Speed Boat Developments From the Past Into the Future

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eed Boat Developments From The Past Into The Future

p://www.lesliefield.com/other_history/speed_boat_developments_from_the_past_into_the_future.htm[26/10/2010 17.42.45]

Figure 1.Standard, 1903

Speed Boat Developments From The Past Into The FutureMorley S. Smith

Abstract

Speed boat development began in the early 1900's with the development of the first gasoline powered pistonengines. These engines were large and heavy. Boat hulls were long narrow round bottomed displacement hulls.

As engine design improved, the Vee bottomed, hard chined planing hull and the stepped hull were developed. Drivesystems included the direct drive, Vee drive, stern drive, and surface drive.

The performance of each combination of hull type and drive system can be illustrated by a graph of PerformanceFactors. (Power Factor vs. Speed Factor) These Performance Factors take into account the running weight, enginehorsepower, and measured maximum speed.

For each combination of hull type and drive system, there is a maximum performance line or Limit Line. Bycomparing the Limit Lines of different combinations of hull type and drive system, we can also see how theefficiency of boats has increased over the years.

The final'-step is to look at combinations and configurations which might produce even greater improvements inthe future. The combination of surface drive and stepped hull will be significantly more efficient than currentpleasure craft.

Morley S. SmithPerformance Plans, Freeville, N.Y. Member

* * *

The history recorded herein, begins at about 1900 when internal combustion piston engines began to replace heavysteam engines and boilers. Many of the examples given herein are race boats. These-boats were the most efficienttypes of their day. The speeds weights and horsepower are often recorded.

Displacement Hulls

At the turn of the century when internal combustion engines were physically large and very heavy, the hulls usedwere displacement hulls. When a displacement hull moves forward, the sharp narrow bow pushes or displaces thewater out to the sides. As the hull passes, the water closes in behind it.

Figure 1shows the lines ofStandard, one ofthe fastest displacement hulls at that time.[1]

Length 60 ft. Beam 7'-6' 110 hp. Speed 30mph. in 1903

These displacement hulls were roundbottomed hulls. Viewed in cross section, thetops of the sides were more or less vertical.As the sides move downward, they curvegently into a nearly flat bottom at the keel.

Direct Drive:

The direct drive engine is located at aboutmid length in the hull. The propeller shaft goes aft from the engine transmission, through the bottom of the boat, tothe propeller which is located aft under the transom.

Refer to Figure 13, for a sketch of Alternate Propulsion Systems.


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Figure 2. Displacement Hulls

Showing the effect of of waterline

length

Engine weight = 30 lbs/HP

[Photo of Dixietemporarily not

available]

Figure 3.Dixie, displacement hull

Figure 4. Displacement Hulls

Showing the effect of weight

Twenty foot waterline length

The performance of a displacement hull is such that the power requirements increase as the speed increases. Athigher speeds, the power requirements increase more rapidly, until a speed is reached where great increases inpower produce negligible increase in speed.

Figure. 2.illustrates the first basic characteristic of displacement hulls: 'Thegreater the length, the higher the limiting speed." If you want to go faster, youbuild a longer hull.

Luckily the racing rule makers saw what was about to happen. Race boats werealready getting absurdly long. New racing rules in 1905 limited the length ofGold Cup and Harmsworth Trophy boats to 40 ft..

Dixie ILength 40' x Beam 5'-6' [1] 150 Hp. Dry weight = 5,150 Lb.29.8 Mph in 1904; 32 Mph. in 1906Designer: Clinton Crane

Note how the very sharp bow cuts through the water, rolling back a nice bowwave. The hull runs level. The ideal displacement hull. (Figure 3)

Since the length of these hulls had been limited, the only way to increase the

speed further was to reduce the weight.

Figure 4illustrates the second basic characteristic of displacement hulls: 'Thelighter the weight, the higher the limiting speed.' As before, power requirementsincrease more rapidly as speed increases.

Henry Crane designed and built a large and phenomenally light 220 Hp. enginefor anew boat to be namedDixie II. The V-8 engine had 2,477 cubic inchdisplacement, weighing less than 0.9 pounds per cubic inch. [1] Compare this tomodern automotive engines which weigh about 2 pounds per cubic inch withouttransmission or exhaust manifolds. The extremely light weight of Crane's 1908

engine was a major engineering achievement. The reason the power output wasso low is because maximum speed of the engine was only 900 Rpm. (about idlespeed for a modern automotive engine).

This light weight engine was placed in an extremely light hull. The 39'-3" hullweighed only 1,130 Lb. It was a huge canoe with planking of only 1/4" on thesides, and 3/8" on the bottom. [1]

In running trim this hull and engine weighed about 15 pounds per horsepower.Less than half of the Weight/Hp. ofDixie I.

Dixie IILength 39'-3" X Beam 5'-4"220 Hp. Speed = 37 Mph. in 1908

This combination of high power and low weight allowed the hull to be pushedbeyond displacement speeds. At these speeds the bow wave is pushed to theside so forcefully that the water does not close in behind the hull. The bow ineffect cuts a trough, and the stern sinks into the trough. The bow rises and triesto climb up on top of the bow wave. The bow is out of the water, and waterlinelength is no longer the hull length. This round bottomed hull had been pushed to

planing speeds.

Dixie IIdominated racing; winning the Gold Cup in 1908, 1909, and 1910, and the Harmsworth Trophy in 1908.

(Dixie IIhas been restored and is now in the Antique Boat Museum at Clayton N.Y.)

The next step was to develop a hull shape which would do a better job of climbing up on top of the bow wave and'plane'.


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Figure 6 The lines of a typical planing hull.

Figure 7. Trim and Drag

characteristics of a typical direct

drive monohull.

Planing Hull Characteristics

Figure 6shows the lines of a typical planinghull. A patrol boat designed by John Hacker.[2]

(a) Hard chined hulls have asharp edge between the hull sidepanel and the bottom panel. At

speed, these hulls tend to throwthe bow wave downward ratherthan push the bow wave out tothe side as a displacement hulldoes. The hard chine keeps thebow wave from wetting the sidepanel, thus reducing the wettedarea. Less wetted area means

less hull drag.

Nearly all of the fiberglass planing pleasure boats made today have a vee bottomed, hard

chine configuration.

The concept of the hard chined hull was not new. E. W. Graef published plans for a hardchined, vee bottomed displacement hull namedDolphin.

(b) It was also discovered at about this time that a flat deadrise or flat bottomed hull willhave less drag than a highly veed hull. [3]

Figure 7 shows the Trim and Drag characteristics of a typical direct drivemonohull. (Running weight = 2,500 lb.)

As the speed of a planing hull increases, the trim angle, or angle of attackdecreases.

(a) This increases the wetted area and greatly Increases the drag.

(b) The propeller shaft, shaft strut and rudder also cause drag.Appendage drag. The appendage drag increases as the square of thespeed. Total drag increases very rapidly as speed increases.

When designing very high speed craft, it is important to consider all of the liftand drag forces acting on the appendages.

(Propeller, propeller shaft, shaft strut and rudder.) These forces can greatlyaffect the position of the center of lift of the hull and the optimum hullproportions. Procedures and references for calculating hull drag and appendageforces are given in the appendix.

Planing Hull Limits

Figure (8) Performance Factors for Direct Drive Planing Hulls

Performance Factors are complex mathematical expressions whichcombine running weight, engine power and measured maximum speed.

These mathematical expressions are derived from model scalingprocedures. By using these complex expressions, all size effects areeliminated. (A more detailed explanation of Performance Factors is givenin the appendix.)


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Figure 8. Performance Factors for Direct

Drive Planing Hulls

The Performance Factors show that as the amount of applied power isincreased, the speed increases. At high speeds, considerable increases inpower produce relatively small increases in speed.

By using these complex numbers we can compare the performance ofany size or weight of direct drive planing hull with the maximumperformance possible. [4]

There is a definite limit to how much speed tan be obtained from a directdrive, Yee bottomed hull. This is called the Limit Line, and this limitdepends upon horsepower and weight. The closer the actual performance

comes to the Limit Line, the more efficient the craft.

The graph shows circles which represent test data from actual boats. [5][6] There are numerous possible reasonswhy these data points do not lie on the limit line.

(a) The chine beam of the test boat is not optimum for the power and weight.

(b) The propeller chosen for the test is not optimum for top speed. It might be chosen forbest cruise efficiency, maximum acceleration or a compromise.

The engine might not have been producing it's rated power on the day of the test. (d) The data for running weight isnot always accurate.

Nevertheless, I find it surprising that the test data for very different craft is so closely bunched near the limit line.

The racing rule makers banned stepped hulls from the Gold Cup races in 1920 and limited engine displacement toten litres (or 610 cubic inches). Packard Chris-Craftand similar hard chined monohulls dominated the Gold Cupracing until 1931, when stepped hulls were again allowed to compete.

Prominant names include,Baby Bootlegger, Imp, Hotsy Totsy, Rainbow IX, Miss Columbia, Arab VI,andBabyHorace III. [1] The boats listed here have been beautifully restored or replicated and are in running condition, oftenappearing at antique boat shows.

The boats were about 25' to 27' long, and averaged about 40 to 45 Mph. around the course. Speeds changed verylittle over a 13 year period, and this is to be expected from looking at the Limit Line on the Performance Factorgraph.

Performance Factors and Limit lines can be used to predict the performance of a given, hull with different amountsof power.Rainbow IX(Length 25'-10', beam 5'-10') is a good example. [7]

Originally built in 1923 as Packard ChrisCraft II, this hard chined, vet bottomed hull was powered by a 6 cylinderPackard engine producing 250 hp. and achieved a maximum speed of about 45 mph. This craft has since beenrestored and repowered with a modern, light weight, 650 hp. V-12 BPM.engine. Even though the craft has about2 times it's original power, it does not run much more than 10 mph. faster than it's original speed, (as would be

predicted by the limit line).

How do we overcome the performance limitations of direct drive monohulls? We either change the shape of thehull, or we change the drive system. Regardless of wether we change the hull or the drive, one of the goals is tomake the hull run at a higher angle of attack at high speeds. (Refer back to Fig. 7.)


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Figure 10. Newg

Figure 9. Planing Hulls - Hard Chine

Let us first look at a different hull shape. The stepped hull.

Stepped Hulls

The concept was originally proposed by Rev. Ramus of Sussex England in 1872. He proposed both a single stopwith tandem planing surfaces, and a combination of three pontoons with one forward and two aft. Indications arethat these shapes were derived from model tests. Unfortunately, the heavy steam power plants of that day could notpush a hull fast enough to plane, and take advantage of the new concept.

As early as 1906 there were published drawings for small stepped hulls with hard chines. William Henry Fauber [8]obtained a U.S. patent for hulls with multiple steps in 1908, but could find few people in the U.S.A. interested, sohe moved to Europe.

Two small boats Solair(12') and Flapper(15') demonstrated the potential of stepped hulls as did the Harmsworthchallenger Pioneer(5 steps) in 1910. (See Data Chart, Figure 12.)

The stepped hull began practical development about at the same time as the hard chined planing hull. A step in thebottom of a hull, raises part of the bottom surface so that it is no longer touching the water. Less wetted area. At thesame time, the planing surfaces meet the water at a near optimum angle of attack over a wide range of speeds. Thestepped hull is very efficient hydrodynamically.

Refer to Figure 9 : Sketch of Hard Chined Planing Hulls

In the early days of stepped hulls, it was not certain just how manysteps should be incorporated. Pioneerhad 5 steps in 1910.Maple Leaf

IVhad 5 steps.

Maple Leaf IV: Length 39'-11' x Beam 8' . Two V-8 engines 350 Hp.each.

In 1912,Maple Leaf IVcame over, from England, won the

Harmsworth Trophy, and took it home. She had no less than five steps,and the driver sat on a pedestal high above the transom in order to seeover the bow.

Some hulls had so many steps that they were called "shingled'.Rainbow IV(12 steps);

Eventually, model tests showed that a single step would be mostefficient if you could locate it in the right position and give.it the

proper depth.

The lines shown in Figure 10are typical ofstepped hulls in the 1920's. Note the veryflat bottom. This boat raced in a classlimited to engines with 1.5 litredisplacements.

Gar wood brought the Harmsworth Trophyback to United States in 1920 with the firstof hisMiss America's . These single steppedcraft so dominated the Gold Cup andHarmsworth racing that few other boatsattempted to compete. TheMiss Americaseries were not really efficient boats, just

big boats with huge amounts of power from multiple V-12 Packard engines.

Between about 1915 and 1940, a great many motor torpedo boats and fast patrol boats were built world wide,


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Figure 11 : Performance Factors for

Racing Stepped Hulls.

with stepped hulls. [10] The performance of these craft varied considerably, with some being very inefficient.

Stepped Hull Limitations

The stepped hull maintains a nearly optimum angle of attack over most of the speed range. The hydrodynamic hulldrag is almost constant. The drag of the propeller shaft, shaft strut and rudder, (appendage drag) increase as thesquare of the speed.

Figure 11: Performance Factors for Racing Stepped Hulls.

The graph of Performance Factors shows actual speed data of differentprominent racing stepped hulls. The data points are numbered and refer tonumbers on the data chart Figure 12. The boats are numbered in sequenceaccording to the year when the speeds were established. The sequentialincreases in power factor reflect engine development and not hulldevelopment. Notice that most of these boats perform almost on the limit line.Gar Wood'sMiss Americas were really quite inefficient. Many stepped hullsfrom England were significantly more efficient and often faster. They failed to

win races because of a lack of strength and mechanical reliability. The verystreamlinedAlagi was slightly more efficient than the others.

Stepped hulls are difficult to design. There are many design variablescompared to the design of a Vee bottomed monohull. I do not know of anyaccurate method available to optimize stepped hull design other than by modeltesting.

Stepped hulls dominated race boat design until about 1938 when Adolph Apelpatented the three point hydroplane configuration. Ventnor three point

hydroplanes dominated small limited class racing, yet stepped hulls were running competitively in Unlimited class

racing up until 1949. In 1950, Slo-Mo-Shun demonstrated 'prop riding' and boosted the world speed recordsignificantly. (More on 'prop riding" later.)

Stepped hulls definitely have the potential of being significantly more efficient than rnonohulls.

Compare the Limit Lines on the Performance Factor graphs. There are a number of reasons why stepped hulls didnot become popular for pleasure boats.

(a) Complexity of design, and the costs of development.

(b) Stepped hulls were banned from gold cup racing from 1920 to 1931. Wealthy race boatowners were not investing in stepped hull development.

(c) There were quite a few relatively small stepped 'gentleman's racers' built, but few ofthese were really efficient.

(d) There were many huge war-surplus aircraft engines available after the first world war,at reasonable prices, and few light weight marine engines available. It was easier, (andpossibly cheaper) to buy a big engine for a monohull, than to develop an efficient steppedhull.

Fig.12

RACING HULLS

NAME YEAR LENGTH BEAM

RU N

WEIGHT

LB

POWER

HP

SPEED

MPH

POWER

FACTOR

SPEED

FACTOR


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DISPLACEMENT HULLS (Round Bot t omed)

(a) Standard 04 60' 7'-6" 5400 110 30 15.38 22.65

(b) Vingt et Un II 04 38-9" 4-7" 4500 75 24 12.97 18.68

(c) Chip 05 27'-3" 2230 15.3 20.7 6.0 18.1

(d) Dixie 05 40' 5'-6" 5725 150 27.4 19.59 20.48

(e) Dixie II 08 39-9" 5'-4" 4020 220 35.8 43.4 35.8

HARD CHINED PLANINGHULL

(1) Vida IV 07 15' 1630 14 25 7.92 23.04

STEPPEDHULLS

(1) Solair 10 12' 1255 70 46 53.7 44.29

(2) Flapper 10 15' 680 40 47 62.7 50.1

(3) Miranda IV 26' 5-11" 3500 115 40.3 26.67 32.

(4) Dixie IV 11 39'-6" 6'-11" 7854 440 45.2 39.74 32.06

(5) Newg 25 18'-6" 4-10" 1785 90 45 45.78 40.86

(6) Amer VI 28 6450 2200 80 249.9 58.64

(7) Estelle IV 29 35' 9'-6" 9350 2000 105 147.4 72.34

(8) England II 30 38'-6" 10'-6" 14150 3600 99 163.59 63.66

(9) England III 32 35' 9-6" 10500 4400 120.5 283.2 81.43

(10) Amer X 33 38' 9-8" 14150 7600 125 345.35 80.37

(11) DelphineIX 33 26' 8-6" 4600 550 75 92.7 58.16

(12) Britan III 33 24' 4397 1375 110 244.3 85.94

(13) Blue Bird 37 23' 5925 2150 120 269.75 89.2

(14) Alagi 38 20' 2600 450 91.4 147.6 77.9

(15) Canada III 39 25' 3250 1000 100 252.8 82.2

(16) Canada III 3800 1650 120 347.6 96.1

(17) Canada IIIR 3250 450 77 113.8 63

(18) Canada IV 50 33' 5500 3000 143 410.5 107.6

(19) Pepsi 50 36' 12'-6" 12430 2 x 3500 160 185 105.

Aerodynamics

Three point hydroplanes and other more modern hull configurations such as tunnel hulls, use aerodynamic lift toimprove the efficiency of the craft. Any weight supported by air, does not have to be supported by the water. Airhas much less drag than the water. In order to obtain significant aerodynamic lift, it is necessary to have lightweight and to run at very high speeds. (Race boats with modified engines or outboard motors.)

Aerodynamics of high speed boats is an extensive subject all it's own and will not be dealt with further in thispaper. [11]

Drive Systems

Let us look at the different drive systems and see how they affected the performance of planing hulls. We haveseen how stepped hulls increased the angle of attack of the hull at high speeds, (compared to the direct drivemonohull). An increase in trim angle can also be achieved by moving weight, (the engine) aft, or by changing thedirection of propeller thrust. This is what alternate drive systems do.


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Figure 13 . Alternate Drive

Systems

Figure 13shows Alternate Drive Systems

Direct Drive

Engine weight is at about mid length in the hull. Propeller thrust is upward.

Having the engine weight forward on a monohull, helps the boat get up ontoplane more quickly, as does the lifting component of propeller thrust. The directdrive layout is still in use today on boats which specialize in towing water skiers.

At high speed, the forward weight and upward propeller thrust reduce the angleof attack and increase the hull drag.

The direct drive is the least expensive of the drive systems.

Vee Drive

The engine is located aft in the hull. The output shaft runs forward to a gear box,and then aft from the gear box, through the hull bottom to the propeller locatedaft under the transom.

(a) Some early stepped hull race boats were equipped with Yeedrives and located the crew aft of the engine. Liberty The Second, Miss Daytona, Miss

Minneapolis, Arab Iv, Prowler Jr.,[1](These boats have been restored or replicated.) andmost of Gar Wood'sMiss America's..

(b) Other layouts put the engine right back against the transom. The drive shaft runsforward between the crew which sits forward of the engine.

This was a common layout for English stepped hulls such asMiss England II, Miss Britain III, Delphine IX, andBluebird I. [1][2][3]

The use of a Vee drive in a stepped hull shows no increase in performance compared to a direct drive stepped hull.

Vee drive systems were common on many modern flat bottomed racing monohulls and drag boats.

Engine weight is aft of mid length, and the propeller shaft angle is less inclined than with a direct drive. Having theweight aft tends to lift the bow of the boat, as does the more level thrust line of the propeller. The hull becomesmore efficient at high speed than the direct drive monohull.

Stern-Drive

The engine is aft against the transom with the drive shaft going aft through the transom above the water line into aright angle gear box mounted aft of the transom. The drive goes down into another right angle gear box which

contains the propeller shaft.

Engine weight is full aft, and the propeller thrust line is basically parallel to the keel. Modern designs arehydraulically adjustable so that the propeller shaft angle can be varied up or down. Upward thrust of the propellerhelps a monohull get up on plane. Downward thrust of the propeller helps to lift the bow at high speeds. (Moreefficient for this hull.) The stern drive is the most common drive system (with inboard engine) for modern planingpleasure craft. A hard chined monohull with a stern drive, is almost as efficient as a stepped hull. (With the samedeadrise and a practical power range.)

The monohull is much easier to design than a stepped hull. When we consider that the number of designers of goodstepped racing hulls in the past was probably no more than a half dozen, we can understand why the large volume

boat manufacturers of today avoid such complex designs.

Figure 14, shows Performance Factors for Stern Drive Pleasure Boats Thesmall circles represent data points from actual boat tests published in 1992.The limit line is also shown. The boats tested in 1984 and prior years were


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Figure 14 , shows Performance Factors

for Stern Drive Pleasure Boats

significantly less efficient. The limit line was further to the left. [3]

The general characteristics of the Limit Line are similar to what we haveseen for direct drive monohulls and direct drive stepped hulls. As the speedincreases, the power requirements increase.

Surface Drives

A surface drive is one in which only the lower half of the propeller is in thewater. This was tried by Albert Hickman on his Sea Sleds in the late teensand onRainbow IVin 1924. [1] In these applications, the propeller shaftwent aft from the engine and through the transom just above the bottom ofthe boat. As the propeller rotates, only one half of the blades are in the waterat a time. A three or four bladed propeller is used in order to reduce thevibrations caused by blade impacts. It is a characteristic of surface piercingpropellers to shoot a great plume of water out behind the boat.

The 'Roostertail' is evident in photos ofRainbow IV(1924) and of Hickman's SeaSleds (1920- ). [l] Surfacepiercing propellers must have a larger diameter than submerged propellers because not all of the blade area isworking at any one time.

The advantage of the surface drive is that It eliminates the drag of the propeller shaft and shaft strut, and part of therudder area. Neither craft just described exhibited any really significant gains in speed. (This will be explainedlater.)

Modern Developments

The surface piercing propeller was rediscovered almost by accident by three different race boats, in three differentcountries in the late 1940's.

(A)Bluebird

In 1939 Sir Malcolm Campbell set a world speed record of 141.7 mph. in a three point hydroplane designed byAdolph Apel of Ventnor fame. [12][13] In 1949, his son Donald began testing the same boat, and found that at 145mph. the transom started to lift. The transom mounted engine cooling water pickup would come out of the waterand the engine would overheat. With the water pickup relocated to a forward sponson, a speed of 160 mph. wasachieved. When the stern lifted, the propeller came part way out of the water and became a surface piercingpropeller.

As the propeller rotates, the blades come out of the water, travel through the air, and then come down out of the air

and into the water with considerable impact force. This impact force is seen as a lifting force on the propeller shaft.It is this lifting force which supports the aft end of the boat. The aft end of the boat rides on the propeller force,thus the name 'Prop Rider'.

The propeller shaft, shaft strut and part of the rudder are lifted out of the water. This eliminates much of theappendage drag and allows a considerable increase in speed.

The angle of the Bluebird's sponson bottoms was then changed so that they would have an efficient angle of attackafter the stern lifted off the water. A speed of 170 mph. was reached before the craft hit a floating log and was toobadly damaged to rebuild.

(B)Miss Canada.IV

In 1948 Harold Wilson established a North American record of 138 Mph. in a two step hydroplane designed byDoug Van Patten. [14] Almost the same speed that Malcolm Campbell had achieved with a three point hydroplanein 1939.


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With some changes in propeller, this boat achieved 173 mph. before the propeller shaft broke. Photos of this runshow a distinct roostertail. The boat was prop riding but no one seems to have been aware of it.

Harold made one more try and exceeded the 170 mph speed, but an overspeeding engine destroyed the gear boxbefore speed measurements could be made. The expense of rebuilding the equipment to withstand thesesignificantly higher speeds prompted Harold to retire from racing.

(C) Slo-Mo-Shun IV

The story of how this boat was developed by designer Ted Jones and owner Stanley Sayers has not been revealedin detail. It just seemed to appear in June 1950, and set a world speed record of 160.3 mph. Again, the roostertailrevealed that this boat was 'prop riding'. After having the angle of the sponsons adjusted, Slo-Mo boosted therecord to 178.5 mph in 1952.

Here we have three boats which exhibited almost 40 mph. (or 29%) increase in speed as a result of prop riding.These three boats demonstrate the gains to be had by combining a surface drive with a stepped hull.

Why not apply a surface drive to a pleasure boat ?

In recent times, manufacturers such as Arneson and Dan Arena have packaged surface drive systems which locate

the engine back at the transom and put the propeller about 30" aft of the transom. On the Arneson system the shortpropeller shaft is pivoted about a vertical axis for steering. These drives exhibited some speed increase wheninstalled on Offshore racing tunnel hulls, but nothing near the speed increases seen on three point hydroplanes.Rainbow IVand the Seasleds did not exhibit great increases in speed in their day either.

Modern surface drives use supercavitating propellers. On a supercavitating propeller, the water separates from thesuction face of the blade and leaves an air cavity between the water and the blade face. The cavity extends aft ofthe trailing edge of the blade. Sometimes the trailing edge of the blade is made very blunt or flat. These are calledcleaver propellers.

Future

It all goes back to the factors which limit the performance of any vee bottomed planing hull. The angle of attack ofthe hull planing surface relative to the water surface.

The surface drive has two factors working against the monohull. The propeller lift forces (which are well aft of thetransom on the Arneson drive), and the propeller thrust line which is high and near the bottom of the hull. Boththese factors tend to push the bow of the hull down, flatten the trim angle and make the hull less efficient. Anygains from reduction in appendage drag are offset by an increase in hull drag. The boat does not travel significantlyfaster. The reduction of one set of drag forces is offset by the increase in another set of drag forces.

The idea is to combine the efficiency of the surface drive with a hull that will have maximum efficiency, in spite of

the prop lift of the surface drive. This requires a stepped hull.

I call the combination of surface drive and stepped hull, a Surf-Step. The potential gains from such a combinationare considerable.

Now you can see why a study of hull design history is desirable. It enables us to look at the overall advance oftechnology without getting buried in minute details.

Performance Comparison

Figure 16shows the potential speeds for four different types of boat inthe 18' to 20' length range. The hull weight is fixed. The running weight

is adjusted for engine size, and drive type weight. 8y using a single boatsize, the numbers should be more meaningful to the average reader.

The Surf-Step is about 8 mph. faster than a stepped hull, or 12 mphfaster than a stern drive with the same power. This magnitude of gain is


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Figure 15 , Performance Factors of Different

Configurations

Figure 16 , Horsepower vs. Speed for

different configurations.

worth pursuing.

Surf-Step

General design Characteristics On modern surface drives, the propelleris located 30' or more aft of a hull planing surface. At low speeds,especially when the boat is 'getting up onto a plane', the hull assumes asteep angle of attack. This submerges the aft located propeller morecompletely so that more blade area is working. The pressure loading persquare inch of blade area is reduced and 'runaway cavitation' is lesslikely to occur.

I prefer to extend the hull bottom aft on either side of the propeller to

reduce hull drag during the process of getting up onto a plane. Thisshould further reduce propeller loading at that critical speed.

A further enhancement would be to place a shroud around the uppersection of the propeller. A lip on the shroud aft of the propeller will helppressurize the water at the propeller diameter.

If the shroud extends out to the hull side extensions, the propeller will beoperating in a truncated tunnel.

The designer must be aware of the fact that a propeller draws in waterfrom a disc area which is significantly larger than the propeller diameter.

The primary hull step must be located forward of the effective center ofpressure in order to prevent porpoising.

The large lifting forces produced by the surface piercing propeller, movethe center of pressure well forward of the static center of gravity.

Dual Deadrise

Consider the cross section of the hull at the primary step. This portion of the hull will be running at near optimumtrim when at maximum hull speed. A conventional vee section would be running chine-dry. The outboard portions

of the bottom near the chine provide no lift, but are wetted by spray. I prefer to cut away that portion of the bottomwhich is not working. The wetted area is given a relatively low deadrise of about 10 degrees. Research has shownthat wave impact forces are greatly reduced when the beam is reduced. [15][17] The edge of the wetted area isdefined by a spray fence. Model tests have shown that spray fences can reduce the drag of a stepped hull by asmuch as 10 percent. Lift strakes do not have this effect.

The portion of the hull outboard of the spray fence will have a much higher deadrise. (Thirty degrees is shown.) Inrough water, the craft can be expected to operate at lower speeds. There will be more wetted area. The outboardportions of the hull with their high deadrise will be operating at much less than optimum trim. These portions ofthe hull will contribute very little to wave impact forces.

An alternative to a dual deadrise surface is a convex hull section. The continuous curvature will be more rigid thana dual deadrise surface. The spray fence which defines the beam of the high speed wetted area can be moveddepending upon the power and loading of any given particular application.

Summary


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The performance of any combination of hull type and drive system has definite Performance Limits. In order toincrease performance beyond the limits of presently common boats, we must develop new combinations of driveand hull.

Some combinations do not promise significant improvements in Performance Limits. A stepped hull with a sterndrive is only slightly better than a rnonohull with a stern drive. The monohull with a surface drive is only slightlybetter than a monohull with a stern drive.

It is the combination of surface drive and stepped hull which can produce significant improvements in efficiencyfor pleasure craft. (Up to 40% less power required.)

The task of designing an effective Surf-Step craft will require the combined ,effort of hull designer and propulsionsystem designer. Much of the technology is available. The drive system is complex. Stepped hulls are much moredifficult to design than monohulls, and some of the design secrets which have passed on with the old designersmight have to be relearned. Don't expect the first prototype to perform at the Limit Line. The potential gains arestill substantial.

References

[l] D.W.Fostle, 'Speedboat' Mystic Seaport Museum, 1988

[2] Lindsay Lord, 'Naval Architecture of Planing Hulls', Cornell Maritime Press. 1954

[3] Daniel Savitsky, 'Hydrodynamic Design of Planing Hulls' Marine Technology, October, 1964

[4] Morley S. Smith 'How Fast Will It Go ?' Society of Small Craft Designers, 1986

[5] Powerboat Magazine, Gerald Christian Nordskog Publisher

[6] Trailerboat Magazine Poole Publications

[7] William T. Campbell Jr. 'A Speedboat Scrapbook' Mystic Seaport Museum, 1992

[8] Kevin Desmond, Power Boat Orion Books (Crown Publishers)1988

[9] Uffa Fox, 'Sail And Power' Peter Davies Ltd., 1936

[10] 'Fast Fighting Boats'

[11] Morley S. Smith 'The Aerodynamics of High Speed Boats' S.N.A.M.E. Oct.1985

[12] Leo Villa & Kevin Desmond, 'The World Water Speed Record', Pitman Press., 1955

[13] Donald Campbell, 'Into The Water Barrier', Odhams Press. 1955

[14] Harold Wilson, 'Boats Unlimited' Boston Mills Press., 1990

[15] Savitsky, 'On The Seakeeping of Planing Hulls', Marine Tech. April, 1968

[16] Ward P. Brown & R.L.Van Dyk 'An Experimental Investigation of Deadrise Planing Surfaces with ReentrantVee Step', Davidson Lab Report 664, Stevens Inst. Dec. 1964

[17] Eshbach, 'Handbook of Engineering Fundamentals', John Wiley & Sons, New York, 1952

[18] Eugene P. Clement & James D. Pope 'Stepless and Stepped Planing Hulls' Hydrodynamics Lab R & D Report1490, 1961

[19] Daniel Savitsky, 'Procedures for Hydrodynamic Evaluation of planing Hulls in Smooth and Rough Water'Marine Tech. Oct. 1976


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[20] Donald L.Blount & David L. Fox, "Small-Craft Power prediction" Marine Technology January, 1976

Appendix

Performance Factors

Performance Factors were developed so that the performance or efficiency of one boat design can be compared tothat of another. Performance is expressed in terms of maximum speed, rated power and running weight. These arefactors which are usually readily available for existing boats. Factors such as overall length and maximum beam

have very little to do with the actual performance of a boat. Factors such as location of the center of gravity, chinebeam at the center of gravity, or deadrise at the center of gravity are seldom known.

Performance Factors are derived from model scaling relationships. When a prototype hull design is scaled down toproduce a model for tank testing, the proportions of the model must maintain a fixed relationship to the proportionsof the prototype. These relationships are controlled by the "Rules of Similarity'. [17]

All length dimensions (length, beam, location of center of gravity etc.), must be decreased in the same proportions.(The length scale factor.)

Areas - decrease as the square of the length scale factor.

Volumes - decrease as the cube of the length scale factor.

Weights - decrease as the cube of the length scale factor.

The model is towed and the drag is measured at different speeds. Because the model has been scaled according tothe rules of similarity, the model drag and power requirements art scaled also.

Horsepower - decreases as the length scale factor to the 3.5 power (Mathematically)

Speed - decreases as the square root of the length scale factor, or the length scale factor to the 1/2 power(mathematically)

Naval architects usually plot the hull drag against a speed factor. The speed coeff. is commonly expressed as aspeed/length ratio or a speed/beam ratio.

Speed Coeff.

or

Where g is the acceleration due to gravity. (32.2 ft/sec2) The introduction of g makes the speed coefficient non-dimensional.

The model and the prototype run at the same speed coefficient.

The problem with such coefficients is that they do not allow the comparison of the performance of two differenthulls with significantly different proportions. Consider a hypothetical example. Two identical hulls. Same weight,

same drag versus speed curve. same power and same top speed.

One hull is given flared sides so that. the measured beam at the shear is greatly increased (without any increase inweight) If these hulls are compared on the basis of drag versus a speed/beam coeff., the performance will not becomparable. The wide hull will appear to have more drag at the same speed/beam ratio. In reality, the speed


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performance of the two hulls will be exactly the same. Similarly, a long raked stem can upset a speed/lengthcomparison.

It is because of these disparities that other speed coefficients needed to be developed.

Froude Number = F =

= volume of the water displaced at rest

This is close to the Speed Factor which I use. I divide the weight by a factor of 1,000 just to produce moremanageable numbers.

Drag/Lift Ratio

Naval architects commonly express the efficiency of a planing surface in terms of the drag produced in order tohydrodynamically support a given amount of weight. The drag/lift ratio. The amount of drag is roughly equal to thecomponent of propeller thrust which is parallel to the keel. The thrust is proportional to the driving horsepower(and speed).

Because the horsepower of an existing hull is known, and the actual hull drag is not known; it is logical to expressthe hull efficiency as a power/weight factor.

Power - varies as the 3.5 power of the length scale factor.

Weight - varies as the 3rd power of the length scale factor.

Power - varies as the (3.5 / 3.0) power or the 1.1667 power (mathematically) of the Weight scale factor.

Speed - varies as the square root or power of the length scale factor.

Weight - varies as the 3rd power of the length scale factor.

Speed - varies as the ( divided by 3) or the 1/6 power of the Weight scale factor.

Figure [18] Shows weight raised to these two power levels.

System Design

At low planing speeds, the appendage drag, the aerodynamic drag, and the appendage lift forces are smallcompared to the hydrodynamic drag. The usual procedure for designing a low speed boat is to optimize thehydrodynamic drag of the hull. There is little to be gained by trying to improve or optimize anything but thehydrodynamic drag. [3][18][19]

On high speed boats, the appendage drag, aerodynamic drag, and appendage lift forces can be very large. (Theseforces increase as the square of the speed.) Lift produced by the angled propeller shaft of a direct drive systemshifts the effective center of gravity (or center of hydrodynamic pressure seen by the hull), forward. The angle ofattack of the hull decreases, and the hull drag increases. The real optimum chine beam for this high speed hull willbe different than it would be if shaft lift were ignored in the calculations. The whole system must be consideredduring the design stage of high speed boats. [20]

Limit Lines

The limit lines shown on the graphs are mathematical expressions which are valid for the particular combination ofhull type and drive system being investigated. One equation should cover all of the craft of a given combination.


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The naval architect can develop a mathematical equation which will closely approximate the limit line. The basis ofthe equation is that the horsepower put out by the engine equals the total of all of the losses and drag forces in thesystem. This equation includes factors such as: transmission losses, propeller slip, aerodynamic drag, hull drag,appendage drag, (rudder, propeller shaft, strut, etc.)

By using such equations, the naval architect knows where the horsepower goes and which losses are greatest.

When calculating the hydrodynamic drag of a planing hull, the designer must take into account the lift forces on

the inclined propeller shaft and rudder, and the suction forces produced by the propeller on the hull.

Consider a direct drive hull. The aerodynamic drag, propeller shaft drag, strut drag and rudder drag all increase asthe square of the speed. Individual drag coefficients can be developed using methods outlined in [18]. When theeffects of propeller shaft and rudder lift, and propeller suction are taken into account, it was determined that the hulldrag also increases as the square of the speed - (in the power range for which I have data).

Horsepower is calculated at different speeds using this equation. This data is then converted to Power Factor andSpeed Factor for plotting on the graph.

Similar procedures can be used to establish the mathematical equation for the limit line of other configurations.

Stepped Race Boats

The data collected usually provides total weight of hull and engine. I have estimated crew and fuel weight to arriveat an approximate running weight. The hull hydrodynamic drag is assumed to be constant at maximum speed.Equal to the drag of the step deadrise at optimum trim plus ten percent.

Most of the boats recorded, used an aft propeller shaft bearing mounted in the rudder. This essentially eliminatesthe drag of the shaft strut.

Reference [4] shows test data and limit lines for offshore performance boats, outboard powered vet bottomed hullsand outboard powered tunnel hulls.

(Reprinted from Speed Boat Developments from the Past Into the Future by Morley S. Smith, Freeville, NY)

Hydroplane History Home PageThis page was last revised Thursday, April 01, 2010 .

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