 I O zj'~

z

/ Z

i

5 4 5 G 7 5 9 10 IZ 13 14 15 Veloci# 5, in Knots

FIG. 10 SHAFT HORSEPOWER, R P M , AND DERIVED CURVES FOR 100-FT T X 5 TUG (Model 4088--Propel ler 1832, displacement 482.33 tons ; wetted surface 3605 sq ft. )

IG 17

800

700

600

500

400 ~

g I

300

200

100

services such as ice-freeing the superstructure, s team main propulsion may very well be a pre- ferred choice.

Direct-Connected Diesel

In late years there have been few tugs built in this country tha t were not propelled by Diesels, the most common installation being either the direct-reversing or the clutch-reversing type.

The direct-reversing Diesel is an a t t emp t to secure the advantage of the s team engine in tha t m a n y of these plants are slow heavy-du ty engines directly connected to the propeller shaft. There

can be little doubt tha t this is one of the most economical from the s tandpoint of first cost, simplicity, and dependability. However, other factors should be considered before choosing this installation. The direct-reversing Diesel is normally air-started. In a harbor tug where maneuvering is almost constant, the demand for air is large and necessitates large air- tank capaci ty and compressors capable of charging the tanks in a very short period Of time. The loss of air can very well be dangerous.

The USCG requires bott le capaci ty sufficient for 12 starts and a charging rate of 1 hr for the


FIG. 10(a)

• M O D E L R E S I S T A N C E D A T A

SHIP 8 ~ . 6 7 ' x _ Z ~ 6 ~ f , ~ _ LABORATORY TMB TEMPERATURE 7 5 " F

, .~ . HARBDR T.UG~JJ~SJ~_ BASIN ~ E E L W A T ~ R _ _ _ W A T E R COND. ~ T I L L = FRESH

MODEL NO. ~ O B B ~ BASIN SIZE ~ 2 2 ' MODEL MATERIAL WOOD

APPENDAGES R U . P D J ~ R MODEL LENGTH ! l , l l ' MODEL FINISH PAINT

T E S T ~ D A T E 8°1.~-49 TURBULENCE _ _ INDUCED

REMARKS,_T~J~tBULENCF INflLICFD RY SAND STRIPS ON R O W

MAX. WLB/Bx_I 0 0 0 M A X I M B / B x I . O O O _ HS/Bx.OO625_ DR/B x .0620_ BR/Bx_.23~B__ BKW/B x NONE_

b 4 S O L ~ ~40L - - ~t

'~ BO ~ / ' ~ . ~ " ~ A x / A M = 1 .006 WATERLINE BEAM "~,.T~,~, " ' % . BX /BM" L O 0 4

,o I , / " <" / " - S tT'°'t "lEAS i-- "-. " - . ,. _ o _

18 16 14 12 [ IO ] 8 " 4 2 iA /A x 0 .12g .2B*~ 1442 .59";' ; .745 8 6 0 LOOC .994 . 965 .920 8 5 2 .766 . 6 6 0 . 540 .404 .25(l A l l CI B / B x 0 ' .93s .9~6

.559 . 734 8 3 5 . 9 0 3 1 . 9 5 0 9 7 6 .991 1.000 LOOO .996 9 8 5 .967 9 5 3 , 885 , 8 0 6 . 703 .572 .408 .216 0 ~ l O / ~ l 0 .214 . 363 . 4 8 2 5 9 1 1 . 6 8 2 .760 815 .851 .861 851 I )31 801 7 6 2 , 702 ,631 ,551 .446 3 2 3 .181 0

d A / d L 2.2£ 2 .93 3.15 3.15 3 ,02 !2 .67 L 9 8 L 2 9 0 .62 0 0 .37 0.76 1.17 1.56 1.96 ;=.27 2 5 7 2 .85 2 .95 2.68 1 .70

1.00 0.t0 /

0.90 / 0.09

P = "pitch, ft / D = diameter, ft

0.10 _ _ _

0

Fro. t0(b) 0.80

CHARACTERISTIC CURVES

Diameter, in. .13.00 >" . . . . . . . . . . . . . . . . . . . . u 0.'70 Pitch, in . . . . . . . . . . . . . . . . . . . . . . . . . . 11.75 No. blades . . . . . . . . . . . . . . . . . . . . . . . . 4 "~ Speed of advance . . . . . . . . . . . . . . . . . . 3 . 4 - 5 . 0 8 ",.7- Linear ratio, X . . . . . . . . . . . . . . . . . . . . 9 .0 ~ 0.60 P / D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0. 904 x~ M W R . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.271 P A / D A . . . . . . . . . . . . . . . . . . . . . . . . . . 0.48 "~ 0.50 B T F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0. 042 T X 5

Cq nZP3D2 Q = torque, lb-ft "5 0.40

T C, n2P2D2 T = th rus t , lb ~ 0.30

n = RPS Tv

e = 2Q--n v = speed of advance, fps

P n -- v 0 2 0 S

P n

0.08

0.07

o o ~

cl 0.05

/ / / \ ° 0.04 c g

/ 0.03 ~ /

-0.10 0

0.02-

0 OAO 0.20 0.30 0.40 0.50 0.60

Slip Ra{io

0.0I


130

125

IZO

115 i

II

I0

100

95

9O

85

80

-/O

G5

GO

55

50

45

40

35

30

25

/ EHP

/ /

. = /

0 2 3 4 5 G 7 8 9 10 II 12 13 Velocity in Kno#s

FIG. 11 SHAFT HORSEPOWER, R P M , AND DERIVED CURVES FOR 100-FT TX7 TUG (Model 4094--Propeller 1006, displacement 462.57 tons; wetted surface 3653 sq ft.)

14

700

G50

6OO

55O

5OO

45O

4OO

350 o cp

300 ~

Z50

ZOO

150

I00

5O

0

compressors . Usual ly , wi th p r o p e r l y m a n a g e d tugs th is is sufficient; however , wi th on ly a few sl ips on the p a r t of the engineer or s t a r t i ng up on a cold day , the a i r m a y be d i s s ipa ted in e m b a r r a s s i n g l y few s ta r t s .

Of l a te yea r s the re has been a l eg i t ima te effort to p lace all cont ro l s a t the h a n d s of the t ug mas te r .

M a n y t imes when d i rec t - revers ing Diesels are s t a r t e d and reve r sed f rom the p i lo thouse the tug m a s t e r has b lown off all of the a i r w i thou t real iz ing the engine had ach ieved enough r p m to s t a r t . M a n y t imes too, the m a s t e r has no t no t iced t h a t a i r was no t ava i l ab le to pe r fo rm the m a n e u v e r s and found himself wi th a dead ship.

TUGBOAT DESIGN 611

FIG. l l ( a )


SHIP Sl.l' x ~KO' x IO I~' I~)K LABORATORY TMB TEMPERATURE SS*F

COAC, TWISE AND HARBOR TUG BASIN DEEP WATER WATER GOND. ST ILL : FRESH

MODEL NO. 4OR4 BASIN SiZE 2775 ' x 51'x : ~ ' MODEL MATERIAL WOOD

APPENDAGES RUDDER MODEL LENGTH I1.11' MODEL FINISH PAINT

T E S T ~ D A T E ~, - 14 - F*O TURBULENCE INflU~FD

REMARKS,TURBULENCE INDUCED BY SAND STRIPS ON B O W

~u ,~ ,'8 ,It ,Is ,1~ """ , ,

MAX. WLB/BxlLQ_0_L MAX.IMB/BxA~O_OI_ HS/Bx_~0J~_ DR/Bx x~5#D_ BR/Bx~LS~_ eKw/Bx_~toJ~__

4 6 0 L 'I" . ~ 4 0 L " I .Ioo L----'-'-'---I- ~ - - ' - ' - "~ [ [~"~]:::::~ ~ I 1

u. " / i --V ~ ~ - WATERLINE BEAM! ~ ~ ~ IB~ /R , , . l ~O~ 0 60 - - / ' 1 . . . . . . ~ f ~ " ' - ~ " - - [ - - - -SEGTION AREAS - '-- '~-- ' / '1 "" '~ ~ . t , ' ' ~ " z 40 . . . . . . . . . . . . . I I I I " - L B O S,,L AFT STAO I I , - " - - ~: 2 " / - - , - ~ " ~ _ ~ . ( _ ~ t ~ L _ _ l - P O I N T S OF I N F L E G T I O N - - - , [ - - ' ~ . ~ I

IIA/IIL[I,I III Z,ll2]'t.l] 3.10 ~I.0] ~,64 2,04 I ,~I0,~I I I O.Ol O,,llO e t02 1.47 1,91 ~,~0 9 1 0 ~,TO t*.*ttt '2. ' l ,2 1,9~ 1.411,

l.O0 0. I0

Q = t o r q u e , lb - f t

T = t h r u s t , lb n = R P S

v = s pe e d of a d v a n c e , fps

P = p i t ch , f t D = d i a m e t e r , f t

0.g0

FIG. l l ( b ) 0.80

CHARACTERISTIC C U R V E S P R O P E L L E R N O . 1 0 0 6

D i a m e t e r , in . . . . . . . . . . . . . . . . . . . . . . 1 3 . 5 5 o>"0.70 P i t c h , in . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 . 2 5 c No . b l a d e s . . . . . . . . . . . . . . . . . . . . . . . 4 '5 S p e e d of a d v a n c e . . . . . . . . . . . . . . . . . 1 . 8 - 3 . 8 K ~ 0.' 0 L i n e a r r a t i o , X . . . . . . . . . . . . . . . . . . . . 9 . 0 P / D . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 9 0 4 M W R . . . . . . . . . . . . . . . . . . . . . . . . . . 0. 250 P A / D A . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 4 5 3 "-~ 0.50 B T F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T X 7 -~

O .~ 0.40 C~ n2P~D~

T Cr n2p2D2 o 0.30

Tv ?- e = 20--n ~ 0.20

P n -- v Pn

0. I0

0 -0.I0

/

// / /

/ /

0 0.I0 0.20 0.30 SI;p Ra+{o.

0.40 0.50

0.09

0.08

0.07

,~ o.o~

0.05 "~ %-

0.04

0.03

0.02_

6,o ,:, "

, 0 0.60

612 TUGBOAT DESIGN

IO00

g00

800

q00

60(

~ 50C

o I

400

300

ZOO

00

0 I ?_ 4 5 6 7 8 9 10 II 12 13 14 15 16 Veloci fy in Kno¢s

FIG. 12 SHAFT HORSEPOWER, I~PM, AND DERIVED CURVES FOR 100-FT~TX8 TUG (Model 409 i--Propeller 2937, displacement 347.7 tons; wetted surface 3050 sq ft.)

200

19(

18C

i7C

150

i40

130

i20

i l O

IO0

9O

80

70

60

50

40

3O

2O

I0

0

The solution to these problems lies in elimina- t ion of pilothofise control and re turn ing to an engineer answering bells, or if this is no t ac- ceptable, at least placing low-air a larms in the pilothouse. An indicator showing the engine has achieved enough rpm to be taken off air and

placed on fuel also should be located in the pilothouse, or be t te r yet, make the s tar t ing control au tomat ic so tha t when the engine control lever is .placed in s ta r t ing position, the air is cut off and fuel is injected with no fur ther m o v e m e n t of the pilothouse control.


F I G . 12(a )

M O D E L R E S I S T A N C E DATA

SHIP 114 ~R' ~ ~ I' It I I ~ LABORATORY TMS TEMPERATURE 6 2 " F

~¢ OCEAN'GOING TUG_ I 2 K BASIN OFFP WATER WATER COND. S T I L L F R F ~ H

MODEL NO. 4091 BASIN SIZE ] ) 7 7 ~ ' x ~ l ' x ~ ' MODEL MATERIAL w o o n

APPENDAGES RUI~I3~R MODEL LENGTH I I I | ' MODEL F INISH PAINT

TEST_J_ iL2._DATE I I - I £ - 4 9 TURBULENCE INnUC~'O

REMARKS, T LIRRLIL~N(~E IN~U(~E~ RY SAND STRIPS ON ROW

I y /o .....

in414,

• . , ..

2 19 18 17 16 15 5 4 3 2 I

MAX. W L B / B x L O 0 3 MAX. I M B / B x J . ~ H S / B x ~ O Q Z B _ _ D R / B x - . L O 3 L - - - B R / B x ~ B K W / B x N n N F

' ~ ' ~ " - ' ~ I 1 - - ~ ~ - - - - i SO . ,~ f / I I I - " ~ ~-. Ax/AM-~C,~

I / ~" WATERLINE BEAMS - - ~ 6 0 " / - / ~ " - SECTION AREAS- ~ ~ ; ~ ~ ' ~ BX/BM'LQQ~

~ " LOB 0 . 5 0 6 L AFT STA. O - - . . t - 0 3 8 20~, / " '~1~" " POINTS OF INFLECTION - J " ~

ISTATIONSt I ~ \ " 16 14 IZ I I0 j 8 6 4 2 ~ --~..~

~ /A x 0 .06i .218 .376 . 530 .675 .797 .894 .964 .995 1.000.972 .920.844.744.6~6 .489 .343 .196 .069 0

~ /B X 0 . 3 3 S . 5 4 3 . 6 9 0 . 8 0 0 8 7 9 . 9 3 1 .973.9941.0031,000.985.960.917.852.767.65| 5 1 6 . 3 5 5 . 1 7 7 0

~D/B~[ 0 A 3 . = .~'75 .397 .503 .608 .696 .763 ,811 .837 ,832 .817 .783 . 7 3 4 . 6 7 3 ; 592 . 5 0 ( .401 .277 .14E 0

dA /dL 0.26 LgC 3.18 3.14 3 .00 ; ' .70 ~).18 1.67 1.06 0.25 0.36 0.81 1.29 1.78 2 .20 :Z .57 2.85~2.95 2 .82 I.gE ~.76

1.00

0.90

Fro. 12(b! 0.80

C H A R A C T E R I S T I C C U R V E S

PROPELLER NO. 2937 ~ /

P i t c h , in . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0 . 5 0 "D No. b l a d e s . . . . . . . . . . . . . . . . . . . . . . . . 4 ",~ Spe ed of a d v a n c e . . . . . . . . . . . . . . . . . . 1 . 6 - 4 . 8 K ~ 0.60 L i n e a r r a t io , X . . . : . . . . . . . . . . . . . . . . 9 . 0 ~c P/D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 897 v M W R . . . . . . . . . . . . . . ' . . . . . . . . . . . . . 0.223 c~ r " 0"50 PA/DA . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 404 B T F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 049 ~'~

"5 0.40

n2P'D~Q ~o.~ / C ~ Cq Q = t o r q u e , lb - f t

T o Cr = nTP2-D2 T = t h r u s t , lb 0.30 /

n = R P S ~ T v ..c / e = 2D~ v = s pe e d of a d v a n c e , fps v-- 0.20

S Pn - v / - Pn P = p i t ch , f t D = d i a m e t e r , f t O. IOi /

/ 0

-OJO 0 O.IO 0.20 0.30 0.40 Sl{p Rai-io

/

X

O.lO

0.09

0.08

0.07

O.OG

'D 'x,~ °'°5 g u

o.o4 L

0.03

0.02

0.01

0 0.50 O.£K)


o _

40~

35(

300

250

200

[50

100

5O

I

EHP SHP

/

?- 3 4 5 G 7 8 9 10 II 12 13 14 .15 Velocity in Knofs

Fro. 13 SHAFT HORSBPOWBR, R P M , AND DSRZVS# Ctmvss FOR 100-FT T X 9 TUC" (Mode l 4085- -P rope l l e r 3057, d i sp l acemen t 302.78 tons ; we t t ed sur face 2857 sq f t . )

16

1500

1400

1300

1200

llOO

I000

900

800 o~

700 o

212

600

500

400

30O

200

I00

0 ~q

Another factor that should be considered when proposing a direct-reversing Diesel is the weight of machinery which necessitates heavy founda- tions and adds to the expense in unexpected ways. Additional crew members are necessary to effect repairs where heads have to be lifted with tackles instead of man handled. Stowage of large pis- tons, liners, heads and rods may take up valuable space. Trolley systems may have to be installed to move spare parts from stowage to the engine, etc.

Reduction Gear Diesels with Clutches

There has been a trend toward lightweight medium- to high-speed Diesels driving through. clutches and reduction gears. This type of arrangement has many advantages over the direct- connected DieSel. The original cost is often less and this is often a determining factor.

The use of reduction gears will allow the propeller rpm to be chosen for maxinmm efficiency. This economy will result in large savings of fuel throughout the life of the tug.

Clutches of the Airflex type have proved them- selves over a number of years of use and can be relied upon to give satisfactory service if a few simple precautions are observed. The use of such clutches allows the much desired pilothouse control. Here a word of caution must be interjected. A rubber tire-type clutch will burn out in a matter of seconds if the air pressure drops. A warning light should be placed in the bridge indicating low clutch air.

Better metallurgy and engineering have allowed the Diesel rpm to be increased without much prejudice to maintenance eosts. When repairs have to be made, the cost of parts and labor are lower than with the heavy-duty low-speed Diesel.

TUGBOAT DESIGN 615

FIG. 13(a)

MODEL RESISTANCE DATA

SHIP 1~4R' x -~-~s' * I ;~,G' LABORATORY "]'MR TEMPERATURE 73" F

.~.5. OCEAN - GOING TUG-15.Sk BASIN DEEP WATER WATER CORD. .~,TILI FRE!;H (972)

MODEL NO, 4 0 R 5 BASIN SIZE ~ 7 5 ' x 51 'x :):)' MODEL MATERIAL woofl APPENOAGE6 RUflB~R MODEL LENGTH IO 9 0 MODEL FINISH PAINT

T F S T ~ D A T E 7 - I:) - 4ej TURBULENCE, NOT ;NOUCEfl

REMARKS

9 18 17 16 15

o

WL lilT.

MAX. WLB/BxJ_O.QQ_ MAX. I M B / B x ~ HS/Bx_.QQ94_ DR/Sx .085 B R / B x . ~ BKW/Bx__JtO]~

l' . 4 8 0 g ,J • . 5 2 0 C

SO / , , / ~ :>... Ax/AM "J~O~- / ~ -" WA RUNE BIEAMS " )~ .

60 I / i" ~" -- SECTION AREAS / ~ , - , ~" ,, BX/BM'.LDO~

0 , / LGB 0 . 5 0 2 L AFT STA. 0 ~ \ I - . 2 9 5 POINTS OF INFL~GTION- ~

ISTATIONSI [ " ~ "118 IS 14 12 I I0 I 8 6 4 2 ~1~

l ] .563 .698 .814 .910 968 .997 ,999 .977 .g34 .870 .785 .675 .546 .31)8 2 1 9 . 0 T 3 0 &/A x 0 .061 .232 AO9 1.50 1,25 .995 .998 .9174 .93§ I ] .816 .11111 .930 .965 987 .992 .87~:.793 .685 .548 ,363 .199 .001 ~ / 6 X 0 .39"/ .605 .732

I.,,o , , , , , , ' " " " : : : o , . oo, I ,535 .633 0.73 .860 .866 .855 .123 .7T7 .'tZl .650 ,561 .44g 321 . t77 .001m :IA/dLIO.2OIt.46 i.75 t.so o.9~, 0.32 0,~ ~, 0.71~ 0.97 1.19 40 i.61 i.65 i . f4 0.511

F I G . 13(b) 0.8C

CHARACTERISTIC CURVES r ~ P R O P E L L E R N O . 3 0 5 7

D i a m e t e r , in . . . . . . . . . . . . . . . . . . . . . . . 9. 466 ~ 0.7( P i t c h , in . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. 120 ,c No. b l a d e s . . . . . . . . . . . . . . . . . . . . . . . . 3 :-~ S p e e d of a d v a n c e . . . . . . . . . . . . . . . . . . 2 . 1 - 6 . 1 K ~ 0.6I L i n e a r r a t io , X . . . . . . . . . . . . . . . . . . . . . 9 . 0 P / D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 7 5 2 M W R . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 3 5 1 d P A / D A . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 4 8 8 '-20.51 B T F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 0 5 T X 9 -,~

_ _ _ _ "5 0.40 Ca - Q Q = t o r q u e , l b - f t ~= n 2P3D2

T o Cr = n2P~D------- 2 ,T = t h r u s t , l b

n = R P S ~ 0 .30

r v ~- e = 2Q--n v ~ s p e e d of a d v a n c e , fps ~- O. ZO

P n - - v P -~ p i t c h , f t

S P n D = d i a m e t e r , f t 0.10

1.00 0 . I 0 /

0.g( / 0.0g

0.08 /

/ / ' //, ,)"

/

/ 0

-0.10 0 0.10 0.20 0.30 0.40 0.50 Sl ip Ra ' f i o

).07

~.0~

0 . 0 5 ~

0.04 o-

_ _ 0.03

O.OZ

0.01

0 0.60


7_40

230

220

ZIO

200

190

180

170

160

150

14(

13- 130

120'

I10

too

90

80

70

6o

50

40

30

I I L i I - - I

t - - ! 0 I 2 3 4 5 g 7 8 9 10 II I?_ 13

Veloci'l-y in Kno÷s

FIG. 14 SHAFT HORSEPO%VER, R P M , AND DERIVED CURVES FOR 100-FT rXl0 TUG (Model 4086--Propeller 3058, displacement 3.50.40 tons; wetted surface 30 t5 sq ft. )

I?.00

I100

I000

900

800

700

600 ~_

o

5003::

400

300

200

100

Many of the manufacturers of lightweight high- speed Diesels have adopted the system of unit replacement so that only very serious repairs need hold up a tug for any extended period.

In some recent tugs two or more engines of the high-speed type are geared together to drive a single propeller. This arrangement provides at

low cost a reliability very desirable in a single- screw tug.

With the clutch-operated reverse-reduction- gear Diesel, the engine is running at all times; therefore, the . t ime for effecting maneuvering is cut markedly. Concomitants of this are the advantages of warming up the Diesel before loading

TUGBOAT DESIGN 617

MODEL RESISTANCE DATA

SHIP 140.o' x 53.0' x 13.5 ' L A B O R A T O R Y _ _ _ _ ~ L TEMPERATURE 64" F

55. OCEAN-GOING TUG-I4K BASIN DEEP WATER WATER GOND. STILE. FRESH

MODEL NO. 4086 BASIN SIZE ?775' x 51' X 22' MODEL MATERIAL WOOD

APPENDAGES RHnnER MODEL LENGTH II I1' MODEL FINISH PAINT

T E S T ~ D A T E 4 - ~ 7 - 4 9 TURBULENCE N~T INOLICED

REMARKS _

FIG. 14(a)

2 0 19 18 17 16 15 ~H.W

4

5 2

: • V L ENT. rtt ~,14 S"

MAX. WLB/Bx IDOOMAX. IMB/Bx~A~10_ H S / B x ~ D R / B x ~ Q _ 7 _ 6 1 _ B R / B x _ . 2 J ~ Q _ _ B K W / B x _ h I D I t E _

~) 60 / / ~ WATERLINE BEAM, c ,. - SECTION AREAS ,,. ~ .

/ I I I 40 , /~ LOB 0 .506L AFT STA. 0 . / " x ~ 2 0 / , / ~ -POINTS OF I N F L E C T I O N - - -

I ISTATIONSI [ ~,.~, / m ~s t¢ ~z [ ~o I 8 6 t 4 z - ~ ,

IA/A X 0 1 1 9 . 2 7 2 . 4 2 3 ~ 7 4 . 7 1 5 8 3 3 , 9 2 2 . 9 7 3 . 9 9 6 L 0 0 0 . 9 8 4 . 9 5 5 . 8 9 7 . 817 . 6 8 5 . 5 3 8 .37'6 .221 .OoJI 0

B ( B ~ 0 .510 . 7 0 9 . 8 2 3 , 8 9 8 9 4 8 9 7 7 . 9 9 2 . 9 9 7 1.000 LOOO . 9 9 7 9 8 9 9 5 9 9 0 0 7 9 9 . 657 . 4 B 4 . 3 0 2 .130 . 0 0 4

~ID/B{)I 0 .210 . 3 5 2 . 4 6 7 . 5 7 2 . 6 6 9 .75C . 8 1 4 . 8 6 2 . 8 8 4 . 8 7 6 . 8 6 6 . 8 3 8 . 7 9 8 7 3 8 6 G Z . 5 5 0 . 4 1 2 . 2 6 2 . 1 2 4 0

dA/ .dL 1.86 2 .80 3 , 0 7 3 . 0 7 ; L 9 5 2 . 6 2 : Z . 0 9 1.44 O.TB 0 . 1 3 0 . 1 9 0 . 4 4 0 . 7 8 1.31 2.0C t . 7 7 3 . 1 4 3.2q 2 . 9 0 2 . 2 0 1 .04

&x/AM = L . ~

f=_O~ t • O~SZ

F m . 14(b)

CHARACTERISTIC CURVES PROPELLER NO. 3058

D i a m e t e r , in . . . . . . . . . . . . . . . . 10. 476 P i t c h , in . . . . . . . . . . . . . . . . . . . . 9 . 524 No. b l a d e s . . . . . . . . . . . . . . . . . 4 S p e e d of a d v a n c e . . . . . . . . . . . . 2. 056 L i n e a r r a t i o , h . . . . . . . . . . . . . . 9 . 0 P / D . . . . . . . . . . . . . . . . . . . . . . . 0 . 9 0 9 M W R . . . . . . . . . . . . . . . . . . . . . 0. 299 P A / D A . . . . . . . . . . . . . . . . . . . 0 . 512 B T F . . . . . . . . . . . . . . . . . . . . . . . 0 . 0 5 1 T X 1 0

C~ Q n2P3D':

T n2P2D 2

Tv 2Qn P n - - v

S P n

Q = t o r q u e , l b - f t

T = t h r u s t , lb n = R P S

to 5 . 8 5 K

v = s p e e d of a d v a n c e , fps

P = p i t ch , f t D = d i a m e t e r , f t

1.00

0.90

0.80 %-

>3 ~ 0.70 _ _

.__o

O.GO

f~ ,-0.50

"5 0.40

,$

8 0.30

o.?o /

0.10 /

0 -0.I0 0 0.I0

/4

0.20 0.30 0.40 0.50 Sl ip R a f l o

010

0.09

0.08

0.01

u _ _ _ _ 0.05

0.04 V

0.03

0.02

0.01

0 0.60

618

400

TUGBOAT DESIGN

350

300

25O

ZOO o¢

150

100

50

ZO

10

0

- I 0

-ZO

\ /

/ /

/ /

EHP

" . " ~ ,

f "--*---.Z

/

/ /

I

2 3 4 5 6 -/ 8 9 IO II IZ 13 14 15 16 Velocif~' in ~ n o f s

FIG. 15 SHAFT HORSEPOWER, RPM, AND DERIVED CURVES FOR 100-FT T X I I TUG (Model 4090--propeller 2804, displacement 286.35 tons; wetted surface 2865 sq ft.)

1500

1400

1300

I?.00

I100

I000

1]

go0

800 o~

qO0 L o I

GO0

500

400

300

200

100

it down, reduced air demands allowing smaller compressors and bottles and reduced complexity of the engine due to its rotating in only one direction.

Direct-connected Diesels having a relatively high starting rpm start the tug with a lurch.

Unless the tug master is very careful, broken hawsers and lines will result. This problem is minimized with the clutch-operated Diesel where the engaging rpm may, and should be, the idling rpm of the engine.

While the controllable-pitch propeller, per se,

F m , 15(a)



SHIP ' ' ' LABORATORY TMR TEMPERATURE KT, F

15 K OCFAN - GOinG TUG BASIN [)~'FP WATF'R WATER GOND. ~TH 1 FRF~,H

MOOEL NO. 4 0 9 0 BASIN SIZE 2775'~ Sl 'x ~9' MODEL MATERIAL WOOO

APPENDAGES RUDIM:'R MODEL LENGTH It I1' MODEL FIN(SH PAINT TEST ~ DATE 10-10-49 TURBULENCE INnUdF(~ REMARKS' TURRLIL~'NCE IN~LIC~D BY SAND STRIPS ON ROW

SINGLE SCREW 140 KNOT

5

/

0

MAX. WLB/BxL..O£uD_ MAX. IMB/BxJJ~_QQ_ HS/Bx_.OIZ9__. O R / B x ~ B R / B x ~ BKW/Bx_NORF.~

.435L "1"

/ WATERLINE BEAM¢. ' / SECTION AREA,*

i / . x . . , , 4o LGB o.so, • APT STY. o ,~. 2 0 1 / 1 / " - PCh%'TS OF INFLEGTI ) N -

ISTATIOt~I o. p - I0 m ~4 12 I ,o I 8 A / A x [.O~Sl .131 . 326 , ,54] , 7 2 2 . 8 5 4 .g3G 9 8 2 . B i g . 0 0 0 . 9 9 5 . 9 8 0 . 969 . 9 3 0 B / B ~ . 0 3 6 . 4 1 0 .S' t2 . 221 ,1) 13 .96(] .112| I ) 9 3 1.00¢ . 0001 .000 1 , 0 0 0 . 9 9 4 . 9 7 6

~ 1 / 1 ~ 1 . 0 3 2 . 2 1 6 .411 .57S .'705 . 7 9 2 1 1 4 0 , 8 6 5 .881 .1121 .881 . 8 7 3 . 1 1 6 2 . 8 3 ~ G A / d L 0 . 7 3 2 . 8 3 4 . 3 5 3 , 0 | 3 .10 2 .14 1 .20 0 , 5 7 0 .16 0 0 , 0 7 0 .23 0 , 5 3 1 ,03

- - ,565 L -~-

Ax/A M • J . Q f l ~ ">. ~ GX/eM ..LO~.O

~ \ f '_O__ - , ~ \ t - i i a

,,,,\

9 Z 9 . 8 5 2 . 7 4 2 ,~191 , 4 0 3 . 1 9 8

"I'89 . 7 1 1 .611 .4119 . 3 4 6 .1114

1.75 2.41 .?,,.go 3.22 3.3 w' '1'.08

1.0(

o.g(

Fro. 15(b) ~,~ 0.80

C H A R A C T E R I S T I C C U R V E S PROPELLER NO. 2804

"u 0.70 D i a m e t e r , in . . . . " . . . . . . . . . . . . . . . . . . . 8 . 0 0 P i t c h , in . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 9 8 2 N o . b l a d e s . . . . . . . . . . . . . . . . . . . . . . . . . 4 S p e e d of a d v a n c e . . . . . . . . . . . . . . . . . . . 2 . 2 - 6 . 0 K ~ 0 .90 L i n e a r r a t i o , X . . . . . . . . . . . . . . . . . . . . . 9 . 0 o P / D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O. 8 7 3 ~-~ M W R . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 2 4 P A / D A . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . 4 6 ~ 0.50 B T F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V a r . T X l l 13

Q ~ 0.40 C~ = n~p3D2 Q = t o r q u e , l b - f t u

T % Cr = n 2 P 2 D ~ T = thrust , lb _c ~ 0 .30

n = R P S ~-

Tv e = 2Q~ v = s p e e d of a d v a n c e , fp s 0.20

P n - - v S P = p i t c h , f t

P n D = d i a m e t e r , f t 0.1q

-0 . I0

i O. to

0.09

0.08

t

0 0.10 0.?.0 0.30 S l ip Rct'fi o

0.07

0.06

0.05

£.)

0.04 g

0.03

0.02

0.01

0 0.40 " 0.50 0.80

620

IZ.0

IO.0

8.0

5 6.0

o

4.0

z.o

FIG. 16

TUGBOAT DESIGN

0 ZOO

Cp

0.64.

250 300 350 400 450 500 &

Displacemen+- Leng+h R~+io (0.01 L) ~

TUG RESISTANCE CONTOURS--CONTOURS OF PRISMATIC COEFFICIENT

(Cp)--SPEED LENGTH RATIO V/%/-L = 0.9

12.0

lO.O

8.0

~.0

4.0

Z.O

0 ZOO

/ / \

r

o.~o , ~ 0.58

250 300 350 ,00

Displacemen+ - Lencjfh Ra~io (0.01 L) - - ~ ' - - - ]

450 500 &

FIG. 17 TUG RESISTANCE CONTOURS--CONTOURS OF PRISMATIC COEFFICIENT

(Cp)--SPEED LENGTH RATIO V/VZL = 1.00

will be considered later in this paper, it also must be considered as a propulsion system. While propeller experts may argue the hydrodynamics of twisting the blades and the serious effect upon

efficiency, the possible reduction of efficiency due to large hub diameter and other factors, the patent advantages must be admitted by even prejudiced advocates of the fixed-pitch wheel.

TUGBOAT DESIGN 621

12.0

I0.0

8.0

6 . 6.0 _(2

4.0

Z.0

200

FIG. 18

250 300 :350 400 450 500 Displacemewt. _ Leng.~ h Ra~rio A

(o.ol L) 3


(Cp)--SPEED LENGTH RATIO V/V~ = 1.10

12.0

10.0

8.0

8 ~.0

o

4.0

2.0

FIG. 19

0 200 50

I

\ \0g 0 5# .58

Cp

300 350 ,00 450 /X 0isplacemen~r - Lengl 'h Ra~-io (0.01 L) ~

TUG RESISTANCE CONTOURS--CONT0URS OV PRISMATIC COEFFICIENT ( C p ) - - S P E E D LENGTH RATIO V/v/L = 1.20

The major advan t age of the control lable-pi tch propeller is the facil i ty with which a vessel may be bridge-control led. Here the pilot is free from worry as to avai lable s t a r t ing or c lu tching air.

He does no t have to concern himself as to whether or no t the engine will start , b u t can place all of his mind and not "inconsiderable ta lents to the problem which he and the tug are there to solve.


12.0

iO.O

8.0

× 6.0 %

4.0

Z.0

0 200

FIG. 20

. ~ 0 . 6 2 " /

~x'-~. 0.84 ~ \ 0 . 6 6 /

" 0 . 6 8 / Cp

I f

5

250 300 ~50 400 450 500

Displctcement- - Leng fh Rat-i o ZX (0.0t L?


( C p ) - - S P E E D LENGTH RATIO V/~/L = 1.30

The elimination of the necessity for reversing an engine gives positive operation and results in longer engine life. Reverse gears and clutches with the room and weight consumed are eliminated by using controllable-pitch propellers.

Tugs equipped with controllable-pitch pro-" pellers can for prolonged periods operate at reduced speeds. This is extremely impor tan t when oceangoing tugs are required to heave to. Dur- ing docking, the ability to exert small thrust for relatively long periods is also useful. The controllable-pitch installation allows the gradual take-up of slack in hawsers which results in savings and safety. All of these advantages will be mentioned as pertaining to the Diesel-electric system.

The major advantages of the controllable- pitch propeller over the Diesel-electric drive appear to be the great reduction of weight possible and the ability to match (to some extent at least) the propeller to the main engine at all power requirements and rpm.

Diesel-Electric Propulsion Diesel-electric propulsion has been very popu-

lar in recent years and just ly so since this system provides in a measure much of the flexibility which had hitherto been found only in s team tugs. The elimination of reduction gears and remote control are mat te rs tha t war ran t consideration.

Automot ive- type high rpm Diesels can be used in multiple to give a very flexible power availability.

There are, however, as in all mechanisms, certain disadvantages tha t must be weighed by both owner-operator and the designer before deciding on the correct system. The efficiency of the Diesel-electric drive from prime mover to propeller is at best 85 per cent and more often 80 per cent. This order of loss is not occasioned by any of the other types of machinery mentioned when operating at designed op t imum condition. This differential in efficiency is reduced considerably when the propeller is operating at other than opt imum. When compared to the direct-reversing Diesel system which must operate at about 75 per cent of its available horsepower when at 100 per cent propeller slip, the Diesel-electric drive proves its superiority by allowing the full engine horsepower, less the transmission efficiency, to be applied to the wheel.

The owner will find tha t a higher class, or at least higher salaried, engineer is necessary to keep a Diesel-electric plant running than would be required with the other types. He also will find tha t an extraordinary amount of money can be spent in repairs, largely a t t r ibuted to the con- trollers.

Be these facts as they may, the ease of control by the pilot, the abili ty to have full command of the thrust from zero rpm to maximum have made,


19.0

12.0

15.0

o 13.0

I1.0

9.0 200

Fio. 21

f /

f

/ #

_____

0.56

0.66

0.68

/

v /

/

S /

250 300 350 400 450 500 D]sp lacemen+- Lenc~t-h R~÷]o /k

(o.ol L) ~

T u c RnSIST^NCn CONTOURs--CONTOURS OV PRISM^TIC COEVVICIENT (Cp)--SPI~ED LI~NGTH RATIO V/%/L = 1.40

at least for the t ime being, the Diesel-electric propulsion system a very desirable one.

Tug Propellers The sole function of a tug is to pull or push.

The most impor tant component of the entire tug system is, therefore, the mechanism tha t produces the t h r u s t - - t h e propeller. Unfortu- nately, too little consideration is given to this i tem even by competent designers. Indeed, the importance of the propeller selection should be given first consideration and priority. The cost of present day harbor tugs is on the order of $15.00 to $17.00 per lb of bollard pull. The difference between a well-designed wheel, and a wheel designed for other than op t imum towing conditions, may well result in differences of 20 to 30 per cent in bollard pull. In a 1200-hp tug this could result in about a $90,000 loss if figured a t 20 per cent loss of pull and $15.00 per lb. Surely this is worth considering.

M a n y designers calculate propellers to be correct at free-route speed. This would be ac- ceptable only in a rescue tug and debatable even then. The wheel should be designed for towing speeds which are normally in the range of 5 to 6 knots. At these speeds, the engine should turn full rated rpm and produce full rated horsepower. At bollard, under these conditions, the engine may be expected to put out about 90 per cent rated horsepower with a proport ionate reduction in rpm. At free running wi th this:wheel, the engine

must be governed to prevent overspeed (rpm). A penal ty will be taken at top speed but the tug will be working harder a t towing speeds.

There are very few propeller problems tha t could not be solved satisfaetorily if given a low enough rpm and a large enough diameter. Fig. 22 gives the results of bollard pull in pounds per horsepower on a normal harbor tug of small size. I t will be noted tha t at lowered rpm the pounds of bollard pull per horsepower is many times tha t experienced a t the rated rpm. This indicates but one thing; tha t if we are to improve material ly the tow-rope pull of tugs, every consideration must be given to larger diameter propellers operating a t lower rpm.

Fig. 23 illustrates the advantages of low propeller rpm. These curves are based upon con- s tant torque such as would be found in a s team- reciprocating or Diesel installation. The s team tug could raise the entire tow-rope pull f rom bollard to top speed by altering the cutoff point of the valves, and therein lies a distinct advantage.

Since tow-rope pull is of such interest to the tug designer, something should be said regarding the K o r t nozzle. This device has much to recom- mend it under certain conditions. Where draf t is restricted and the designer is foreed to a small- diameter propeller, where diameter is small due to use of high rpm engines, or where the towing speed is low and little free running might be expected, the Kor t nozzle m a y very well be the

62t T U G B O A T D E S I G N

100

g o

80

212 ci3 g 7O CL

O- GO

"6 c~ 50

4b

\

\

30 , ~ iO0 ~20 ~40 200 Z40 260

\

IGO 180 Sh~f~ RPM

FIG. 2 2 BOLLARD PULL VERSUS BRAKE HORSEPOWER ( D e s i g n e d m a x i m u m o u t p u t : B H P = 400 , R P M =

290 . M a x i m u m d e v e l o p e d a t b o l l a r d : B H P = 356 , R P M = 240. )

30

o"

"o

g Is C3-

c~ l0

o 5

0 0

FIG. 23

~ - - . . - . . . ~ f f . . / 2 0 RPM Pro ~eller

300 RPM Pro ~leller>

2 4 G 8 tO t2 14 Veloc i fy in Knofs

T o w - R o v E P U L L - - 1 0 0 0 B H P IN 1 0 0 - F T TUG

answer to increasing the thrust. The design of the nozzle and its propeller have been very well covered by A. M. Riddell (4) and by A. J. Daw- son (5). Suffice it to say that the principal advantage of the system is to provide the propeller with a flow field of sufficient velocity to reduce the propeller slip to a point where propeller efficiency is good. This is caused by a venturi effect with the propeller located in the throat. When the tug backs, the venturi effect is missing and no great advantage is gained. To overcome this, modern Kort nozzles are designed with some e;xpansion

in cross-section area abaft the wheel disk. If the designer should expand the outlet area to equal the inlet area, no advantage would be gained at all and indeed the tug thrust would be reduced by the amount of the friction drag of the nozzle. I t therefore appears that the tug owner must be convinced of the relative handicap in backing power before specifying the Kort nozzle.

There has been much speculation as to prob- ability of damage to propeller and nozzle by sucking in drift. An informal survey of yards along the Ohio and Mississippi Rivers, where many Kort nozzles are used, indicates that, in general, the frequency of damage is less but the extent of damage when it does occur is somewhat more. The nozzles also are subject to rather severe erosion in way of the propeller tips. Most yards now are specifying a stainless-steel insert of 30- to 40-1b plate in this area. One yard is facing the wearing area with a hard steel using a welding procedure.

In a similar craft the author has faced the ring with neoprene rubber cemented to mild steel and in another case has brushed on a flexible poly- ester resin. Both systems seem to be working M1 right after about a year 's operation, the flexible lining not resisting the abrasion but yielding to it.

Kort nozzles on oceangoing tugs because of their entrained water act to damp out pitching and because of the cover for the wheel act to prevent racing of the propeller. The deep-sea tug operator then could expect to maintain higher towing speeds in rough water by utilizing the Kort nozzle. This in itself should be a strong recommendation for its selection for oceangoing tugs.

Controllable-Pitch Propellers While this type of propeller was discussed

u.~der the "propulsion system," some few design cJnsiderations are here submitted. This propeller, like the adjustable-pitch propeller, has blades that can be twisted on their radial axis to accomplish increase, decrease, or reversal of pitch. I t must be realized that while a fixed- pitch screw of constant face pitch varies from a coarse angle at the root to a fine angle at the tip, the change of pitch is accomplished by adding or subtracting a fixed angle thus making the tip have a greater change of pitch than the root. When the propeller is feathered the blade elements at the root are still driving ahead while the elements at the tip are driving astern. The situation is of like order when the propeller is full astern. In astern condition the efficiency is quite low. But then we are not talking about great efficiency in this condition any way. For instance, a fixed pitch Taylor ogival blade with blunt leading and


trailing edges as exist in tug practice, was tested recently in astern condition a n d the efficiency curves were as follows:

Efficiency Efficiency Slip, ahead, astern,

per cent per cent per cent

0 51.5 - - 10 61.0 - - 2 0 6 2 . 2 - -

30 59.5 15.0 4O 54.5 29.5 50 47.4 30.0 60 39.3 26.6 70 3{).3 21.2 80 20.0 14.7 90 10.4 7.6

100 0 0

This was for a 4-bladed wheel of 1.00 pitch ratio with a M W R of 0.30.

In designing controllable-pitch propellers, the remarks of Prof. L. Troost 4 and discussion by Prof. L. C. Burrill, might well guide the engineer. The liberty is taken of quoting these remarks ver- batim: '.'I think we are learning more and more that changes in local pitch from root to tip do not seem to have a marked effect on total efficiency and that it is the pitch or blade angle at about 0.7 radius that governs the blade efficiency, provided always that the pitch changes at root and tip are not extreme thus giving negative or extremely high angles of incidence.

" In my view, the Betz condition, and the radial variation of pitch which can be derived there- from, can quite easily be departed from without appreciably affecting the screw efficiency . . . . "

I t then appears that from the observations of these gentlemen and from experience with adjustable-pitch wheels, the uniform pitch should be set for some low pitch/diameter setting (say at towing speed), then allowing the blade to be turned to its higher than uniform pitch setting for higher speeds. This procedure will raise the maximum efficiency at towing speed and will not be seriously detrimental to efficiency at free- route speed.

TUG RUDDERS

Naval architects and marine engineers lately have begun to realize that what goes on behind the propeller is important enough to warrant consideration. The tugboat man of course has been experimenting with rudders since the advent of the screw tug. Unlike the scientist, the tugboat man has always been prone to accept his personal

4 " O p e n W a t e r T e s t Series wi th Modern Propel ler Fo rms , P a r t 3," by L. Troos t , p resen ted at a mee t i ng of the N o r t h E a s t Coas t Ins t i tu t ion , December 15, 1950; and subsequen t discussion by Prof. L. C. Burrill .

45

40

~35 n~

30 I

,? z5 g~

15

10 50

F I G . 2 4

Leff Rudder , ~ j

~ R u d d e r 45 °

f J

f J

100 150 200 250 3 0 0 3 5 0 400 Broke H o r s e p o w e r

T U R N I N G FORCE D U E TO R U D D E R AT 4 5 D E G AND T U G AT BOLLARD

observations colored by his private prejudices. Be that as it may, he has adopted rudders that are quite unlike the ordinary ship rudder.

The tug, restricted in both length and draft, must often move and turn tows many times its length and displacement during adverse wind and weather conditions. This effort must be ex- erted at very low ship speeds. No other ship is called upon to meet such rigorous conditions save river towboats. To accomplish this mission, the rudder necessarily is as large as possible. In modern normal form tugs it reaches from very near the keel line to sometimes above the at-rest waterline.

In section, the tug rudder may vary from single- plate type to fully streamlined rudders and it is some of these variations that will be discussed.

At first thought, a modern streamlined balanced rudder would appear to be the-most advantageous. I t must be remembered, however, that this rudder while having a favorable lift-drag ratio and therefore a rather low resistance, by no nmans has the highest lift. Lift, or turning force, is what the tug master is after.

In his inscrutable manner , the tug captain arrived at a design that, while a streamline abor- tion, has lift of great consequence. The tug was fitted with a plate (or at least parallel sided) rudder to which were appended wedges along the trailing edge. This device does give remarkable lift and allows the tug to maneuver large tows with ease while traveling ahead. Astern, because of the perpendicular rectangle presented to


the water, the tug does not steer as well as if it were fitted with a streamline rudder.

One tug of Army design, having a streamline rudder, amazed the tug master with its astern steering ability, the particular tug master being accustomed to a wedge rudder.

I t is believed that most of the obvious advantages of both the wedge type and the streamline rudder could be realized by simply reversing the present streamline rudder.

Too little occasion has been taken to streamline the propeller hub which necessarily is close to the rudder in this type vessel. Here distinct advantages could be gained. This device might serve to overcome some of the reluctance to accept the controllable-pitch propeller where several per cent efficiency is lost in hub turbulence.

As a design criterion the following formula is suggested to approximate rudder area:

A I~ = L d c

where c = 0.05 for utility tugs

= 0.045 for harbor tugs = 0.04 for oceangoing tugs

An = rudder area, sq ft L = length between perpendiculars, ft d = draft (mean), ft

Turning effort was measured oll one tug having the following characteristics :

LOA -- 70 ft 3 in. LBP = 64 ft 8a//4 in.

B (mid) = 1 9 f t 6 i n . d = 7 ft 5a/4 in. k = 122 long tons

BHP = 400 Shaft rpm =- 298

Wheel diam = 5.65 ft Pitch = 4.12 ft

Rudder area = 22 sq ft

The results of the test are given in Fig. 24. The tug was secured between two piers by bow lines so that the tug could not move forward. The side thrust was measured by means of a Chatillion dynamometer mounted on the pier and secured to the stern of the tug directly opposite the rudder post.

Unfortunately, no tug of comparable size with wedge-type rudder has been available. I t was hoped that this tug could be fitted with wedge rudders and the test repeated and compared but budget limitations precluded this work.

FUTURE DEVELOPMENTS

I t seems apparent to the author that the line of future development in tugs must follow several definite courses.

Normal hulls have reached a state of development where little more can be expected from them. This is not to say that eventually some choice of hull characteristics may not allow an increase in speed. At this stage, the normal type tug does not consume in E H P a tenth part of its available horsepower when operating at towing speeds. Therefore, the efficiency of the hull as a resistful machinery platform is rather good. I t is believed, however, that the present type hull form for the average small and harbor tug unduly prejudices the efficient operation of the propeller. Increases of 15 to 20 per cent in tow-rope pull are possible for most tug operations by providing sufficient cover for the propeller and by selecting the proper propulsion-propeller system.

B I B L I O G R A P H Y

1 "Principles of Naval Architecture," edited by H. E. P, ossell and L. B. Chapman, THE SOCIETY OF NAVAL ARCHITECTS AND ~/[ARINE ENGINEERS, vol. 2, 1942.

2 "Small Craft, Construction and D e s i g n , "

by D. S. Simpson, Trans. SNAMTE, vol..59, 1951, pp. 554-582, 610-611.

3 "Resistance and Trim of Heavy Displace- ment Standard Series Ships," by A. B. Murray and J. A. Barklie, Stevens Insti tute of Tech- nology, Hoboken, N. J., Experimental Towing Tank Report No. 279, January, 1945.

4 "The Theory and Practice of the Kort System of Propulsion," by A. M. Riddell, In- stitution of Naval Architects, June, 1942.

5 "Recent Towboat Development, with Par- ticular Reference to Kort Nozzle Propulsion," by A. J. Dawson, Trans. SNAME, vol. 50, 1942, pp. 33-48, 53-54.

Discussion

MR. PAUL G. TOMALIN, ~[ember : The author Machinery and propellers are the essential presents some very interesting information on the working elements of tugs. They must receive design of tugs. ~ Information on resistance, wake high consideration, in the design. The writer's fraction, and other hydrodynamic data is a comments are mainly on the marine-engineering valuable contribution, aspects of the paper.


The author draws some very broad conclusions on characteristics of various plants which, in the writer's opinion, are not justified. He gives the impression that he has a specific installation in mind for each of the comments and discusses the pros and cons on this basis.

The enthusiastic endorsement of reciprocating steam engines overlooks many disadvantages, the largest being the cost of s tandby operation. This disadvantage is an almost overriding one. In addition to the cost of the s tandby crew mentioned, is the item of s tandby fuel, a major item in the over-all cost of operation. Space and weight occupied by the boilers, condensers and other auxiliaries are not as important as the com- plications of the plant. The plant, contrary to the impression given, is complicated by its boilers, feed pumps, condensate pumps, feed heaters, fuel pumps, fuel heaters, condensers, vacuum pumps, blowers, etc. I t only seems more simple because we are accustomed to it.

As to the advantages of low rpm, ability to maintain dead slow speeds, rapid reversals, and flexibility, these can be easily equaled or bettered by several Diesel drives. These facts are confirmed by the operators' choice of power plants of tugs built in recent years and is further confirmed by the large number of conversions made from steam to Diesel. For tugs for export, perhaps there may be merit to the steam plant, but the writer cannot think of any location, where export of tugs is probable, where bunker C will be found to be proportionately lower in cost than Diesel fuel.

In the writer's opinion, the direct-connected Diesel engine served a very useful purpose, in the transition between the era of steam and that of Diesel propulsion. By today 's standards, considering availability of types of transmissions, the direct Diesel is outmoded and has many disadvantages. The starting-air requirement is, of course, one of the major drawbacks. I t is not believed to be proper for a direct Diesel tug to be pilothouse controlled and the writer is a great advocate of the pilot house control. As a matter of fact, very few, if any, vessels of the direct- reversing type have what may be considered, pilothouse control. Pilothouse control consists of a lever which the operator can push forward for going ahead at any desired speed, or pull back to go any desired speed astern and operate the control lever at any speed and in any sequence. Practically all di/eet-reversible Diesels have extension of engin}-room controls to the pilothouse, not a pilothouse control.

Reduction-gear Diesels with clutches are discussed and some of the advantages are pointed

out, but only for one drive. In addition to the mechanical pneumatic-operated clutches mentioned, there are electromagnetic clutches and hydraulic clutches which can give very excellent control of the propeller. As a matter of fact, the eddy-current type of coupling can give speed control down to as low as the operator wishes to go, so that the advantage of flexibility, speed of reversal and dead slow operation can be obtained easily. The same is true basically with hydraulic transmissions, although there should be further development on this type of control. The airflex and eddy-current clutches are entirely satisfactory. Mention is made of possible damage to air clutches by low air pressure; no excuse would seem to exist for low air pressure. When air controls are used and, in fact, even for whistles, it is always a sound policy to have a pressure gage in the wheelhouse, showing the pressure that is available on the line.

In the geared, clutch-reversing engine drive, the writer concurs with the author that the full power of the engine should be absorbed under the designed towing condition. On page 61{3, the first paragraph is considered to belong under the discussion of the direct-reversing Diesel rather than reduction gear Diesels with clutches.

Diesel-electric propulsion can do practically anything any other plant can do and more besides. I t has tremendous advantages, it can develop full power at any rpm, it can be simple, uncomplicated and reliable but unfortunately it is very expen- sive. We have had many years experience with Diesel-electric propulsion in the Coast Guard and we have not found any undue maintenance as quoted by the author. The type of Diesel- electric propulsion used is a modified Ward Leonard variable-speed system, so that the engine is loaded at all speeds at its optimum rating for minimum maintenance and will operate at any speed of shaft desired. We have not experienced any appreciable amount of maintenance to con- trollers, because in the propulsion scheme de- scribed they are not used. The only currents handled are very small in size, on the order of 5 kw for 1000 hp. Efficiency of the Diesel-electric plant should not be in the order of SO per cent by any means. Figures in excess of $5 per cent are not difficult to obtain.

Under the section of "steam propulsion," the author makes the statement that the pitch- diameter ratio can be set about unity where propeller efficiency is the highest. The discusser does not agree with that premise. Unity pitch ratios are desirable at certain speeds but usually at speeds considerably in excess of that for which towing is designed. I t is noted that in Figs.


6(b) to 15(b), inclusive, there is only one propeller with a pitch ratio as high as unity. The average of all is about 0.885. The propeller should be selected on the basis of its maximum efficiency at the towing speed, whatever the particular circumstances dictate.

The propeller calculation is quite simple and the estimate of wake and thrust deduction that can be made from the data of model tests presented is very valuable in this respect. I t should be noted that in the various model tests that a high E H P / S H P is maintained over a wide speed range. Where it has large variations it indicates that re- finement of the lines could result in easier flow to the wheel and hence a higher towing efficiency can result. Figs. 13, 14, and 15 should have lower speeds cut off at 5 knots because it is not believed, to be the author 's intention to show E H P / S H P equal to or exceeding 100 per cent. In the Diesel- electric drive, with a simple automatic torque control a propeller can absorb full horsepower running free with the speeds of advance down to zero. This is common practice in icebreaker design.

The information given as the efficiency ahead and astern of a controllable-pitch propeller does not make it look particularly attractive for astern operation. I t is possible to design symmetrical propellers which will give excellent ahead efficiency and equal astern efficiency and, if of reasonably large size, they do not tend to cavitate as easily as the airfoil type of propeller.

Several times the author mentions sufficient cover for the propeller. Sufficient cover for the propeller is a very vague term. If the inflow of water to the propeller is good, very little, if any, cover is needed for the propeller. Large coverage for the propeller is necessary for poorly designed stern lines. The writer cannot agree with the procedure advocated by the author that p r o - viding as full waterlines aft as possible will assure cover for the wheel. This can only result in full lines in the upper waterlines and a poor flow into the propeller. The concept advocated of a hard knuckle that "could act to strip off the aerated water" cannot be visualized by the discusser. The increases of 15 to 20 per cent by the use of cover of the propeller is questioned; as is also the increase of 20 to 30 per cent of bollard pull due to low rpm of the propeller. This is an overly optomistic estimate. This is confirmed by Fig. 23, which indicates that at 5 knots an increase in tow-rope pull of approximately 5 per cent by cutting the rpm to less than one-half and an increase at zero speed of advance or bollard pull of 2 to 3 per cent, not the 20 to 30 per cent noted in the paper.

MR. JOHN S. BARRY, Member: This informa- tive and practical paper is very timely inasmuch as we are enjoying a mild boom in tugboat building on the Eastern Seaboard.

There are three important characteristics for a well-designed tug: (1) A pronounced drag to the keel with a well rounded fore foot; (2) a continuous curve in the deck line; and (3) as large a propeller as possible. The author has clearly explained the reason for keel drag and cutaway at the fore foot with their effect on steering. British tugs for years have located their towing bitts well forward to take advantage of this effect and recent American tugs have located them further forward than has been customary in the past. The deck line of a tugboat should curve all the way from bow to stern with no straight or flat spot amidships. This enables the tug to roll away easily from a dock or the side of another vessel. The round stern is perfect for a tug as it is often used to pivot the tug around on the piling of a dock and the overhang protects the rudder.

With respect to the lines of a tug we might qualify the author 's statement on the area of the midship section by saying it should be as large as necessary to allow proper machinery arrangement, but no larger, with the result that fairer waterlines and a better distribution of displacement can be had, making for a better performing tug, especially in rough weather. The indicated values for PC of 0.58 to 0.60, may possibly be a little low as in actual practice they often run higher. Values of PC for three successful wooden tugs are 0.62 for a 66-ft harbor tug, 0.6 ° for a 127- ft seagoing tug, and 0.686 for a 165-ft salvage tug.

I t is vital that the machinery spaces on oceangoing tugs be protected from an influx of the sea since they occupy such a large part of the tug. Accordingly, engine-room doors should have 24- in-high coamings above the deck, as decks are often awash in rough weather. Other doors in the main deck should have 18-in-high coamings.

Freeboard on harbor tugs has increased in recent years as many tugs in the past had too little freeboard and ran virtually awash.

Since the result of the author 's tests indicates larger propellers turning at lower revolutions we would like to cite as an example a. harbor tug having this feature which resulted in superior performance. Designed for the Navy during the war these wooden tugs were 66-ft overall by 17-ft 4-in. beam by 6-ft S-in. mean draft. Extreme draft was 8 ft 7 in. The Navy requirements were for a 3-bladed propeller, 6 ft 4 in. in diam, to turn about 200 rpm, the tip of the blade to be immersed 20 in. below the surface of the water at rest. The machinery setup consisted,~of two 150-


hp Buda 6-cylinder Diesels turning 1S00 rpm and fitted with approximately 3 to 1 reverse and reduction gears, which connected into a Morse chain reduction gear with a 3 to 1 reduction. The total reduction was therefore about 9 to 1, giving a propeller speed of 200 to 210 rpm. Separate Westinghouse air controls in the wheelhouse operated the engines. In order to accommodate such a large propeller, a reverse curve had to be worked into the bot tom of the hull to avoid excessive displacement and a large down- dipping skeg was fitted under the propeller to take the rudder pintle. The performance of these tugs exceeded expectations and they demonstrated great towing power and were especially good in stopping and backing. Navy yard cap- tains of these tugs thought well of them and claimed they could almost do the work of the much larger and heavier 95-ft yard tugs. I t is believed we were very close to the optimum propeller for this size and type of tug.

With respect to vibration, a class of wooden 127-ft seagoing tugs was converted from twin screw to single screw with the propeller aperture none too large. The blade-tip clearance at the top of the aperture was 31/2) in. for a propeller 9 ft in diameter turning 326 rpm. The result of this inadequate clearance was severe vibration in the stern and steering-gear compartment.

The author has given us criteria for average characteristics of LBD, HP, Freeboard, Keel Drag, and Displacement. If he could supple- ment these with average values for GM and "rudder areas" using his modified formulas the paper would have added value.

DR. F. H. TODD, ~fember: The author has collected a great deal of information which will be extremely valuable to designers working in the field 6f tugboat design. Small craft of this type have been considerably neglected in the past and model results on tugs have been particularly rare.

The author is to be particularly congratulated on giving the data on the series of models of tugs run for the Army Transportation Corps. Al- though this series did not consist of geometrically similar models, they were, in general, sufficiently close to enable contours of resistance to be plotted showing the variation with prismatic coefficient and displacement-length ratio. These will be extremely valuable to designers. They indicate that, as the speed-length ratio is increased, whereas at first the reduction in prismatic coefficient brings about a corresponding reduction in resistance, for still higher speeds the reverse is true, and the'fuller prismatic gives the lower resist-

ance. This, of course, is well known in the area of destroyer design.

The author refers to the use of the Kort nozzle on tugs and, in particular, to its effect on propeller efficiency. I t is well known that the fitting of such a nozzle can increase the towing pull at the bollard, i.e., when the tug is stationary, by amounts up to 40 per cent or more for the same horsepower.

rn model experiments of this kind, it is possible to measure both the pull in the tow line and the thrust on the propeller. I t has been demonstrated clearly from such tests that the greater part of the increase in pull does not result from a greater thrust from the propeller but in a forward reaction on the shell of the nozzle. The nozzle has contracting sectional areas from the entrance to the propeller disk and, owing to the greater velocity induced by the propeller, the pressure on the nozzle over this area is reduced with a consequent forward force on the nozzle.

"The increase in pull in the stationary condition is greater the thicker the nozzle; but when the tug is running at speed, and the drag of the nozzle becomes important, the reduction in speed is greater. The amount of increase in pull at low towing speeds and the loss of speed at high towing speeds, or when free running, can, in fact, to a large extent be controlled by the shape of the nozzle section. The difficulties in ~stern maneuvering experienced with Kort nozzles and mentioned by the author can, to a great extent, be overcome by fitting small flanking rudders in the nozzle ahead of the propeller.

CAPTAIN C. P. MURPHY, USCG, ]tfember: The author gives a criterion for stability, which he states is not adequate to insure safe tugs. The criterion referred to is the general weather- stability criterion which has been used by the Coast Guard for various classes of vessels, and is specifically included as part of the rules applicable to passenger vessels. Wide experience with this criterion appears to substantiate it as a criterion of GM required for seaworthiness for small and moderate size vessels, but it is agreed that this is not the only criterion necessary to insure adequate stability for certain classes of vessels. Passenger ships must have stability to withstand collision damage and to prevent capsizing if a large number of passengers should crowd to one side. Heavy- lift vessels must have adequate stability to swing out their booms with their rated loads. Similarly, in the case of relatively higher powered tugs, a heeling-moment criterion, based on horsepower, should be considered.

Such a criterion has been tentatively used by


the U. S. Coast Guard. This rudder-heel criterion is in some respects similar to that proposed by the author, but at tempts to take into account the pertinent effects of propeller diameter and rudder size and location, and treats the maximum rudder heeling moment as a static; i.e., a gradually applied moment, not a dynamic, suddenly applied moment. The corresponding formula which has been used by the Coast Guard is as follows :

(SHP X D)"I3Sh GM = 3 8 A ~

where SHP = shaft horsepower

D = propeller diameter, ft [(SHP X D)2/a]/38 = approximate propeller

thrust, long tons S = effective decimal fraction of propeller

slip stream which is deflected by rudder; this is arbitrarily assumed to be equal to that fraction of the propeller circle cylinder which would be intercepted by the rudder if turned to 4,5 deg

h = vertical distance from propeller shaft centerline at rudder to towing bitts, ft

z~ = displacement, long tons 2f/B = least tangent of heel to deck edge

A nmnerical comparison using the data of Fig. 1 of the paper, assuming a propeller diameter equal to approximately SO per cent of the molded draft amidships and using an S-value of 0.6, which about corresponds to the normal balanced rudder, indicates the formula which the Coast Guard has used to require about half the GM of that required by the formula proposed by the author. Thus the essential difference is entirely in the use of 2fiB in lieu of f /B; that is, in the treatment of rudder heel as static, not dynamic.

Since the Army Transportation Corps has had extensive tug-operating experience it would be of great value to have a little more information concerning the operational background of the formula proposed by the author. I t is possible that the statical basis which has been applied by the Coast Guard is too moderate.

This writer is in thorough agreement with the author 's comments concerning the importanee of providing ample initial GM and freeboard. The minimum freeboards given by Fig. 2 of the paper are believed to be somewhat better than the average present practice and should provide adequate stability characteristics without the need of excessive initial GM. The author refers to the importance of a high degree of watertight

integrity in the deck and deck erections and this cannot be overemphasized. However, there is another important feature which the author does not mention; namely, the provision of ample freeing-port area in the bulwarks.

The author refers to the opinion, "One verv able tug operator of long experience stated that no tug would work well unless there was an inch or two of salt water over the after deck." No tug should be designed with sufficient tankage to make it possible for any operator, however experienced, to indulge in such foolhardy practice.

~/[R. OLIN J. STEPHENS II , Member: This paper provides a very valuable outline of the fundamentals of tug design and should be extremely useful to anyone engaged in that work. Also the Army Transportation Corps deserves thanks for arranging tank tests and obtaining much of the data which the author has organized in preparing his paper.

The paper also serves as a reminder of the fact that we have a great deal of available information on the resistance of various forms and it seems likely that there is not too much new to be dis- covered in that immediate field. On the other hand there is plenty of room for over-all improve- ment, and in the case of tugs this would appear to lie in the area of combined hull, propeller, and rudder design. In seagoing tugs, or harbor tugs which may be expected to make deep-water voyages, the seakeeping characteristics must be an important consideration.

More information on rudders would be helpful and possibly the author can provide specific information on the type of rudder for which tests are plotted in his Fig. 24. Mention of the lack of tests of a comparable wedge-type rudder sug- gests that this may have been a streamlined rudder or possibly a streamlined rudder reversed. I t would be interesting if the author could advise us on this point.

The plots of bollard and towrope pull (Figs. 22 and 23) are also helpful but the conditions of the tests plotted in Fig. 22 are not made clear. As these appear to be tests of an actual vessel it is assumed that, as shaft rpm was reduced, the bollard pull was measured at various intervals. I t would be helpful if the author could describe how the shaft horsepower was determined as one cannot help but wonder whether the high values associated with the low shaft revolutions are alto- gether to be relied upon particularly in view of the data plotted in Fig. 23, which illustrate the value of a large-diameter wheel but not ap- parently to the degree suggested by the previous figure.


This question is raised only with the thought that full information might be helpful and not with any intention of questioning the author 's recommendation that large-diameter wheels should be used, as this is fully concurred in. Time has not permitted the analysis of Fig. 22 by the usual methods of propeller calculation.

MR. THOMAS D. Bow~s, 21/lember: This is an interesting and valuable paper on tugboat design. Not a great deal of information in general is available on this subject.

In the past, tugboat design followed a conventional relation of length to beam, beam to depth, and draft, with limited horsepower. To a great extent these tugs were satisfactory for the work they had to do. Ships to be docked were small, very few even being as large as 10,000 deadweight tons. The largest that were handled were small. Carfloats were small. However, the ever-increasing labor costs and the increasing size of ships made it necessary to consider tugs from an entirely different economic setup. Today tugs have to dock ships of twice the tonnage they did in the past, as a.earfloat tug has to handle over twice as many loaded freight cars as it did 10 or 12 years ago. In coal service, tugs are today handling 12,000 to 20,000 tons in harbor work where they were handling 4000 to 6000 tons a few years ago.

To meet the economics of business, tugs had to go to a radical change in design. Our tugs today are of far less displacement than they were in the past, the horsepower is anywhere from 2 to 4 times what was considered desirable a few years ago. Less draft has been necessary to enable the tug to handle equipment in places that they did not do so before. In turn, this lesser draft has made it necessary for us to do considerable research and experimentation in the design of prdpellers.

The lower displacement has been made possible by the strong, light construction we have been able to develop using the longitudinal system, as well as the fact that none of our tugs carries fixed ballast. The displacement of the writer's latest tugs is based entirely on the weight of the ship light, all fuel, stores, outfit and crew, but no ballast. The fuel capacity in these modern Diesel tugs for harbor service and short coastwise runs is on the basis of 40 days of practically continuous operation. In turn this means that we must have numerous fore-and-aft trim tanks that can be filled to make up for the fuel and lube oil as used.

Our after peak trim tanks are set up so that when necessary they can be used for fresh-water

cooling when operating in foul waters that might cause trouble with our intercoolers, in which case the outside feed connections to the cooler are cut off and we use the water from the after peak tank for cooling. This takes care of the moderately short time the harbor tugs operate under such conditions.

The tugs designed in the writer's office may be divided into the following classes:

1 Coastwise and off shore. These tugs today run from 125 to 135 ft in length. However, we also are towing short coastwise from Norfolk to Philadelphia with our standard l l0- f t tugs developing 1600 bhp. These tugs tow coal barges of 8500 tons capacity.

2 Harbor tugs. These include commercial tugs for ship docking and hawser work, railroad tugs for carfloats and covered-barge work, and railroad tugs for hawser work. In addition to this are smaller tugs approximately 70 ft in length with 500 to 900 hp that are used for dredge tenders and such work.

A modern tug must handle much more quickly and turn in a smaller diameter circle. Our hawser boats have a modified, streamline rudder of rather large area. Our carfloat and barge tugs have a streamline rudder with a modified fishtail. These designs of rudder have been the result of considerable experimentation in the wind tunnels. In designing a streamline fishtail rudder great care must be taken with the two after edges. If the wrong type of curve is used at the after edge of the rudder, vortices will be set up and, in turn, this will cause excessive vibration and shattering.

Stability must be increased over what is considered desirable just from the profile and center of gravity of the ship. As horsepower is increased and as rudder area is increased, it is necessary to increase stability. Our tugs run about 60 per cent more GM than would be found in using the Coast Guard formula.

We also have found it highly desirable to cut away the fore foot on a modern tug to a slightly greater extent than ever was done in the past. We also have done away with the old-fashioned stem. All of the writer's tugs today have well- rounded bows that extend well under the load line. All of our tugs are built today with blisters instead of guards. These blisters extend out about 8 in. on either side beyond the molded line of the tug. They are made of very heavy plating, are rounded into the deck and angled into the hull. With blisters, truck tires can be used for fenders and have a fair life. Our bulwarks are set back about 8 in. from the side of the vessel and then raked in about 12 deg. We have done away with davits and other top hamper


tha t bother a tug in maneuvering around a big ship. Ins tead of davits we have a single crane for handling lifeboats tha t can be turned inboard against the funnel. Also the masts on these modern harbor tugs are set in a tabernacle so tha t they can be pulled down when operat ing aft on a large steamer.

The last 30 large tugs with 1200 hp and over from the wri ter 's designs have been of the high- speed Diesel type with reduction gears. The last twenty-seven l l0-footers, designed by the writer are equipped with 10-cylinder 0 P engines, operat ing at 720 rpm. Our propellers turn at 200 rpm. These engines are one-way engines and are equipped with Airflex clutches and reduction gears. These gears and clutches are sat isfactory if they are buil t for the part icular service in which they are to operate and the controls t ha t are set up are such tha t the operat ing personnel can do no damage. There has been considerable trouble with equipment of this type in the past but tha t .has been most ly due to wrong materials in the drums, friction blocks and very poor types of controls. Today we insist on separate cast-iron drums tha t can be replaced very quickly and special type of nylon tube, and then we design our own control equipment to meet the part icular requirements.

The controls on these tugs are interlocked so tha t it is impossible to get air in both clutches at the same time. One clutch cannot be i n fa t ed until the other one is deflated. To prevent the captain of a tug doing damage not only to the clutches but also to the ship, the throt t le and control handle is so set up tha t no mat te r how quickly he moves it ahead or astern the engine will not accelerate beyond idling speed until the clutches have been inflated to at least S0 lb - -120 lb being operat ing pressure.

Late ly we have designed a new type of moderate size tug for special dock work. This tug has no fantail, the blister is carried down to the waterline giving us about the same effect as we had in much longer tugs. In addit ion the blister shrouds the propeller and prevents surface water and air being sucked down when the tug is moving slowly and the engine is wide open. For ships over 1S,000 to 20,000 tons deadweight, it is sometimes necessary to get a tug into the dock between two ships to push the bow in. The writer has designed 70-ft tugs Of 900 bhp for this purpose. The same type of tug is used in connection with dredging the head of a dock up by the bulkhead line. These tugs have to be short to maneuver in the cramped space . They also must have power to handle the large dredges and, a t the same time, tow the large side-dump scows.

MR. ROBERT TAGGART, Jl4ember: The designer has been at an extreme disadvantage in at- tempting to predict tugboat horsepower. I t can be seen from the 10 models tested with displacement-length ratios ranging from 294 to 481, tha t the Taylor S tandard Series contours are quite inapplicable. The writer would like to explain at greater length how the resistance contours given in Figs. 16 through 21 were derived.

In 1944, the Office of the Chief of Transpor ta- tion requested the Navy Depar tment to obtain an extension of Taylor contours to greater displacement-length ratios. As a result, tests were run at the Experimental Towing Tank of Stevens Ins t i tu te of Technology on six models with displacement-length ratios of 300 and 400 and with prismatic coefficients of 0.50, 0.60, and 0.70. These models were developed from the Taylor parent form with a beam-draf t rat io of 2.25. The results are presented in E T T Repor t No. 279, as curves of effective horsepower for a 400-ft ship. In spite of this addit ional information there is still a decided lack of resistance information in the tug range. To fill in this gap, the series of tug-resistance contours was developed. I t must be emphasized tha t these contours were produced from a very small amount of actual test da ta with a great deal of fairing, interpolat ion and extrapolation. However, it was hoped tha t they might serve as an interim guide until a series of related models covering this range could be tested. These contours were derived in the following manner :

(a) Revised S tandard Series da ta were plot ted for prismatic coefficients of 0.50, 0.60, and 0.70 as contours of speed-length rat io plot ted against volumetric coefficient and residual-resistance coefficient. The Experimental Towing Tank results were also plot ted and the contours faired through to a value of a volumetric coefficient of 0.017. Cross curves were then drawn as contours of volmnetric coefficient plot ted against prismatic coefficient and residual-resistance coefficient. The shape of these contours was determined by the more complete da ta at volumetric coefficients of 0.003, 0.005, and 0.007.

(b) From these curves contours of prismatic coefficient from 0.56 to 0.6,q were plot ted against volumetric coefficient and residual-resistance coefficient.

(c) Using these eontours, curves of resistance for the 10 tugs tested were obtained. The rat ios of the actual tug model test resistance to the resistance obtained from the contours were ealeu- lated. The general trend indicated a higher resistance at low-speed ratios and a lower resistance at high-speed ratios. The ratios were then faired


1 ~ . 0 _ _

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I - (D . . . .

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6 . 0 _ _ _ _ j m '

C

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0 ! I 0.60 o.~o o.8o 0.90 1.00 LI0

v/4Z FIG. 25

8 ~

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TUG RESISTANCE QONTOURS--CONTOURS OF PRISMATIC COEFFICIENT (Cp) ; VOLUMETRIC COEFFICIENT = 0 .007

-Iv

C--v' L '~

against volumetric coefficient and applied to the resistance contours.

(d) The final results were presented as contours of prismatic coefficient plotted against speed-length ratio and residual-resistance coefficient.

The results given in Figs. 16 through 21 of the paper, as the author states, have been replotted on the basis of displacement-length ratio. Al- though this may be a more familiar type of plot to those accustomed to the Taylor contours, as published in the "Speed and Power of Ships," the present trend is to utilize nondimensional coefficients for the bases of such plots. I t also has been determined that a more useful method is the plotting of continuous contours of resistance coefficient against speed-length ratio as the abscissa. This permits the interpolation between curves in a much more rapid manner. I t also permits the user to picture the complete power contours bounding the vessel which he has selected. The Taylor contours extended and reevaluated, are presently being reproduced by the Government Printing Office in this latter form.

Further information of this nature is available in a series of tests run by the Japanese.. 5 This series, although not strictly adaptable to tugs, does encompass to some extent, the tug form. In this book the volumetric residual-resistance coefficient is employed which utilizes the two- thirds power of the displacement in the denomina- tor in the place of the square of the wetted surface. This gives a resistance comparison based on the actual amount of volume which a given hull encompasses rather than the secondary factor of wetted surface, thus indicating nondimensionally the resistance in pounds per unit volume. I t also has gone somewhat further in the use of nondimensional coefficients by utilizing the Froude number as the basis for the speed, "To avoid the useless confusions coming from the units of speed or length adopted."

The original tug resistance contours plotted on the basis of volumetric coefficient are shown in Figs.25-29. Accompanying these contours is a

These are published in "Graphical Methods for Power Estima- tion of Fishing Boats," by Atsushi Takagi , Chief of the Fishing Boat Section of the Fisheries Agency, Tokyo, Nippon Oyo Print- ing Company, Ltd., 1950.

16.0

14.0

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FIO. 26

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TUG RESISTANCE CONTOURS--CoNTOURS OF PRISMATIC COEFFICIENT (Cp); \IOLUMETRIC COEFFICIENT = 0.009

16.0

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x

% 8.0

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. ?

0.68 /I 0.66 Ill o 6 ~ / / 7 / o.6z."--~ i f/A o . ~ d / , ~ i <~°-.~ L~

72~" /

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v/,/-C

FIG. 27 TUG RESISTANCE CONTOURS--CONTOURS OF PRISMATIC COEFFICIENT (Us,); \;OLUMETRIC COEFFICIENT = 0.011

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T U G R E S I S T A N C E C O N T O U R S - - C O N T O U R S OF PRISMATIC C O E F F I C I E N T ( C p ) ; VOLUMETRIC C O E F F I C I E N T = 0 . 0 1 3

conversion graph (Fig. 30) from the volumetric coefficient to the more familiar displacement-length ratio.

The self-propelled tests which were conducted on these 10 models were run with stock propellers at the Taylor Model Basin. These stock wheels were selected primarily for size based on the

diameter which would fit withi~ the de- largest signed propeller aperture. Therefore, the curves

/

in Figs. 6-15 of the paper must be utilized with some caution• These propellers, although of

• , [ such diameter as to gtve a probably correct thrust-deduction coefficient and wake fraction do not represent the most efficient propeller either in the towing condition or in a free-running condition. However, a very good measure of these two conditions can be obtained. At the free- running condition probably the best comparison of tug performance can be obtained by dividing the effective horsepower by the hull efficiency. This will give a good measure of what can be obtained with a well-designed propeller for the free- running condition.

In the towing condition, however, another fac-

tor applies• The thrust-deduction coefficient represents the loss of thrust due to the inter- action between the propeller and the hull. This is thrust which is not available for propulsion of the vessel or for towing. Therefore, the higher the thrust-deduction coefficient the less efficient is the vessel in the towing condition• Experiments conducted in the past have indicated, although it cannot be stated positively, tha t the thrust- deduction coefficient for a given speed and for a given propeller diameter will remain constant for tha t speed regardless of the amount of thrust loading placed on the propeller• Therefore, although the curves given in Figs. 6-15 are based on the free-running condition, the factor of 1 minus the thrust-deduction coefficient is a good measure of the relative residual power available for towing.

There are many factors which affect the thrust- deduction coefficient, all of which are not known. In general, however, it appears tha t the area of the stations directly ahead of the propeller have an ex- t reme influence on the resulting thrust deduction. The wider these sections, tha t is, the greater area

16.0

14.0

12-.0

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FIG. 29

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TUG RESISTANCE CONTOURS--CONTOURS OF PRISMATIC COEFFICIENT (Cp); aVOLUMETRIC COEFFICIENT = 0.015 , ~ ~r_-.d

IZO 3.8 4.0

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FIG. 30

860 300 340 380 420 460 SO( &

Displacemen+-Length Ratio (O.OI L') 3

GRAPIt SIIOWING CONVERSION FROM VOLUMETRIC COEFFICIENT TO DIS- PLACEMENT--LENGTH RATIO

C 0

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O0

Z


which they have in line with the propeller disk, the greater will be the thrust-deduction coefficient. The sectional area of rudder post and rudder behind the propeller are also factors in determining the thrust-deduction coefficient. I t can be noted that Model T X 11 has by far the highest thrust- deduction coefficient at 6 knots of any of the vessels tested. This is very probably due to the large wooden stern post and rudder post installed on this model. These results would therefore indicate that fairing away the material as much as possible forward of the propeller and making the rudder as narrow as possible directly aft of the propeller would be highly desirable. The idea of using the reversed streamlined rudder appears highly desirable in order to obtain a low thrust- deduction coefficient. I t is highly recommended that this approach be tried in the future.

The author is to be complimented on an excellent paper on the subject of tug design. The naval architect can learn a great many lessons in the study of these more extreme forms. A further detailed study of this type of form, as well as the many other types in existence, such as the bulk carriers of the Great Lakes, double- ended ferry boats, barges, and the like, may well lead us to information which will prove highly useful in the more conventional merchant cargo- ship and passenger-ship, design.

MR. EUGENE F. MORAN, JR., ~kfember: This paper opens with a discussion of two formulas for checking the sufficiency of GM. The first one from the United States Coast Guard has to do with the beam wind effect on the above water fore-and-aft profile. The second one submitted by the author considers the effect of the strain of the towing hawser when not leading directly aft, in producing a list. Both of these formulas have become somewhat outmoded in recent years for several reasons: (1) The switch from steam to Diesel power plant no longer requires large smoke stacks with extreme heights for increased draft and removal of smoke. (2) The awareness of the advantages of streamlining have reduced deck structures to the smallest amount required for proper operation; also, in modern design, the deckhouse aft has been shortened to allow the installation of towing bitts as far forward of the rudder as possible for easier maneuvering while under hawser strain; (3), and perhaps the most significant, the beam of a modern tug of given length is much greater than in former designs. The acquired increase of stability more than offsets the slight increase in residual resistance when running light. The increased horsepower now being installed is more than necessary for maxi-

mum Speed when running light. When engaged in towing at speeds of 5 to 6 knots, the residual resistance, as the author points out, is of minor importance.

With respect to freeboard-and reserve buoyancy, it has not been felt necessary even on seagoing tugs to provide a to'gallant forecastle, as ample reserve buoyancy can be provided by fulling the deck line and increasing the sheer forward of admidships. More than ample reserve buoyancy is provided aft through continuing the increased beam at the deck line to well aft of amidships to produce a nmeh fuller curve at the stern. This fulling of the deck line at the stern gives the added advantage of better immersion of the propeller in solid water, as the author points out. This fullness does not prevent the design of an extremely fine run to the underwater body, per- mitting an easy flow of water to the propeller. With respect to the depth of the bulwarks and tumble home or canting inboard to protect them from damage, that design problem would be controlled solely by the work intended for the tug. The fender rail or guard rail, plus the hanging side fenders furnish some protection when along- side a high-sided vessel, but not enough when under the flair at the bow or the counter at the stern.

The author makes reference to the speed-length ratio, giving values of 1.0 to 1.2 or even higher. Most tugs of recent design are very little more than 100 ft in length, with a speed of 12 to 14 knots. This will give a speed-length ratio of 1.2 to 1.4. The reason for this high ratio is that the length is purposely held to approximately 100 ft to improve maneuverability in congested areas. The maximum speed light has been raised in recent years from the former 8 to 10 knots to the present 12 to 14 knots because a large percentage of the time is spent running light from one assignment to the next. I t frequently happens that without this additional speed, the second assignment could not be handled on time and might be lost.

The author states: "There is little doubt that the most effective power can be obtained by uslng steam reciprocating machinery." This statement is somewhat overreaching. There is no doubt that the old compound and triple-expansion steam power plants were simple to operate and permitted close control of power output from 0 to 100 per cent. This control, however, was exercised in the engine room by an operator generally un- aware of surrounding conditions. His control was exercised second-hand after receiving instructions by signal from the pilothouse. This same eom- plete control of power output has been available in recent years through the development of the


Diesel-electric power plant, with pilothouse control. An additional development, but still in the process of experiment, is the variable-pitch propeller. The experience and data on this second method of control are not sufficient for any determination at this time.

Further, the author states that a better tug can be produced "if ballasting is not resorted to." No distinction is made between permanent ballast which can be changed only at great time and expense and water ballast which can be changed at almost no expense and with no loss in operating time. With respect to permanent ballast, as the author points out, the design limitations on length, beam and draft frequently require the permanent installation of ballast to acquire the proper trim. Service requirements such as operating on inland waters with varying draft limitations, and also at sea where more freeboard is desirable, make it ahnost necessary to install a water-ballasting arrangement of tanks and pumps. The same designed tug can then operate in deep harbors with a draft of 12 ft or more and in shallower rivers where the tug can be tipped to 11 ft, 10 ft, and possibly 91//_9 ft. This is accomplished through the filling of the ballast tanks forward and the discharging of ballast tanks aft.

When the tug is assigned to ocean towing the process is reversed. With the forward ballast tanks empty, the bow rides higher in the water and with the after tanks filled, a greater and steadier thrust produces a higher tow-line pull. I t is also frequently necessary to correct the trim of the tug because of the consumption of fuel oil during a trip. This flexibility of draft and freeboard is of such advantage that quite a number of tugs were designed and built to the specifi- cations of the writer's company for operation principally in the New York State Barge Canal Sys- tem with its limitations of draft, beam, and maximum height above water, but with such seaworthy characteristics that they were capable of operations in New York Harbor and along the entire coast. M a n y long-distance ocean tows were completed by this type of tug, and one tug in particular after years of operations in the canal system was sold to the Suez Canal Company and delivered to them by us under her own power.

Consideration of the advantages of Diesel- electric and also direct-connected Diesel power plants as against a steam power plant, either tur- bine or reciprocating, are so many and varied that it would require a completely separate paper to consider them properly. The net result appears to be overwhelmingly in favor of the Diesel- electric installation, in spite of its higher first cost. The reduction in over-all operating ex-

penses, including maintenance and repair with the Diesel plant over the steam plant has brought about a nearly 100 per cent selection of that type of power plant in new tugboat construction in the last 15 years. Even ashore, coal-hauling rail- roads in the past several years have been forced, reluctantly, to shift over to this type of prime mover.

The author points out the slight drop in over- all efficiency of the Diesel-electric drive, particularly when operating at full power. From a practical operating point of view, this loss in dollars of increased fuel expense is more than offset by a reduction in damage claims through better control of vessel maneuvers and more proper strain on lines.

With respect to tug rudders, the installation of "appended wedges" on both sides of the trailing edge of a plate rudder will give to the tug a considerable increase in maneuverability, and for tugs principally engaged in operations requiring this feature such wedges or "cheek pieces" are generally installed. For tugs engaged principally in long-distance towing, the increased drag more than offsets that advantage, and the cheek pieces generally are not installed. With respect to going astern, the effect of the rudder on steering is not too great and the tendency of the tug's stern to move to port in the ca.se of right-hand propellers is a help in such maneuvering. I t is not recommended that tugs be maneuvered astern for any considerable distance, as with high velocity astern considerable damage might be caused to the rudder installation.

MR. JAMES A. WASMUND, ?vfember: The writer agrees with all of the good things which the author has said about electric-drive tugs and reluctantly admits to the disadvantages, although he feels that some of these may not be as bad as might appear. This applies particularly to the high cost of electrical maintenance and high-salaried engineer requirements.

In addition to the flexible power availability pointed out by the author, the use of multiple- Diesel-engine electric drive permits the maximum of physical location flexibility of the Diesel- generator sets for proper weight and space distribution.

If desired, the Diesel engine air-starting system with its air compressor and storage tanks can be eliminated by providing starting windings on the propulsion generators and a suitable storage b a t t e r y . In any event, starting will be infre- quent since it is not necessary to reverse engines. This also assures rapid reversals with simple control located in the pilothouse.


Diesel-electric drive provides smooth vibration- less operation not obtainable with direct-Diesel drive. Electric transmission does not t ransmit the propeller vibrations back to the prime mover. Critical speeds and associated torsional vibrations can be eliminated. For seagoing tugs particularly, automat ic electrical control can be provided which will prevent the propeller and con- sequently the engines from overspeeding when the propeller comes out of the water in a heavy sea.

We feel tha t Diesel-electric drive is an efficient, modern- type propulsion system, already proved on the basis of many successful installa- tions, and tha t on a tugboat , which must operate in restricted waters, pilothouse control easily obtainable with electric drive provides the ac- curate and rapid maneuver ing tha t is manda tory to this class vessel. Damage and tow-line break- age will be reduced to a minimum.

COMMANDER J. A. BROWN, USN Member: The New England Section of the Society is indeed for tunate to be the sponsor of this paper. The author has gone a long way to fill in one of the great voids in the published design data of our profession. There is, however, one point in this paper which the writer would like to debate, and tha t is the proposed stabil i ty criterion.

The writer agrees tha t the judging of the stability of a tug solely on the basis of a relationship between initial stabil i ty and beam winds is inadequate. Certainly the list caused by bollard pull when towing abeam is a more impor- t an t measure of the stabil i ty characteristics of a tug, particularly a harbor tug. But he cannot agree tha t the formula

B H P )< 15h G M -

&f/B

is entirely adequate either: (a) Using B H P neglects the differences in

transmission efficiency of the various types of engines. (b) This relationship is based on current ratio of B H P to displacement. Installa- tion of a radically different propulsive system such as the free-piston gasifier might require a different relationship for adequate stability.

The most impor tan t argument the writer has against this formula, however, is tha t it a t t empts to measure dynamical behavior by a statical criterion. Except in a few cases of slow flooding, losses of small ships by capsizing are the result of dynamic forces due to rudder action, waves, winds, passenger crowding, or cargo shifting either acting alone or in combination.

We should measure the resistance to these

forces by studying its dynamic characteristics. As Jaakko Rahola states in "The Judging of the Stabili ty of Ships :"

"For determining the sufficiency or insuf- ficiency of the stabili ty of a vessel, it is not enough tha t one knows the amount either of the initial metacentric height, the statical critical heeling angle, the capsizing angle, or any other stabil i ty factor separately; it is necessary to know several of these factors at the same t ime."

I t is recommended then tha t we judge the stabili ty of a tug or any other ship by means of the r ight ing-moment curve. For the normal operating condition of the tug the dynamical stabili ty of the ship should be adequate to limit the heel due to the max imum abeam bollard pull to a certain angle. The reserve of stabili ty should be sufficient to prevent capsizing under the influence of an additional moment caused by waves or beam wind. A reasonable figure for the limit- ing angle might be 7 deg or the angle at which 1/~ the freeboard on the low side is immersed. At the same t ime the area under the r ight ing-moment curve representing the reserve of stability should be limited to a certain percentage of the total area, and the heeling arm should be less than a certain percentage of the maximum righting arm. The exact values of these criteria should be based on a careful analysis of existing tugs and could be varied for the different types of duties. Bu t once these criteria have been established they would form a means of measuring the stabil i ty of a tug which could be defended technically. These would be foolproof against any abnormal change in a design, either in weight distribution or form.

The writer would like to add tha t Jaakko Rahola, in the book mentioned previously, gives the case history of 34 ships which are known or believed to have capsized. These s tar t with Captain Coles' monitor Captain which sank in 1870, and end with the motorship Monica which sank in 1938. One of these was a Nordenham tugboat which sank in 1925 as a result of a towing-gear casualty. In addition, he describes the stabil i ty of two Finnish ships, auxiliary vessels of fleet-tug type, which capsized and sank off,Finland in the 1930's. These Finnish tugs displaced about 65 tons and both were carrying a deck cargo when they capsized. The first capsized while turning in heavy seas to return to port. She was carrying a deck load of 7.83 tons. Her G M was 1.04 ft, her max imum righting arm of 0.49 ft occurred at 35 deg with deck edge immersion at 15 deg. The second tug had a G M of 0.95 ft, a maximum righting arm of 0.66 f t at about 20 deg with deck edge immersion a t 8.2 deg. To add to the difficulties, the door to the after crew's quarters was


open and since the sill was on ly 0.4 in. high this tug obv ious ly was unfi t for nav iga t ing the open seas.

T h e fo rmula p roposed b y the a u t h o r appea r s to have real mer i t for use in the p r e l im ina ry design of a tug. Here we are in te res ted in compar i sons be tween a new design and an exis t ing ship. A t th is s tage we do not know the form of the tug, hence we canno t de t e rmine the curve of r igh t ing arms. B u t once we have the l ines comple ted , le t us use the curve of r igh t ing a rms or m o m e n t s to de t e rmine the a d e q u a c y of t he s t ab i l i t y of the tug.

MR. DWIGHT S. SIMPSON, A4ember: T h e a u t h o r gives va luab le d a t a in th is p a p e r which should be of cons iderab le help. H o w e v e r the wr i t e r bel ieves the younge r member s should be given a word of cau t ion .

T h e tes t d a t a do no t cover a r e l a t ed series, as the a u t h o r notes, and the re a re m a n y reasons for the differences in per formance , never the less the t e n d e n c y would be to pick the bes t form and e x p a n d or d iminish i t to sui t some new condi t ion .

I t will be no t iced in Figs. 1 and 2 of the p a p e r t h a t the curves of t he severa l e lements , such as b e a m and f reeboard , r e la ted to length do not "ze ro ; " t h a t is to say t h e y do no t v a r y in the fo rm o f X = C Y b u t a s X = CY=t= K o r b y m o r e compl i ca t ed fo rnmlas as t h e y d e p a r t f rom a s t r a i g h t line. If, for ins tance we reduce all the hul ls shown to 100 f t in l eng th we would have an a s s o r t m e n t of beams and d i sp l acemen t for our lO0-ft tug as follows :

Actual Beam Displacement length, 100-ft L, 100-ft L,

ft ft tons

42.1 29.78 368.6 51.0 30.40 433.0 59.0 32.55 447.8 74.33 27.56 373.0 87". 67 26.92 360.5 91.10 27.46 365.3

114.58 26.25 347.7 134.80 25.06 302.78 140. O0 23.56 350.40 155. O0 22.40 286.35

Again , if we expand the 42 . l - f t hull to 1,55 f t we would have a b e a m of 46 f t or 44 per cent more t h a n in the ac tua l vessel.

Of course the re are m a n y reasons o the r t h a n h y d r o d y n a m i c for the select ion of beam, draf t , d i sp lacement , b u t exper ience shows t h a t i t is unwise and gives unce r t a in resul t s to use a g iven design wi th more t h a n a b o u t 10 per cen t increase or r educ t ion in length.

The Coas t G u a r d fo rmula is obv ious ly no t m e a n t for smal l c ra f t where the r e l a t ive windage

is small . T h e a u t h o r ' s fo rmula checks ve ry well wi th the ac tua l G M of severa l of the wr i t e r ' s tugs.

T h e wr i t e r is in f avor of cons iderab le d r ag es- pec ia l ly on the smal ler boa t s where the towing b i t t s are no t v e r y far fo rward of the rudder . I t is no t i ceab le t h a t Eu ropean tugs towing f rom a hook a t nea r ly mid- l eng th of the ship, have al- mos t no drag. On our own larger tugs with the b i t t s fa i r ly well forward , less d rag is requ i red as Fig . 4 of the pape r would indica te .

R e g a r d i n g power p lants , the a u t h o r is l ike ly to f ind cons iderab le d i s a g re e me n t wi th his ana lys i s of the s t eam plant . T h e days of the Amazon @teen are no longer wi th us and a s t eam engine canno t now be kep t runn ing wi th the a id of a horseshoe nail and a piece of ba l ing wire. A d d the mul t ip l i c i t i e s of boi ler accessories and exper ienced and we l l - t r a ined personnel a re requi red , bo th ashore and afloat . Hence, m a n y s t e a m-pow e red tugs have rep laced the i r engines wi th Diesels.

A n u m b e r of s t eam t rawlers r ecen t ly i m p o r t e d into N o v a Scot ia have p roved ve ry cos t ly in opera t ion . ~Zhile the l a y u p per iod is on ly t h ree days e v e r y two weeks or so, an engine room crew and s t a n d b y fuel mus t be pa id and much t r oub l e and repa i r cost are due to inexper ienced opera tors .

T h e Diesel -e lec t r ic p l an t can do a n y t h i n g the s t eam p l a n t can do (except poss ib ly h e a t the ship) and owing to i ts suscep t ib i l i ty to full p i lo thouse control , can do i t be t t e r . Those wi th in th is discusser ' s knowledge do no t r e p o r t a b n o r m a l l y high m a i n t e n a n c e costs.

R e p o r t s f rom the severa l e lect r ica l coupl ings in use ind ica te t h a t t h e y are well wor th invest i - ga t ion for tug use.

F r o m persona l experience, the cont ro l lab le - p i tch propel le r has a ve ry defini te p lace in tug ope ra t i on and goes far t owards b r ing ing o t h e r t y p e s of in s t a l l a t ion up to electr ic per formance . Blade sect ions can be des igned to give b e t t e r back ing pe r fo rmance t han the a u t h o r shows. In one ins tance, where a t h r e e - b l a d e d CP, prope l - ler r ep laced -a fou r -b laded so l id-b lade wheel of good charac te r i s t i c s d i rec t connec ted to a 700-hp engine, the ship speed, bo th ahead and as tern , was the same; the t ime requ i red for reversa l of sh ip ' s mot ion was the same; b u t the range of the vessel f rom full ahead to full s top was cu t in two b y the C P propel ler .

T h e wedge or f i shta i l -sect ion r n d d e r is ex ten- s ively used, bo th in t ug and t r awle r work. I f a s t r eaml ined r u d d e r is used the th ickness r a t i o should be v e r y small , then the wedges need no t be large enough to affect the a s t e rn s teer ing to a no t iceab le degree. Often t hey are seen to be much wider t h a n necessary .

T h e a u t h o r ' s b i b l i o g r a p h y m a y well be supple-


mented by a very useful work "Screw Tug De- sign," bv A. Caldwell published by Hutchinson in New York, 1946.

.~./[R. ROACH: The lack of published information, concerning the design of small craft in general and of tugs in particular, belies the number of people qualified to discuss such subjects. The experiences and data recounted by the discussers is welcome and helps greatly to bring into focus a rather neglected phase of our profession.

Before replying to the discussers, the author feels tha t he must qualify the statements made in the paper concerning power plants and, to a measure, explain the feeling and preferences he ad- mittedly has.

Regardless of the ' efficiencies a given power plant might have while operating, the efficiency when the tug is deadlined, is zero. And if personnel costs and depreciation continue, and cost of spare parts can be considered in the equation, the efficiencies may even be negative. In military operations, reliability, not efficiency, must be considered foremost. I t is primarily for this reason that the gross reliability of certain power plants received a warm reception and others were treated rather casually. Where well-trained crews are available, spare parts are within tele- phone reach, and well-equipped repair yards are bidding for work, the more efficient plant is desired and optimum. However, this situation is not true in many places where military tugs are called upon to operate.

Mr. Tomalin very effectively has presented the brief for Diesel-electric propulsion. As stated in the paper, this system admits of the utmost finesse in control plus certain advantages in being able to maintain the engine at that RPM best from either a maintenance or a maxinmm horsepower output. In fact, the only major disadvantages are those of high original cost and maintenance which in the experience of the Army with quite a few Diesel-electric tugs in operation is high. Here again, these maintenance costs could be expected to be much less with highly trained crews often unobtainable to the Army.

As for the comment regarding pitch ratio for tug wheels, it is felt that perhaps we have become accustomed to higher shaft RPIk~ to the extent that pitch ratios of unity are not often encountered on tugs in recent years. The fact remains that the higher thrusts at bollard or towing speeds are obtained with the higher pitch ratios. This of course, is not to advise any designer arbitrarily to pick a high pitch ratio regardless of the R P M and speed, but to advise, when possible, to adjust the shaft R P M to a low enough figure to give the

high thrusts tha t are available with high p i t c h - ratio propellers.

Mr. Tomalin's remarks concerning the possible use of symmetrical blades on controllable= pitch propellers is of interest. Even with such blade profiles the problem of blade angle near the root exceeding the blade angle at the tip and the at tendant disparity of pitch angle when this blade is bodily Totated through a fixed angle must be recognized. Here the symmetrical blade profile would only partially mitigate the problem.

The possibility of a fixed-angle controllable- pitch propeller with symmetrical blade profiles is intriguing and may possess sufficient merit to warrant its investigation for tugs, icebreakers, and similar type vessels.

A clarification was requested as to what the author meant by "sufficient cover" for the propeller. I would like to refer to a paper by L. A. Baler and J. Ormondroyd, G discussing vibration that occurred on lake freighters when their heavily loaded propellers literally sucked vortices through the propeller disks. The propellers of a tug often are working at near 100 per cent slip. Here the water will move into the propeller disk by the hydrau- lically easiest route. If not prevented by the hull, the propeller will reduce the pressure above it to the extent that air will be drawn down or vortices formed from the surface that destroy the even flow to the wheel. These factors are particularly noticeable when driving astern as the upsweep of the buttock lines act as a trap to force air and disturbed water down into the propeller.

For harbor work, the author proposes a stern similar to the flat run of the river towboats; the waterline extending far enough from the tips of the propeller to restrict or prevent flow from the surface.

The author is indebted to Mr. Barry, for his observations concerning the reasons for some of the characteristic shapes of tugs that we so often take for granted but that are in fact necessary for the performance of their everyday tasks.

Captain Murphy in commenting on the stability criterion proposed by the author has another but similar formula which in essence allows a heel about double that allowed by the author 's criterion for a constant heeling moment. I t is felt that the formula proposed by Captain Murphy is unrealistic from the dynamic standpoint. Com- mander Brown, as does the author, recommends that stability be considered from dynamic considerations.

I t must be remembered that few tugs ever have

6 " V i b r a t i o n a t the Stern of Single Screw Vessels," by L. A. Baier and J. Ormondroyd, Trans. S N A M E , Vol. 60, 1952, pp. 10- 25.


capsized from purely statical heeling moments. Whether GM is a proper measure of stability, is indeed, debatable as it has only the merit of being measurable and an implication that at least for limited angles of heel the righting arm is of a given order. Equally important is the angle at which the righting arm is a maximum and the range of stability.

To develop a set of rules to judge the stability of a tug, taking into account the initial moment to heel, the maximum moment to heel, and the range of stability, would be a tremendous task and one not easily enforced. For example, one tug was recently calculated to have at light-ship displacement, a maximum righting arm of 0.625 ft at 30 deg of heel, and a range of stability of 54 deg. At full load and ballasted, she had a maximum righting arm of 1.125 ft at 31.5 deg of heel and a range of stability of 71.5 deg. Clearly a set of standard displacement, ballast, and trim conditions would have to precede the establishment of a righting- moment criterion.

In the meantime, the GM gives a rapidly determined indication of the desired stability. I t appears that Captain Murphy 's criterion will not provide enough initial stability to insure safety. The following is a list of existing tugs with the GM as designated by the author 's and Captain Murphy ' s eriterion :

Length of Actual GM by Roach's GM by Murphy's tug, ft GM, ft criterion, ft criterion, ft

45 1.81 2.44 1.12 65 2.12 1.93 1.05

100 2.55 2.40 1.10 143 3.66 2.32 1.09

In reply to Mr. Stephens, the tests plotted for Fig. 24 are those for a streamlined rudder of 15 per cent camber. The plots of bollard pull in Fig. 22 was carried out at the Transportation Research and Development Command and do represent actual measured results. The horse- powers were measured by a shaft line torsiograph of the variable-induction type while the tow line- pulls were measured with a Cox and Stevens' dynometer of the strain-gage type. In Fig. 22, the throttle was reduced giving varying bollard pulls.

Fig. 23 illustrates calculated results from two wheels considered optimum for their R P M at a towing speed of ,5 knots. In mentioning potential gains in the rope pull the author had reference to the lamentable fact that many tugs have propellers designed for free route speeds rather than towing speeds; here the difference between a towing wheel and a free wheel can easily result in significant amounts.

Mr. Bowes has contributed in a large measure to the progress of this country in tug design. The blister guard is one of many significant inno- vations from his board. His elimination of the fantail stern by carrying the blister guard down to the waterline has the effect of covering the propeller as recommended by the author. I t is gratifying to find confirmation by a naval architect of Mr. Bowes' reputation.

The author is indebted to Mr. Taggart for many of the calculations involved in this paper and particularly for the derivation of the tug "resistance contours" Figs. 16-21. Mr. Taggart also has explained very clearly the history of the studies in tug hull forms carried on by the Transportation Corps of the U. S. Army. His comments regarding the configuration of the after sections should be' given serious thought as a heavy hand here easily destroys all the effort a designer may put into design of propeller and machinery.

Mr. Moran has brought to the attention of the naval architect, the practical considerations of what it takes to make money with a tug. Perti- nent are his remarks on the desirability of high free-running speeds in order to gain business that otherwise might be lost. Here is a textbook illus- tration of a compromise the designer and the owner must make. In answer to Mr. Moran 's request for a definition of the type of ballasting referred to in the paper, the author considered only permanent ballasting. Liquid ballast for trim- ming purposes, stability, and to maintain draft by compensating for spent fuel is of course, a necessity. Indeed, tankage should be sufficient to allow marked changes in draft where the tug is expected to operate in both canals and harbors. In ocean-going tugs carrying large amounts of fuel, water ballast is more often than not a necessity to insure adequate stability when approaching a burnt out condition.

Mr. Simpson is quite correct in cautioning against extreme expansion or contraction of the hull dimensions given for the various tugs mentioned. The reduction to a single waterline length of 100 ft was done solely for comparative purposes and should be considered only as an academic approach to investigation of hull shapes.

Both Mr. Simpson and Mr. Tomalin call attention to the eddy-current coupling. Several years ago the Coast Guard installed such a coupling on one of its buoy tenders. This installation was observed with considerable interest. Re- cently a similar installation was made on a small twin-screw tanker built for the Transportation Corps. The results of the trial trip were gratifying both as to speed of response to bridge control and to the general handiness of the vessel.

Roach C D.tugboat Design.1954.TRANS

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Transcript of Roach C D.tugboat Design.1954.TRANS