Proceedings of the Symposium Smallcraft Hydrodynamics, South East, Section of the SNAME

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Southeast Section

THE SOCIETY OF NAVAL ARCHITECTS AND MARINE ENGINEERS

SYMPOSIUM

SMALLCRAFT HYDRODYNAMICS

45PGa1pd Pi-inting,

September

1966

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TECHNISCHE UNIVERSITEIT Laboratorium voor Scheepshydromechanice Archief Mekelweg 2, 2628 CD Delft Tel: O15- 786873- Fax: 015 Z8183(1

Key Biscayne Hotel

Key Biscayne

Miami, Florida

May 27, 1966

CONTENTS

Pk966_

TECHNISCHEUNIVERSITEIT Laboratorium veer Scheepshydromechartice Archief Mekelweg 2, 2628 CD Delft Te L: 015 786873 Fax: 015 781836

RESEARCH AND TESTING

Pages

Performance Testing

A 1

A

5

Peter C. Ball,

Chief Research and Development

Engineer, ChrisCraft Corporation, Pompano

Beach, Florida

The Detail Design of Planing Hull Forms

B 1

B 26

Joseph G. Koelbel, Jr.,

TRG Control Data

Corporation, Melville, New York

HULLFORM CHARACTERISTICS

The Performance Limits of the Steppless

C 1

C 14

75-17ning Boat and-TR7Toteranlities of the

Stepped

MIT

Eugene P. Clement,

Supervisory Naval Architect,

David Taylor Model Basin, U. S. Navy,

Washington, D. C.

On the SeakeepinE of Planing Hulls

D 1

D 25

Daniel Savitsky,

Manager, Applied Mechanics

Group, Davidson Laboratory, Stevens Institute

of Technology, Hoboken, New Jersey

SMALLCRAFT PROPULSION

An Approach to Propeller Selection for

E 1

E 16

'Mall Craft

Jean E. Buhler,

Miami Manager, J. B. Hargrave,

Naval Architects, West Palm Beach & Miami, Florida

Paul D. Arthur,

Professor of Engineering,

University of Florida, Cape Kennedy, Florida

Water'et Propulsion for Small Craft

F 1

F 26

Lionel Arcand,

Assistant Project Engineer

Florida Research and Development Center

Pratt & Whitney Aircraft, West Palm Beach, Florida

Bibliotheek van de

Onderakkiing der Scheepsbouwkunde Te: hnische Hooeschool, Delft

D. COMENiAliE

I:

jr 2 6

Liz./

PERFORMANCE TESTING

by

Peter C. Ball

One of the weakest links in the process of designing

and producing small planing boats is in the actual evaluation

of performance after the boat has been completed.

I hope that

this meeting may be a step toward improving this situation.

It is interesting to me, and I think significant, that even

though it is probably easier and less expensive to construct

a full size small boat than a model, most of the new

informa-tion on planing hulls is coming from the model testing tanks.

The majority of performance testing in the small boat

field consists mainly of a series of tests to which a

proto-type or first hull of a series is subjected.

The basic aim

of these tests is to be sure the boat performs adequately

to meet its design goals and to insure that the boat is safe

enough to offer to the general boating public, often

unexperi-enced.

Generally this type of testing develops very little

information which can be used to measure the efficiency of the

hull or measure its performance in relation to other boats,

especially those of a different conformation.

After the boat builder has completed a new boat and it

is launched and is ready for "test", the engine is tuned and

several runs made to select the optimum propeller.

_The

pro-cedure following this probably consists of some speed runs

over a measured course, some high speed and low speed

maneuverability checks and probably some runs in rough water.

The only measurements actually taken are speeds over a

measured course and perhaps determination of running angles.

When these tests are completed and the builder is

satis-fied that the boat is a safe, satisfactory handling boat

he can let the boat go to the customer.

This type of test which I like to call an "adequacy

test" is certainly necessary and especially from the safety

standpoint is perhaps the most important type of testing for

small boats.

But adequacy tests contribute very little to scientific

knowledge not only because of the many important measurements

not taken but also because the general aim of adequacy testing

is "good enough" rather than the best possible.

If it were readily possible to determine the exact power

which the engine is delivering at any time and

if the drag

characteristics of the hull or propulsive coefficient could

be determined, the general adequacy test would be of some

scientific value.

But the measurement of these factors is

_out

of the realm of current testing procedures and the adequacy

test remains just that.

At Chris-Craft we are in a particularly fortunate position

regarding performance testing.

_{The main reason for this is the}

large number of different models we currently have in

produc-tion.

_{Although this does make a great deal of work, it tends}

to improve the quality of our test program in several

_ways.

The first and perhaps most obvious way involves personnel.

The group of people within the corporation that is involved in

testing spends most of its time in this activity.

Because of

this, we have an experienced and stable

_{group of testers whose}

accuracy, reliability and judgment can be relied upon.

Another way in which the testing of a larger number of

boats is of assistance is the broad base it provides for

comparison.

_{We have test records over the years on hundreds}

of boats.

_{When evaluating a new hull we can refer to}

perform-ance records regarding any number of similar boat models.

In

this way, we have a fine measure of the performance to be

desired of any boats we may test.

_{From these records we can i}

determine the effect of relatively minor variations in hull

i

form.

Virtually all Chris-Craft boats

_{are powered by relatively}

few engine options.

_{In gasoline we currently have engines of}

four different displacements in production, and in diesel the

various engine options are also relatively few.

_{The fact that}

all of our test data is based upon a minimum of engine options

practically eliminates this as a variable condition of

_testing.

This is of the utmost

importance since power actually delivered

by an engine is one of the most difficult test

_{conditions to}

control.

The ability to rely on performance records of similar

boats for comparisons also eliminates efficiency factors

which are also normally difficult to determine.

Even though it is feasible to measure hull efficiency

for certain conditions, these conditions probably seldom

exist.

All boat designs represent compromises, even

includ-ing race boats.

The foremost compromise and one which is always the

subject of a great amount of conversation is the rough water

performance vs. smooth water performance compromise.

It is

not possible to design a boat which is optimum under all

conditions.

Some builders attempt to optimize the rough

or smooth water end of the scale and others try to achieve

less spectacular but uniform performance under all conditions.

To date we do not believe it is possible to develop a boat

for use under all conditions without compromising.

I wonder

how we could usefully measure the efficiency of this type

of boat?

The end use for which a boat is intended is also a

compromise.

Whether it is a fishing boat or a sightseeing

boat, everyone seems to want good, high speed performance.

Another area of compromise which is of the utmost

importance especially to production builders is the ability

to perform well under varying loading conditions.

In the

usual approach to high performance hull design, the starting

point is generally weight and location of the longitudinal

center of gravity.

_{In production boats, especially with the}

advent of fiberglass construction, a given hull will be

re-quired to serve under a variety of conditions.

A typical boat

may be sold as a basic boat or may include many major

access-ories, such as lighting plant, fishing bridge and extra fuel.

In addition to this, there is a tremendous variety in owner's

geax which may be added later.

_{In some cases a completely}

different cabin layout may be offered on a particular hull.

Considering the variety of conditions which

_any

particu-lar boat must satisfy, it becomes difficult to arrive at

an

acceptable compromise.

The small boat industry is now in the midst of

_{a change}

from wood to fiberglass construction.

_{In addition to all of}

the problems with materials and procedures which this

_involves,

there are also profound changes in the atmosphere surrounding

performance testing if not in the test procedures

_themselves.

A4

When the mould for a fiberglass hull is begun, the time

for testing is past!

The tremendous tooling costs, especially

on the larger sized boats, and the high cost of mould changes

put much added emphasis on results of tests of the prototype

boat.

To meet this need at Chris-Craft we have used various

methods of constructing prototype hulls.

We have built wooden

prototypes, running plugs and modified the bottoms of

exist-ing hulls.

To date we have had excellent correlation between

the test results of the prototype and the production model.

While the change to fiberglass construction has changed

many factors involving performance testing, the end result

should be beneficial because of the greater demands placed

upon the tests.

Because the cost of mould changes is high,

it demands that more time and effort be devoted to testing so

that the possibility of later changes will be reduced.

The

very importance of these tests requires that they be elevated

well above the traditional level of acceptance testing.

Certainly any expense at this level will be repaid if it

re-duces the necessity for future changes.

In the scheduling of a new hull design for production,

the construction of the mould must be delayed pending

comple-tion of the prototype testing.

This increases the pressure

under which the testing is conducted and necessitates a well

organized, thoroughly planned program.

To meet the demand for better testing procedures brought

on by the advent of fiberglass construction and the continuous

need for improved hull designs, there are several refinements

required that would facilitate testing which should be of

special interest to this group today.

The first and foremost requirements and perhaps the

easiest to achieve are better performance standards and

nomen-clature.

Each person or group involved with performance

testing has developed his own procedure based upon his own

requirements.

There is no broad basis for comparison or

evaluation of test results.

In fact there is no common basis

for discussion of boat handling and performance.

Any effort

to set up a framework within which to define boat performance

would be rewarded not only by better results in the future but

If a system of performance standards were developed,

it would be of great assistance in fulfilling our next

important need, improved instrumentation.

The main

ad-vantage which model testing maintains over full scale boat

testing is the ability to accurately measure varying

quantities.

It is difficult to visualize production boat builders

increasing substantially the time devoted to performance

testing.

Therefore, the ability to take more measurements,

more accurately, in less time is of the utmost desirability

and this need can only be met by improved instrumentation.

The final area we look to for assistance is that of the

model tester or anyone else who can contribute to the

scien-tific understanding of planing hulls.

When new hull form

designs are based on sould scientific theory, we can

antici-pate more predictable results and also greater ability to

achieve better compromises for optimum performance under all

conditions.

These conditions will all improve the quality

of performance testing in the future.

Projecting this line

of thought into the theoretical, perhaps if our boats were

well enough designed we would not have to test them.

In closing, it is my thought that over the years

probably every planing hull configuration has been tested in

every possible sea condition and for every condition of

load-ing, and if this information could be brought together and

assimilated, a treatise could be written completely defining

the performance of planing hulls, bringing the art up to the

level of subsonic aircraft theory.

But this information is

not available to us, basically because of poor testing and

reporting procedures of the past.

_{I hope that we can do better}

in the future.

THE DETAIL DESIGN OF

PLANING HULL FORMS

by

Joseph G. Koelbel, Jr.

A paper presented at the May 27, 1966 meeting of the

Southeast Section of the Society of Naval Architects

and Marine Engineers at Miami, Florida.

This paper is divided into two sections.

Section I

details the evolution of a design through six successive

stages, each of which showed certain specific and planned

improvements over its predecessors.

The emphasis is on

transverse bottom shape and its influence on performance,

particularly in rough water.

There is also some discussion

of the effects of variations in longitudinal bottom shape.

The size range of these boats is from 17 to 22 feet in

overall length.

The body plans shown in Figures 1 through 7

are all the same scale and proportioned to the same overall

length.

Section II will detail the results of tests which were

made to identify and correct many faults in an inverted bell

section boat.

Also included are the results of tests on a

prototype boat designed to replace the faulty boat.

SECTION

Design No. 1

The first boat in this series of designs was a stock

model of a local small production shop.

The body plan in

Figure 1 is only a guess at its shape and does not adequately

convey the poor quality of the design.

There were no

draw-ings for the boat.

It merely evolved by trial and error,

with a preponderance of the latter.

The hull was not fair

having abrupt bends in the chines and bottom planking.

The significant features of this boat are the low

deadrise and general boxiness.

Forward,the chine line, in

plan view, becomes narrow and therefore increases the

dead-rise, but this part of the bottom was never in the water

except with the boat at rest.

In the development of this

shape the builder had first tried a perfectly flat transom,

but tripping was a problem.

When turning the tuMbled-home

sides dug in and caused the boats to capsize.

The small

amount of convexity at the outboard edges of the transom

cured this problem.

The most prominent performance characteristics of this

boat were the pounding and the lack of directional stability.

The boat was also sensitive to fore and aft center of

gravity shifts.

Design No. 2

In an effort to overcome these defects the design

shown in Figure 2 was prepared.

The principal features of

this design (compared with Figure 1) are a little more

deadrise, sides which do not tumble home right from the

chine, and a chine which is narrower at the transom than

amidships.

The performance comparison with the design of Figure 1

is as follows.

There was not much reduction in the pounding,

it banked nicely on turns, had good directional stability

under all usual conditions (this boat was not tested under

severe conditions), carried large loads without difficulty,

and had no noticeable resistance hump.

It was an

improve-ment over the old boat in every way including ease of

construction and appearance.

Design No. 3

The principal deficiency in Design No. 2 is the pounding.

In order to remedy this a new bottom was designed, keeping

the same topside shape.

The resultant design is shown in

B3

Figure 3.

The greatly increased deadrise is evident.

Other-wise the features of the design are the same as for Figure 2.

The outstanding performance characteristic of this

design is the smooth ride.

When it was first built, in 1958,

the rough water performance was truly amazing.1)

When

running it for the first time, people with experience in

conventional runabouts would brace themselves for shocks

that never came.

The directional stability is also

excep-tional, and the deeper-than-usual draft makes handling at

low speed easier.

However, it seemed that a few things could be improved.

The bottom sections are rather straight (though not flat)

near the chine and under some conditions there is considerable

slapping in localized areas.

This usually occurs in bow seas

when the waves are short and steep.

In the author's experience

these conditions usually prevail in shoal water and with wave

heights usually not exceeding two feet.

Also if the boat is

driven hard in open coastal waters with wave heights of

4 to 6 feet, or more, conditions will sometimes be such as

to cause the boat to nearly or completely jump clear of the

water at the wave crest and then strike the face of the

on-coming wave with the bottom slope the same as the wave slope

at the point of impact.

Pounding of this nature can be very

severe.2)

Another undesirable characteristic of this design

is its wetness at displacement speeds due to the lack of

flare in the topsides.

Design No. 4

Therefore an attempt was made to correct some of these

deficiencies in a new design.

The boat shown in Figure 4

is made of molded fiberglass in contrast to the first three

designs which were developed for sheet plywood.

This design

(Figure 4) is based on the testing described in Section II

of this paper as well as the experience gained with the

three boats described above.

1)There was only one other boat at that time which was

sub-stantially different from the standard flat runabout design.

2)It

should be explained that under these conditions, when

the boat jumps clear of the water, it does not always pound

when landing.

If the stern hits first the landing will be

quite comfortable.

If the bow hits first there will usually

be no impact as such, but there will be a rather sudden

reduction of both the forward and vertical velocities because

B4

The principal features of this design are the steep

deadrise forward, the moderate deadrise aft, the roundness

of the sections under the chine in the region subject

to

slapping, the high chine line with molded in spray rail, the

lower spray rail below the static waterline, and the moderate

flare in the topsides, forward.

The handling characteristics of this boat

are as follows:

In turns she banks safely, but not very much.

In a short

steep sea she is easier riding and drier than the former

design (Figure 3) but about the same under the heavy

_pounding

conditions when the boat jumps clear of the

water.

The

rather wide chine beam has several effects.

_{It gives the}

boat good stability and capacity in the displacement

_condition,

the ability to get over the hump without excessive trim and

to plane with heavy loads, but it also causes the boat to

plane at a rather flat trim angle at high speeds and light

loads.

Adding passenger weight in the

_{stern actually}

in-creases the top speed of the boat because it inin-creases the

trim and reduces the wetted

area.

One further disadvantage

of the low trim angle is

_{a slight reduction in directional}

stability in a beam sea.

_{These factors were all weighed}

during the design and prototype testing and the final

com-promise is considered a good one for the purposes for which

the boat was designed.

However, as is always the case, it is

_{not perfect.}

There remained a desire

_{to make further improvements.}

_Also,

other services would have required other compromises.

Design No. 5

This boat was done for a client who was himself a

hydrodynamicist and who was experiencedin designing, building

and racing his own boats in several small runabout classes.

The requirements were for sheet plywood construction, easy

and comfortable running in a short, steep chop, and for

good handling at low speed in rough

_water.

_{Compared with}

the boat in Figure 4, there is less emphasis on initial

stability and carrying capacity, and more on maximum speed.

To achieve these ends the

_{owner was willing to try an}

experiment, which, it will be seen, was not entirely

successful.

_{It was decided to use a double chine}

_hull

which could run on

_{a wide beam at low and intermediate}

speeds and on a narrow beam

_{at high speeds.}

_{In addition,}

the area between the chines would

_{be given some convexity}

or rocker near the stern in the hope of improving

_the

handling qualities at displacement speeds in rough water.

These features are illustrated in Figure 5.

B5

Other important features of the design are the steep

deadrise, a high chine, and narrower than usual beam.

These

features combine to give the boat low initial stability, and

therefore a free-flooding water ballast tank was built into

the keel.

This water not only increases the boat's stability

but adds to its mass and greatly improves its motions in

rough water.

However, it would be undesirable to carry this

weight around all the time, it being required only at low

speed, and so a large hole is cut in the transom permitting

the tank to drain as soon as speed is increased to the point

where the flow breaks clear at the stern.

The boat has been given a narrow flat bottom solely to

facilitate construction of the ballast tank.

It is believed

to have no significant hydrodynamic effects.

One other

advantage of the flat bottom is that it reduces by a couple

of inches the depth to which the transom must be cut out for

the outboard motor.

It may be noted parenthetically that

the outboard motor well is watertight to the sheer line,

thus preventing the shipping of water through the transom

cutout.

The handling characteristics of this boat are, in

general, excellent.

It is very manageable in confined

waters and around docks, as well as in rough inlets and

breaking surf.

Its motions are easy and it is steady an

course.

It is the softest riding of the boats in this

series and the driest with the exception of Design No. 6.

The boat has demonstrated the ability to run in reasonable

comfort at a speed length ratio of 4 1/2 in a sea state

where the significant wave height was 1/2 the boat length.

It could have been driven a lot harder, and would have under

racing conditions.

As noted above, the experimental double chine design

did not work out as planned.

As planing speed is approached

the flow breaks clear at the lower chine but reattaches to

the bottom area between the two chines.

At intermediate

speeds the solid water does not reattach but heavy spray

does.

At high speeds the spray remains clear.

At speeds where the heavy spray strikes the area

between the chines the flow over the rounded buttocks sucks

the stern down, usually on one side only.

Changing course

away from the heel will cause her to heel the other way,

and then she will suck down on the other side.

This gives

her the appearance of being unstable in roll while planing.

At higher speeds, when the spray does not strike the rounded

area between the chines, she remains upright without any

difficulty.

It should be emphasized that it was only the

B6

that was not good.

The principle of planing on a wide

bottom at low speeds and on a narrower bottom at high speeds

is good and works in practice although the extra knuckle is

not always necessary.

Two boats had been built to this design, so one was

kept as a control while changes were made in the other.

First, wider spray strips were applied at the lower chine,

aft, in the hope of keeping the spray down.

There was no

noticeable improvement.

Second, wedges were applied to the

area between the chines to effectively remove the curvature

of the buttocks, so that the running lines all extend

straight aft.

No hook was built in.

Now, when this region

gets wet, it generates lift instead of suction.

The

modified boat runs at a slightly lower trim angle and is a

little slower than the original design.

At displacement

speeds, in a following sea, (when the curvature in the

buttocks was expected to do some good) the modified design

is just as good as the original.

The second deficiency of the design is that it banks

a little too much.

This is a very slight matter but in

another design it will be corrected.

The third deficiency,

which is of a more serious nature, is only occasionaly a

problem.

At low speeds, when negotiating a very short and

steep, but not necessarily breaking sea (for example that

produced by a strong wind blowing against a strong current

in a moderately deep channel) the boat will sometimes take

a couple of inches of water onto the bow deck.

This

wouldn't be bad except that, when she pitches

up to go over

the wave, this water rushes aft, over the windshield and

anto the helmsman.

If the deck in plan were a little fuller,

and the sheer three inches higher this would almost never

happen.

This, of course, does not apply to precipitous

breakers.

No feasible increase in freeboard could keep

heavy surf or large combers from coming aboard.

Design No. 6

This design was done for a stock boat builder.

The

requirements are for sheet plywood construction, large

capacity, adaptability to a wide range of loads and

power

plants, high speed, and good handling at high speed.

_The

last two requirements mean the ability to win open water

races. The body plan of this design is shown in Figure 6.

Comparing this design with No. 5, the four most

obvious differences are the absence of the flat keel, the

absence of the lower chine knuckle, the greater beam, and

the greater depth.

The significant similarity, however,

B7

Station 2, for example, the chine of No. 6 is wider, but also

higher, than that of No. 5.

The new section is roughly an

extension of the old section.

At the transom the deadrise

of No. 6 is approximately that of No. 5 inboard of the lower

chine knuckle.

The handling characteristics of this design are

sur-prisingly good.

It was thought that the greater beam would

cause it to pound more in head seas than No. 5, but actually

there was no noticeable difference.

At high speed in

follow-ing seas offshore (the boat overtakfollow-ing the seas) the bottom

seldom gets wet above the intermediate spray strip, except

for occasional short steep waves which act more like head

seas.

Even under inlet conditions it is surprising how

seldom the chine is immersed.

This phenomenon will be

dis-cussed further in Section II.

There is sufficient deadrise in the stern to give the

boat directional stability, and enough lateral plane at

speed to give good steering control.

Even at speeds above

50 mph. the boat never "skitters" or "dances", but is always

under control.

In regard to handling qualities at displacement speeds

in rough water, this boat would have been considered very

good if it had not been preceded by No. 5.

The motions of

Design No. 6 in pitch and particularly in roll, are quicker

and less comfortable, than those of No. 5

and it does not

have the directional stability of the earlier design.

However, these differences are due not only to the change

in proportions but also to the fact that Design No. 5 has

the free flooding ballast tank which fills at displacement

speeds.

An illustration, although not a proof, of the all

around good qualities of Design No. 6 is the fact that one

boat built to these lines (a stock model)

was used

regularly in sport fishing for blues and stripers under the

usual conditions (open ocean, inlet bars, and surf), and

was then repowered, entered in the Around Long Island

Marathon, and set a new record for the course.

Although this design runs on a very narrow portion of

the bottom at high speeds, it still planes at

a lower trim

angle than we would like to see.

Ways of increasing the

trim angle and reducing the wetted area are being studied.

The boat has sufficient roll stability at planing speeds

(for example, it never heels

over and planes on one half

the bottom) but it seems that if greater stiffness could be

achieved without sacrificing any of the boat's good

This concludes Section I.

Some general observations

about what has been learned from these designs, and what

the author considers to be good features in a planing boat

will be given at the end of Section II.

B9

SECTION II

This section details the results of trials which were

run to identify and correct a number of faults in an

unsatisfactory production boat.

The lines of the boat as

originally drawn are shown in Figure 7.

The intention had

been to produce a molded fiberglass runabout 17 feet long

which would have wide popular appeal.

_{The design was}

in-fluenced by a number of people in the company and then the

boat was not built as designed because of demands of the

manufacturing facility.

The important features of the design are the follawing:

The inverted bell sections with full rounded keel and

hollow chine flare, the sharp chine

_{corners, and the "side}

keels", as they

were called, which run well forward.

The

inner one extends to Station 1 and the outer

one to

Station 2.

To facilitate construction these keels

were

shortened at the forward end to Stations 3 and 4 respectively,

and

_{the chine corner was rounded to 3/8 inch.}

_Because

the manufacturing plant had never made

a hard chine boat

and didn't want to make one, a compromise

_{was reached in}

regard to the chine radius.

_{This was kept at 3/8" in the}

forebody but beginning at Station 3

was gradually increased

to 4" at the transom.

These alterations are shown in

Figure 7.

Plugs and molds were made for the hull, decks and

cockpit interior, and several boats made before

_{one was}

tried out.

_{All concerned were chagrined at the performance}

except the man who drew the lines.

He said the plans had

not been followed, but truthfully did not know exactly why

the boat would not perform.

The writer was called in to make

an independent

evalua-tion.

A number of trials were run under

_{a variety of wind}

and sea conditions.

_{All runs were made with a single 75 h.p.}

outboard motor.

The following poor performance features

were found.

Spray ran around the chine radius (3/8 inch) and

up

the side to the gunwale molding.

_{In following seas, when}

the bow would be immersed deeply for relatively long

_periods,

for example when beginning to climb up the back of a wave,

the spray would sometimes be heavy enough

to carry away the

gunwale molding.

Even in a light chop there was uncomfortable

slapping under the chine flare.

In moderately rough water

there was rather severe pounding.

This was more serious in

bow seas than head seas, and was accentuated when the boat

B 10

Porpoising was a problem when passenger weight was

shifted aft or when the engine was tilted out (producing a

downward component of thrust).

However, under normal

condi-tions of loading and engine angle it did not occur.

In a tight turn at half speed the inside quarter

was sucked in halfway to the gunwale.

At high speed, and

not so tight a turn, the quarter did not sink in as much.

In cross seas (everything from bow seas to

quartering seas) the bow would occasionally "hook"

uncon-trolably to one side or the other.

This would happen most

often in moderately rough water but sometimes also in just

a light chop.

It occurred whenever the forefoot became

immersed.

It seemed that lateral forces were set up because

of transverse components of flow across the rounded sections

at the keel.

The spray strips (or side keels, as they were

called) were extended forward as originally designed and

this completely eliminated the trouble.

It is reasoned

that they broke up the transverse flow pattern and prevented

the formation of high, and/or low pressure

areas which could

swing the bow sideways.

The boats had a tendency to heel to one side or

the other while planing.

One of the boats would heel only

to starboard even with two or three people to port.

A careful examination showed that there were

irregu-larities in the hull contours which developed as the

fiber-glass laminate cured.

The only area where this has any

serious effect is in the bottom near the transom.

_Here

the bottom was supposed to be straight longitudinally but

variations of 0.080 to 0.100 inches in distances of 6" to

24" were found.

These irregularities were more or less

evenly distributed over the bottom and did not have

any

noticeable effect on the performance except in the

case of

one boat.

This boat had the overall waviness of the others

except for a predominance of convexity on the starboard

side and concavity on the port side.

It was this boat

which always heeled to starboard.

Although the magnitude

of the pressure changes due to the bottom

_{curvature was}

not known the forces acted so as to produce the observed

effect and were assumed to be the

cause.

(The propeller

torque acted to heel the boat to port and was therefore

not the cause.)

All other boats were symmetrical and

would heel to either starboard

or port.

The rounded chines aft produced regions of low

pressure.

With the boat upright the forces

were symmetrically disposed,

B 11

but because the chines ran close to the surface of the water

the pressure reductions were sensitive to small heel angles.

The immersed side experienced a greater reduction in pressure

than the emerged side.

Propeller torque was usually the

determining factor in which way they would first heel.

The

list could be changed from one side to the other by changing

course, the banking moment being strong enough to overcome

the chine suction.

Actually both effects are closely related,

being simply the result of changing flow patterns over the

bottom of the boat.

After observing the boat's poor performance and making

the above mentioned guesses at the reasons for it,several

experiments were made.

The first one, that of extending

the side keels, has already been discussed.

In the next experiment the chine radius was built up to

a sharp corner approximating the original lines.

See

Figures 8a and 8b.

This change completely eliminated the

heeling tendency, except that due to propeller torque.

The

boat banked well on turns and showed no tendency to trip.

Although there was not much reduction in the height to which

the spray was thrown, it was all completely clear of the

boat, none running up the sides.

Because squaring the chine corners of the production

boats would have required an expensive reworking of the mold,

two experiments were made with spray rails which could be

added to the unchanged hull.

The spray strips were

7/8 x 3/4" cove molding set with the 3/4" side against the

hull and the 7/8" side projecting normal to the skin.

In the first of these tests the spray strips were set

at the upper tangent to the chine radius from the midsection

forward, but aft of amidships they were gradually worked

down to the middle of the radius at the transom.

_{That is,}

the underside of the spray strip projected outward and

downward at about a 45° angle at the transom.

See Figure

8c.

These strips caused complete separation of the flow

before planing speed was reached, and greatly reduced the

spray height.

They eliminated the heeling tendency, but

also prevented banking on turns.

Although the boat did not

seem likely to trip on turns, the lack of banking was very

uncomfortable and perhaps dangerous.

The strips were then moved up at the after end

so that

the entire length of the strip was at the upper tangent point

of the chine radius.

_{See Figure 8d.}

_{In this configuration}

B 12

reached but the spray was not thrown quite as flat

as with

the spray rail lower.

However, the boat banked well on

turns without any tendency to skid or trip.

It was possible

to put the boat into a hard over turn at full throttle with

no danger of tripping.

To further investigate the matter of spray formation

two additional modifications were made to the boat which had

the chine radius built up to a sharp

corner.

In the first

of these a spray strip was formed on the underside of the

chine having a downward slope of about 45°

as shown in

Figure 9a.

The spray strip was tapered out between station 8

and 9 in order not to affect banking in

turns.

In this

configuration the spray was thrown down sharply, but with

such force, and so close to the boat, that the rebound

actually put more water in the boat than the initial

spray

did.

Next the angle of the underside of the

spray strip was

reduced from 45° to about 15°.

_{This made a great}

improve-ment, throwing the spray out and down in

a very satisfactory

manner.

_{While no further experiments were made}

_{to optimize}

the angle it is felt that 10° to 20° is a good range.

After considering the cost of making the necessary

changes, and the facts that at best the performance

_{was only}

average and the appearance poor, it was decided to

_{scrap the}

whole thing and start with

a new design.

_{The new design is}

that shown in Figure 4, Section I.

This time it was considered advisable

_{to find out as}

early as possible if the design was any good.

_{Consequently,}

when the plug was made a light hull

_{was laid up over it.}

This made the trial boat

_{a little oversize.}

_{After removal}

from the plug a minimum of reinforcement was added inside.

The boat was able to

_{carry great loads and had no}

difficulty getting over the hump, but

_{ran at a very low trim}

angle, and made a poor top speed.

_{In examining the boat it}

was discovered that the glass laminate from the

_{transom was}

lapped over onto the bottom and then the bottom laminate laid

up over this, effectively building in

_{a slight hook all}

across the stern.

_{This amounted to about 1/8" in 6}

_{or 8}

inches.

_{To correct this the shell was built up inside and}

then ground off outside to give the required straight

contour.

_{The boat was then tried out with the}

_{same load}

and center of gravity.

_{The maximum speed had increased}

from about 29 m.p.h. to about 33 m.p.h., and the trim angle

had increased over the entire range of planing speeds.

B 13

To further investigate this effect the bottom was given

a small amount of rocker, about the same in dimensions as

the hook it previously had, that is 1/8" in 6 to 8 inches.

The top speed now was 38 m.p.h. but the trim angle at hump

speed was unacceptable.

It appeared that the straight buttocks were the best

and so the bottom was reworked to this configuration.

A

picture of the experimental boat is shown in Figure 10.

Next the matter of spray wetted area was considered.

The experimental boat had been made with unpigmented resin

and was therefore translucent.

The mottled appearance, in

Figure 10, of the topsides is caused by shadows cast

against the inside of the hull.

Because of the translucence

the entire flow pattern could be observed in detail from

the inside of the boat at all times.

The stagnation line,

the spray wetted area, the passage of bubbles under the

bottom were all clearly visible.

With the spray rail at the chine there was a great deal

of spray wetted area.

In addition the spray was thrown out

fairly high above the waterline and was easily blown back

by the wind.

It was, therefore, desirable to break the

spray off as close to the water as possible.

The first

arrangement of spray strips tried was the conventional one

of having them follow the buttock lines.

This did not do

a satisfactory job of removing spray.

Aft of the stagnation

line they did no good, added a little to the resistance and

seemed to carry additional air bubbles under the boat.

It

seemed desirable to stop the spray rails off near the

stag-nation line.

After trying several arrangements, one of

which can be seen in Figure 10, the one decided

_{upon as}

best was a single strip parallel to the static waterline

and two inches or so below it.

The effectiveness of this

spray rail is illustrated by Figure 11.

A similar low spray rail parallel to the designed

water-line was used on Design No. 5.

It produced a very dry

boat.

Figure 12 illustrates the way the spray is kept close

to the water surface.

In this picture Design No. 5 is

making about 28 m.p.h.

_{In Figure 13, Design No. 6 is shown}

travelling over 50 m.p.h.

Only light spray touches the

B 14

CONCLUSIONS

The most significant feature of a planing hull form

(other than an effective means of flow separation at the

boundaries) is the longitudinal distribution of deadrise.

There is no doubt that constant deadrise is the most efficient

form and that zero deadrise is the ideal amount from this

point of view.

The requirements of directional stability and safe

bank-ing on turns indicate a minimum of about 5° deadrise.

For

operation on bays and lakes much more deadrise than this is

required, because even a 6 inch chop can be very uncomfortable

at 25 or 30 mph in a low deadrise boat.

_{This increased}

dead-rise is required more in the forebody than at the transom.

This indicates that warping the bottom might be a good

solution.

The difficulties and potentialities of this

solution will be discussed below.

A second solution is to

increase the deadrise over the entire length to the

amount

required at the bow.

The difficulty with this is that the minimum desired

at the bow is greater than the maximum desired at the stern.

For example, 25° is not much deadrise at Station 2

_{or 3, but}

at the stern this amount of deadrise can sometimes

_cause

transverse stability problems while planing and

can sometimes

make steering difficult at both planing and displacement

speeds.

In addition, the chine profile with this

arrange-ment is rather straight and not much higher at the bow than

at the stern.

_{The chine planform is usually rather full.}

These features produce rounded buttock lines and waterlines

which cause the boat to run downheaded at displacement

_speeds.

In calm water this is merely unsightly but in rough

water

it tends to make the boat wet and increases the

_{danger of}

putting the bow under.

_{Another disadvantage of the constant}

high deadrise design is that the reduction in pounding

achieved by the deep vee is partially negated by the

corres-ponding increase in running trim angle.

Also there is a

loss of efficiency because of the high deadrise.

The difficulties with the warped bottom

_{are a loss of}

efficiency and a loss of directional stability.

_{The former}

is not easily evaluated but is believed to be small for

moderate amounts of warpage.

_{The latter comes about when}

the angle, in profile, between the

_{mean buttock and the}

keel is greater than the trim angle.

_{Under these conditions}

the keel can draw more water forward than aft while planing.

If this is not balanced by other factors such

as chine

immersion aft, skeg, etc., the boat will be difficult

to

steer.

Even if the boat is directionally stable in smooth

B 15

a little more difficult in a cross sea.

Sometimes, as

dis-cussed in Section II, when the warped bottom is combined

with rounded sections at the forefoot steering can be very

difficult.

To a certain extent trim angle and forefoot

immersion are matters of preference, there being a trade-off

between lessened impact and better appearance on the one

hand and better directional stability on the other.

One way of getting around all these problems is

illus-trated by Figure 6.

In this design the transom deadrise is

about 15°.

This is enough for good directional stability

without resorting to skegs, and not enough to interfere with

steering.

There is a triangular area bounded by the transom,

the keel, and a line from the outboard edge of the transom

to the keel at about Station 2 or 3, in which the deadrise

is constant.

Very little more of the bottom than this is

in the water at cruising speeds and less at very high speeds.

The bottom outboard of this area is curved up a little

near the stern and more toward the bow.

This produces

easily curved buttocks, moderately fine waterlines, a high

chine, and adequate deadrise.

The boat does not run

down-headed at low speeds and shows no tendency to dive in

rough water.

Other performance features are as outlined

in Section I.

This description is not intended as a design procedure

nor is it meant to set limits on any feature of the

configura-tion.

It merely illustrates one successful solution to the

problem of designing a sheet plywood boat for rough water.

There seems to be little argument in regard to section

shape.

Concave sections pound and any advantage they used

to have in keeping the spray down has been completely

eliminated by the proper use of spray strips with convex

sections.

Additional roundness of the sections under the

chines (the opposite of chine flare) eliminates much

uncomfortable slapping in choppy water.

The matter of spray strip location is open to further

experimentation.

The arrangements shown on Designs 4, 5,

and 6 in Section I are good for those designs and for the

trim angles they run at.

They are not placed lower on the

boat in order to keep them above the ripples.

They could

have been lowered at the bow to make them parallel to the

trimmed waterline, but this was not done primarily for aesthetic

reasons.

Other boats may do better with other arrangements

of the spray strips.

The planform of the chine should not be too full forward.

B 16

increases the tendency to pound, is not required for dryness,

and in fact at low speed makes the boat wetter.

On the

other hand too fine a bow is not good either.

A planing

boat must keep her head up as much as possible.

Deep

immersion of the bow sections adds greatly to the resistance

in head seas and is potentially disastrous in following seas.

It is felt that Designs 5 and 6 represent good compromises

for chine shape.

The chine in Design No. 5 could be a

little fuller without detriment.

The chine in Design No. 6

virtually never gets into the water forward of amidships.

The shape of the bow is such that there has never been any

difficulty with operation at any direction to the sea.

The chine beam should be narrower at the transom than

amidships.

This reduces wetted area and improves performance

in a following sea.

In profile the chine should be high enough at the bow

so that it virtually never goes under water.

Normally it

does not have to be very high for this.

During the testing

of the translucent boat it was found that the actual

immersion of the bow in rough water was always much less

than it appeared to the helmsman.

It was also surprising

to find out how much of the boat's length came out of the

water at times.

For example, when negotiating breakers

about 3 or 4 feet high in very shoal water the boat would

sometimes come out to Station 7 or 8 in going over the

crest, then fall into the trough, the deepest immersion

being less than the depth of the chine.

Waves of this type

have steep crests and long flat troughs.

In deeper water, where the crests of the waves can get

closer together for a given height, it is possible to put

the chine under.

The chine is immersed much less frequently

at high speed than at low speed.

The chine profiles of

Designs No. 4 and 5 are considered adequate, and that of

Design No. 6 higher than necessary.

In this discussion

of chine height the chine is considered primarily as the

location of the uppermost spray strip rather than as a

point in the determination of section shape, the object

being to produce a dry boat under as wide a range of

condi-tions as possible.

The height of the chine aft, that is, whether or not

it goes below the waterline, and for what length, will be

determined to a large extent by how the other features of

the design (required section area, beam and deadrise) go

together.

In all probability the most critical item to

consider in determining this feature will be transverse

stability.

B 17

In regard to stability, if this is marginal, as in

Design No. 5, the water ballast tank is a good solution.

In fact, as described in Section I, it is more than just a

solution to the stability problem.

It makes a real boat

out of a plywood outboard runabout, and if it is planned

for in the design stage, it is good naval architecture.

It

is probable that no inboard powered boat ever needs ballast

for transverse stability, but there is a serious possibility

of using a tank forward of the center of gravity to reduce

trim at hump speeds.

When a high enough speed is reached

that moving the center of gravity aft will reduce the

resistance, the ballast can then be released.

Trim tabs or transom flaps are probably a better answer

in many boats but they do not control static trim.

However,

they offer the possibility of improving high speed roll

stability through differential control.

Also the trimming

moment of the flaps is more readily changed than that of

ballast.

The final point is in regard to the design of the

top-sides.

Experience has shown that freeboard and flare in

the after half of the boat are seldom critical.

The

author's preference is for convex topside sections with the

greatest curvature near the chine in order to increase the

stability at small heel angles and the reserve buoyancy at

small immersions.

This helps to keep water out of the motor

well and the cockpit scuppers.

At the bow ample freeboard

and a moderate amount of flare are necessary.

This would

not be so if the boat could maintain planing speeds at all

times, but this is a totally unrealistic design assumption.

A planing boat must be seaworthy at all speeds.

Excessive flare at the bow, particularly when produced

by very concave sections, is an unfortunate styling feature

which, in some mall boats makes it unsafe to step aboard

at the deck edge.

It also increases the vulnerability to

damage from contact with pilings and other boats in a seaway.

The hazards

of the first factor diminish rapidly with

increasing size of the boat, but the hazards

of the second

increase with increasing size.

Excessive flare also reduces

the buoyancy per square foot of deck area and increases the

danger of the boat's being overpowered by a sea breaking

over the bow.

To illustrate what is intended here, the

flare in the forward sections of Design No. 4 is ample but

not dangerous.

That of Designs 3, 5, and 6 is a good

average.

The flare, of course, is more effective in

deflecting spray if it is concave as in Design No. 4, but

B 18

The author wishes to express the conviction that the

first real break-through in modern planing boat design was

made by Ray Hunt in the design of the Hunter-19, or its

prototype.

It was the success of this boat which encouraged

the author to attempt the design shown here as No. 3, and

thereby embark on a continuing program of design improvements.

While free-flooding ballast tanks are not new, it was

Mr. Hunt who conceived of their application to small planing

boats, again in the Hunter-19.

It is hoped that others will be encouraged to submit

the results of their experience in this field either as

discussions of this paper or as additional papers in the

future.

Design No. 1

FIGURE 1.

Design No. 2

FIGURE 2.

Design No. 3

FIGURE 3.

Design No. 4

FIGURE 4.

B 20

5

Sections shown as

originally built

Intermediate

Spray Strip

Lowest Spray Strip

G 8 (0

Design No. 5

FIGURE 5.

Design No. 6

FIGURE 6.

B 21

Intermediate

Spray Strip

'Side Keels"

(e) First spray strip

FIGURE 8.

(d) Second spray strip

(a) Transom as built

(b) Transom as modified

FIGURE 9 .

(a)

FIGURE 10

EXPERIMENTAL BOAT- DESIGN No. 4

FIGURE II

SHOWING EFFECTIVENESS OF SPRAY STRIP

FIGURE 12

DESIGN No. 5 - 28 MPH

FIGURE 13

DESIGN No 6 -50 MPH

THE PERFORMANCE LIMITS OF THE STEPLESS PLANING BOAT AND

THE POTENTIALITIES OF THE STEPPED BOAT

by

Eugene P. Clament

ABSTRACT

Models of a number of designs of stepless planing boats were

tested at a representative standard condition so that meaningful

com-parisons could be made of their relative smooth-water resistance. The

results were then used to guide the development of a systematic series of stepless planing hulls which would have relatively high performance.

EHP values for the initial unrelated hull forms and the hull forms of

the systematic series are given in the paper. The systematic series

was developed from the advantageous position of being able to

incor-porate the desirable features of earlier planing boat designs which

had been developed over an evolutionary period of several decades.

Accordingly, it is considered reasonable to believe that the performance

values for the systematic series represent an approximate limit of

per-formance for stepless hulls beyond which only relatively minor further

improvement will be achieved. On the same performance camparison

graphs, EHP values are also given for a stepped planing hull with an

adjustable rear planing surface. At high speeds the EHP values for

this rudimentary stepped hull are considerably below the values for

the "limiting line" of the stepless hulls. It is concluded that

where-as the stepless hull hwhere-as nearly reached the limit of development, the stepped hull can now demonstrate significantly better performance at

high speed, and also has possibilities for considerable further

improve-ment. A number of details of the particular stepped hull developed at

the David Taylor Model Basin are given.

INTRODUCTION

Most of the great variety of planing boats built in this country

and abroad are of the stepless type, with continuous buttock lines.

This type has been the subject of a large amount of design work and a considerable number of model tests, and as a result it has reached a

relatively high degree of refinement. In the fairly recent past the

David Taylor Model Basin has retested models of a number of these designs at a representative standard condition, so that their relative

merits could more readily be seen. The results of this work were then

used to guide the development of a systematic series of stepless planing

C2

hulls which would have relatively high performance. EHP values for the

initial unrelated hull forms and the hull forms of the series are given

in this paper. Factors which tend to limit the efficiency of the

step-less boat to a level considerably below optimum efficiency are discussed. EHP values are also given for a stepped planing hull with an adjustable

rear planing surface. The lower drag of the stepped hull, the need for

an adjustable rear planing surface, and the potentialities for further improvement of this type are discussed.

EHP VALUES FOR A NUMBER OF PLANING HULLS

EHP values for a number of stepless planing hulls are presented

in Figure 1. The resistance of each design was determined by means of

a model test, and the model data in each case were corrected to

corre-spond to a full-scale boat having a gross weight of 10,000,lb. Since

each test was made at a standard loading condition (Ap/V2/ equaled

7 in each case) the planform area of each hypothetical 10,000 lb

full-scale boat is identical in every case (equals 203 ft2).

The designs represented in Figure 1 are identified in Table 1. SNAME Small Craft Data Sheets have been issued for most of these designs; the identifying numbers of the data sheets are given in the table.

Each section of Figure 1 presents EHP values for a number of designs

at a particular full-scale speed. The open symbols are the unrelated

designs of the past which were retested at a standard condition. The

test results for these and other designs were used to guide the design of a systematic series of stepless planing hulls which would have

relatively good performance. The test results for the series (DTMB

Jeries 62) are indicated by the filled symbols of Figure 1. It can be

seen that the objective of incorporating the desirable features of previous designs in order to develop a systematic series having good

efficiency, has been achieved. Furthermore, since the systematic series

was developed from the advantageous position of being able to select the desirable features of earlier planing boat designs which had been

developed over an evolutionary period of several decades, it is considered reasonable to believe that the performance values for the series repre-sent an approximate limit of performance beyond which only relatively minor further improvement will be attained.

THE STEPLESS VERSUS THE STEPPED PLANING BOAT

For minimum resistance, the wetted area of a planing boat should decrease rapidly with increase in speed (ideally, in the planing con-dition, the wetted area should vary approximately inversely as the square

of the speed). In the case of a stepless planing hull the forward

boundary of the wetted surface moves aft fairly rapidly in the lower part of the planing range, and the wetted area is effectively reduced. The variation of wetted area with speed for several of the designs of