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Chapter VI. Propulsion of Ships

The propulsion system of a ship is to provide thethrustto the ship to overcome the resistance.

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6.1 Introduction Propulsive Devices (reading p205-209)

Paddle-Wheels: hile the draft varying !ith ship displacement"

the immersion of !heels also varies. The !heels may come out

of !ater !hen the ship is rolling" causing erratic course-#eeping"

$ they are li#ely to damage from rough seas.

Propellers: %ts first use !as in a steam-driven &oat at '.. in

*0+. ,dvantages over paddle-!heels are"

) not su&stantially affected &y normal changes in draft

2) not easily damaged) decreasing the !idth of the ship" $

+) good efficiency driven &y lighter engine.

/ince then" propellers have dominatedin use of marine

propulsion.

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Paddle Wheels Propulsion (Stern)

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Paddle Wheels Propulsion (idship)

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Propeller (!-"lade)

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Propeller (!-"lade)# $udder

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%et t&pe: ater is dra!n &y a pump $ delivered stern!ards as a

et at a high velocity. The reaction providing the thrust. %t1s use

has &een restricted to special types of ships.

'ther propulsion Devices:

1. oles (Duct) Propellers*main purpose is to increase thethrust at lo! ship speed (tug" large oil tan#er)

+. Vertical-,is Propellers: ,dvantage is to control the direction

of thrust. Therefore" the ship has good maneuvera&ility.

. Controlla"le-Pitch Propellers (CCP): The pitch of scre! can

&e changed so that it !ill satisfy all !or#ing conditions./. 0andem and Contra-rotatin Propellers: %t is used &ecause

the diameter of a propeller is restricted due to limit of the draft

or other reasons (torpedo). The efficiency of the propeller

usually decreases.

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%et Propulsion

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Vertical-,is Propellers

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Vertical-,is Propellers

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Controlla"le Pitch Propellers (CPP)

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Contra-rotatin Propellers

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0&pe of Ship achiner&

1. Steam 2nine(no longer used in common)

,dvantages: ) good controlla&ility at all loads" 2) to &ereversed easily" $ ) rpm (rotations per minute) matches

that of propellers

isadvantages*.) very heavy 2.) occupy more space

.) the output of po!er per cylinder is limited+.) fuel consumption is high

+. Steam 0ur"ine

,dvantages:

.) deliver a uniform turning tor3ue" good performance for large

unit po!er output" 2.) thermal efficiency is high.

isadvantages:

.) is nonreversi&le 2.) rpm is too high" need a gear &o4 to

reduce its rotating speed

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. Internal com"ustion enines (Diesel enine)

,dvantages: .) are &uilt in all sies" fitted in ships ranging from

small &oats to large super tan#ers" (less 00 hp 6 70"000 hp)

2.) 8igh thermal efficiency.isadvantages: .) 8eavy cf. gas tur&ines

+. 3as 0ur"ines (developed for aeronautical applications)

,dvantages: .) o not need &oiler" very light 2.) ffer continuous

smooth driving" $ need very short !arm; time.

isadvantages: .) e4pensive in cost and maintenance 2.) need a

gear unit to reduce rpm.

5. uclear reactors 4 tur"ine,dvantages .) do not need &oiler" fuel !eight is very small

2.) operate full load for very long time (su&marine)

isadvantages .) !eight of reactor and protection shield are heavy

2)

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Definition of Po5er

Indicated horsepo5er (PI)*is measured in the cylinders (/team

reciprocating engines) &y means of an instrument (an

indicator;) !hich continuously records the gas or steam

pressure throughout the length of the piston travel.

pm- mean effective pressure (psi)

L= >ength of piston stro#e (ft)n= num&er of !or#ing stro#es per second

A= effective piston area (in2)

n= num&er of cylinders

?550I mP p L A n=

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ra7e 8orsepo5er (PB)*is the po!er measured at the cran#shaft

coupling &y means of a mechanical hydraulic or electrical &ra#e.

!here Q= &ra#e tor3ue (l&-ft) $ n= revolutions per second.

Shaft horsepo5er (PS

)*is the po!er transmitted through the shaft

to the propeller. %t is usually measured a&oard ship as close to the

propeller as possi&le &y means of a torsion meter .

!here dS= shaft diameter (in)" G= shear modulus of elasticity of

shaft material (psi)" = measured angle of t!ist (degree)"

LS= length of shaft over !hich @ is measured $ n= revolution per

second

2 ? 550BP nQ=

( )+

"0

S

S

S

d G nP

bL

=

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Delivered horsepo5er (PD)*the po!er delivered to the propeller.

0hrust horsepo5er (PT)*

T= Thrust delivered &y propeller (l&)

VA= advance velocity of propeller (ft?s)

2ffective horsepo5er (PE 9 or 28P)*

RT= total resistance (l&)

Vs= advance velocity of ship (ft?s)

?550T AP T V=

?550E T sP R V=

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Propulsion 2fficienc&

0otal propulsion efficienc&

can also &e replaced &y or

, more meaningful measure of hydrodynamic performance

of a propeller is: a 3uasi-propulsive coefficient"

"

" !here is the shaft

ET S B I

S

D

ED

D

DS S

S

PP P P

P

P

P

P

P

=

=

= transmission efficiency

and thus" .

- 9*A for ships !ith main engine aft

- 9BA for ships !ith main engine amidship

- smaller if a gear &o4 is used.

T D S

S

=

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6.+ Propeller 3eometr& and 0erminolo&

Coss

Cac#

8u&cap

Dace

um"er of lades: 2" 9 /9 !"Eoss

8u"cap

Shaft

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0he face surface of a "lade is a portion of a holicoidal surface

0he helicoidal surface: Fonsidering a lineABperpendicular to

a lineAAand supposing thatABrotates !ith uniform velocitya&outAAand at the same time moves alongAA!ith uniform

velocity" the surface s!ept out &yABis a helicoidal surface.

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Pitch*P!hen the lineABma#es one complete revolution

and arrives atAB. %t traveled an a4ial distanceAA"

!hich represents the pitchof the surface. The propeller&lade is part of that surface and the pitch is also called the

pitch of the &lade.

Pitch anle tan or tan2 2

Gitch ratio: tan

P P

r rP PR

PRD

= =

= =

P

o

,

2 r

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p*0

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Developed ,reaAD

2pended ,reaAE

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oss*(a#a" 8u")

oss diameter= The &lades at their lo!er ends or roots are

attached to a &oss !hich in turn is attached to the propellershaft. 0he maimum diameter of this "oss is called the

"oss diameter . The &oss diameter is usually made as small

as possi"leand should &e no larger than the siesufficient

to accommodate the "lades and satisf&in the

re:uirement of strenth. %t is usually e4pressed as a

fractionof the propeller diameter.

,t one time propeller &lades !ere manufactured separately

from the &oss" &ut modern fi4ed pitch propellers have the&oss and &lades cast together. 8o!ever" in controlla"le

pitch propellersit is of course necessary for &lades and

&oss to &e manufactured separately.

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lade outline*it is decided &y propeller series diarams.

;2panded "lade outlineeading edge

Trailing edge

G* figure 0.5

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$a7e (a &lade isperpendicular or titled !.r.t the &oss )

S7e5 (the s#e!ness of a &lade !.r.t. the center line)

Pitch ratio

%n case that the pitch"P,is not constant" then the pitch is

defined asP = Ptip (the pitch at the tip of a propeller).

Clade area ratio HAD/A0

AD - Total (developed) &lade area clear of that of the &oss

PPR

D=

2

0 ? +A D=

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6. 0heor& of Propeller ,ction

,ssumptions*

) replacing the propeller !ith a stationary actuating dis# across

!hich the pressure is made to rise

2) neglecting the rotational effect of propeller

) neglecting vortices shed from the &lade tip" $ frictional loss.

DVA

VA(1+b)VA(1+a)

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omentum Conservation=orce > net momentum flu

(horiontal)

( )( ) ( )

( )

0

2

0

H H (mass conservation)

A A

A A f A a

A A

T Q V b V

Q a V A V A b V A

T QV b A V a b

= + = + +

= = +

2ner& 2:uation

( )

( )( )

222

2

2

0 0

0

2 2

2 " " 2

2 2

or

2 2

AA

A

A

V bV P

g g

b b VTA P T T A b b V

A g

b ba a

++ =

+ = = = +

+ = + =

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( )

( )

0

0

0

2

0

2 02 2

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2tension of momentum theor&

Fonsider the rotation of the flo! passing through the propeller

disc." the reduced ideal efficiency &ecomes"

2

2

2

I $ I 0.

2 I

2

!here is the rotation velocity of flo! after the propeller"$ is the rotation velocity of the propeller.

I

aa

a

a

= >

+ = =

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lade 2lement 0heor&%n the momentum conservation of a propeller" no detailed

information can &e o&tained !ith regard to the effects of the

&lade section shape on propeller thrust and efficiency.

( )

( )

The total velo. at radius " " 2 .

Thrust: cos sin

Jesistance: sin cos

Koment: $ and

is a function depending on section shape (!in

r A T T

T L D

F L D

F L D

r V V V V rN

d d d

d d d

q d r d d f

f

= + =

=

= +

=

r r r

( )

( )

g

section theory). Dor a propeller" the relative advance velocity

of the fluid at the disc" is $ the rotation velocity is

I .

AV a

r a

+

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aVA

LL1

( ) Ir a Ia r

rV r=

r

AV

a $ aare determined &y e4periments

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6./ Similarit& ?a5 for Propellers

,lthough theoretical studies and C=Don propellers are very

important and provides valua&le guideline for designing propeller"

a great deal of #no!ledge concerning the performance of propellers

has &een o&tained from propeller model tests. 8ence" it is

necessary to e4amine the relation "et5een model and full-scale

resultsas the case of resistance.%n open 5ater(not &ehind a ship)"

( )" " " " " "

- rotational speed" - diameter of propeller- pressure in !ater" - dynamic viscosity

- speed of advancing" - Thrust

A

A

T f D V g n p

n Dp

V T

=

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2 + 2

2

22

2 +

Msing .," the non-dimensinal formula is given &y"

" " "

Droude N: "

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%n open !ater" the propeller efficiency coeff.:

.2 2 2

hen all the dimensionless parameters are the same for the

t!o propellers" the t!o propellers

!ill &

A T A T!

Q Q

TV V J

nQ nD

= = =

1eometricall& similar

2

e .

/cale ratio:

Dor the same Droude N:

Dor the same advance ratio (most important)

indicating the model rotating faster.

s

m

As s

Am m

s As m

m Am s

DD

V D

V D

n V D

n V D

=

= =

= = =

d&namicall& similar

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2 2 2 2

Dor the same < = + a

vacuum (cavitation) tunnel.

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Dor the same Je: "

!hich is contradict to the similarity of Dr. Therefore" it is almost

impossi&le to satisfy the Dr $ Je similarity la!s simutanously.

/imilar to the assumption made

As m s

Am s m

V D $

V D $ = =

in model resisrtance tests" !e

assume viscous force is independent of other dynamic forces.

8ence" it may &e computed separately. %n reality" viscous force

is usually a small portion of the total force. The smilarity of

Je is neglected in propeller model tests.

Therefore" propeller model tests follo!s $ (advance

ratio) similarity la!s. %f the cavitation is relevant" then the

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6.! Propeller odel 0est, test on a model propeller is run either in a to!ing tan# or a

running flo! in a !ater tunnel (cavitation tunnel) !ithout a model

hull in front of it" !hich is called ;open 5ater< tests.

) VA= velo.of flo!

2.) n- rotation ofmotor

.)p!- pressure can

&e controlled

KeasureVA% Q% T%

andn&

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Development

of cavitations

of a propeller

in a

cavitation

tunnel

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Trust coeff Toe3ue coeffT Q

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Q

T

Testing results

0

AV

JnD

=

/lip ratio " Gitch ratio " section types $ N of &lades.AV P

nP D

= =

2 + 2 5Trust coeff. " Toe3ue coeff. "

.2 2

T Q

A T!

Q

Q

n D n D

TV J

nQ

= =

= ='pen- 5ater efficient

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Purpose of open-5ater tests

%t is usually to carry out open !ater tests on standard series of

propellers. Their features (such as N of &lades" &lade outline

shape" &lade area ratio" &lade section shape" &lade thic#ness

fraction" &oss diameter $ pitch-diameter ratio) are

systematically varied. The result data are summaried in a set of

particular diagrams" !hich can &e used for design purposes. e!ill study ho! to use these diagrams later for designing a

propeller.

/tudying the efficiency of a propeller and find a propeller !ith&etter efficiency

/tudying the e4tent and development of cavitations over a

propeller.