The conceptual design of a high performance towing tank for the U.S.Naval Academy

UNITED STATES NAVAL ACADEEIY

Annapolis, Maryland

ENGINEERING DEPARTMENT

Report E-68 - 5

THE CONCEPTUAL DESIGN

OF A HIGH

PERFORMANCE TOWING TANK FOR THE

U. S. NAVAL ACADEMY

Roger H.

Compton1

2

Thomas R. Dyer

Bruce

Johnson3

Revised and Reissued

18 December 1968

Presented to

15th American

Towing Tank Conference

Ottawa, 25 June 1968

Distribution of this

document is unlimited

Assistant Professor,

Naval Engineering Division

Lieutenant, USNR,

Project Officer

Associate Professor,

Director, Hydromechanics

TABLE OF CONTENTS Summary Introduction I. Background A. General Requirements Present Capabilities Present Limitations HydroacoustiC Requirement

B. Investigation of Existing. Facilities 1. Rail Systems

2. Carriage Configuration

3. Carriage Drive Systems

ii. Laboratory Plans 12

Hydr.omeChafliCS Laboratory 12.

The 380' Towing Tank . 13.

1.. Tank Length

.2. Tank.Depth and Ceiling Height

3. Tank Width ..

4. Architectural Features

III. Conventional Equipment

A. Conventional Carriage

Configuration

Speed. Control

3.. Wheels

4. Fast Return System

B. Seakeeping Capabilities

Wavemaker

Wave_damping Beach

C. Model Handling and Technical Support Instrument Modules Drydock. 3 Technical Support D. Rail System Rails Rail Supports 3 28 28 32 35 36

IV. _{Special Capabilities}

39

A. _{High Performance Carriage and}

Drive ₃₉

Configuration

Velocity and Acceleration _Control

Carriage Drive System

B. _{Data Transmission and Reduction Systems}

45

C. _{Acoustic Provisions}

47

V. _{Developmental Engineering}

A.. Developmental Engineering _Contracts

Faculty Studies . 49 Design Review 50 References . 53 Figure 1 54 Figure 2 55 Figure 3 56 Figure 4 57 Figure ₅ . .

Appendix A _{Statistical Analysis of Towing}

Tank Data

A-i

Figure A-I

SUNMARY

The proposed hydromechafliCs laboratory will include: an existing circulating water channel; the existing 5'

tank, extended to 120'; an existing ship stability and model ballasting tank; a new cavitation tunnel, and the 3O' high performance towing tank.

The proposed tank is designed for a maximum steady state speed of 30 knots, with a steady state run of five seconds duration. This speed is required to achieve Reynolds numbers of interest in the study of near field flow noise, while the length of run is required for the gathering of sufficient statistical data to allow

reasonable confidence levels. The required sample time

for low frequency statistical data is long; and casts doubt on the utility of testing ship models in random seas.

The tank is to be construdted with a width of 26

feet and a water depth of 16 feet, in order to minimize blockage and shallow water effects within the bounds of

architectural requirements. Other architectural

considerations will include water treatment, water

tem-perature control, air conditioning, underwater observation

and photography provisions, corrosion control, and a drydock and carriage rigging area.

The tank will be equipped with the conventional

equipment now found in most towing tanks. A large (16'x20') fifteen knot carriage is planned. This carriage will

incorporate an 11' square central well, and will provide

ample deck area for groups of midshipmen. Since this carriage

and seakeeping research, it _{will be of heavy box, girder}

construction with unobstructed viewing, _{and will ride}

on vibration damped steel wheels. A _{precise speed}

control, such as a frequency based _{drive powering four} d.c. or synchronous a.c. motors, will be required. _A

system to allow rapid return runs, while Smoothing the

free surface, is desirable.

A wavernaker able to _{generate three foot high regular} waves, thirty feet long, will be installed. _{It will be}

capable of random sea demonstrations _{and operation with} varying water depths. _{Th& rigid beach will be 'hinged} and segmented to permit _{unobstructed runs, and to offer}

various beach profiles for optimum _{damping and the study}

of near shore wave phenomena. _{Experiments are also} underway with an air bubble curtain beach.

Since a major _{purpose of the tank is the study of}

flow noise, acoustics _{must play a large role in its}

design. Noise due to the carriage, _{models, and towing struts}

must be considered. _{Anechoic material must' line the tank,} perhaps serving also _{as a wave and current damper and} covering. _{'Anechoic, noise free,}

conditions are not

required, so long as an acceptable signal-to-noise 'ratio is achieved at the model.

The rails will be cantilevered from a structure independent of the tank, isolating _{the wheel noise 'and} reducing the required span. _{The tank structure will be}

isolated in similar _{manner from all other structures}

within the 'building.

A high speed carriage will be _{specially designed for} acoustics tests. _{Design Parameters will include velocity,} acceleration, time ra'te of change of acceleration, and

vibrational levels'. _{The carriage will be of lightweight} damped structure ridingon slick rubber tires-or air

11

bearings. Several power systems are under consideration: twin continuous loops of steel tape or cable, pulling

the carriage from external motors and clutches; very high

power to weight ratio induction motors aboard the

carriage a linear induction motor; or a catapult with

sustaining motors aboard the carriage.

Data transmission systems such as telemetry, trailing wires, inductive trolleys, or on-board tape recorders,

will need to be considered.. Vital in this consideration

will be the dynamic range of the equipment relative to that of the expected signals. The data reduction function-must consider real time statistical analysis, optimization

of the number of frequency resolution elements,

pre-processing for a digital computer, and the interfacing with a time sharing computer system.

A CKNOWLEDGEMEN TS

The authors wish to thank Captain

_{R. W. King, USN,}

Associate Professor

_{R. F. Latham, and Associate}

Professor P. R. Van Mater of

_{the Engineering Department}

of the U. S. Naval Academy, for

_{their interest, their}

support, and their ideas.

_{Dr. Karl Remmers of Keinpf}

_and

Rernmers, and Mr. W. F. Brownell,

_{Jr. and Mr. V. W. Miller}

of the Naval Research and

_{Development Center have also}

been of great help,

_{as have many people too}

_{numerous to}

name who have shared freely of their ideas

_{and experiences.}

To Professor Thomas C. Gilitner

_{very special thanks are}

in order, for his instrumental

_{role in making this entire}

project possible.

Since the original issue

_{of this paper, the authors}

have received numerous

_{comments from the delegates to the}

15th American Towing Tank Conference.

_{Many of these ideas}

will be incorporated into the

final design, and have

resulted in changes to this

paper..

_{To all the delegates,}

_and

especially toMr. S. T. Mathews

_{and,Dr.. D}

_Gospodnetic,

we extend grateful acknowledgement for

_{their generous}

sharing of time and ideas,

_{and for an exceptionally fine}

INTRODUCTION

Naval Architecture and Marine Engineering have long

been taught at the U. S. Naval Academy. Since 1956 an

85' towing tank has been in operation, both as a teaching

laboratory and. increasingly as a research tool for

midshipmen and faculty. As part .of the recently approved

modernization of the Naval Academy facilities, a new Engineering Department building is now being planned.

Included in this building will be a modern hydromechanics

laboratory centered about a 380' towing tank with hydracouStic capabilities.

The Naval Academy must educate and train a man for the Navy of the future, and for the technical problems he will encounter. The proposed tank will support this

mission by providing a dynamic atmosphere in which to

educate informed and inquisitive junior officers under the supervision of a first-rate faculty. As the Naval.

Engineering curriculum evolves, instructional use of the hydromechaflics facilities will grow above the already significant levels. But in addition to instruction, the

tank will be used for midshipman research, especially

under the Trident Scholar program. It will serve the vital function of attracting capable midshipmen and faculty.

The tank will also contribute greatly to research in hydro-acouSticS, ocean engineering, hydrodynamics, and naval architecture. Such research will promote faculty

involvement with current naval problems, will allow faculty advancement in the academic community, and will maintain the faculty's technical proficiency. The tank

itself should serve as a source of new ideas for towing tanks of the future.

The faculty of the Naval Academy _{has thoroughly}

investigated the free world's existing _{towing tanks,}

visiting most of the modern tanks _{in the United States,} Europe and Japan. _{The successful features of}

these tanks

have been examined for applicability in the proposed Naval Academy tank, and have led to a conceptual design

SECTION ,1

I. BACKGROUND,

A. General Requirements

1. Present Capabilities

The present 5 foot towing tank at the Naval.

Academy has received increasing use in recent years, both

in a teaching and research capacity. The increased

emphasis on independent student projects and the Trident

Scholar program frequently results in experimental projects

which exceed the capabilities of the present facility. iiidshipmen and their facu.lty advisOrs are naturally

interested in âuch areas as drag reduction, 'the interaction

between turbulence and flow noise, the hydrodynamics of various forms oI advanced marine vehicles from hydrofoils to winged submersibles, and various exotic projc,t in ocean

engineering, naval architecture, and marine engineering.

An attempt has been made to' upgrade the h'rdromechafliCS laboratOry in terms of modern electronic

instrumentation to demonstrate how on-line data acquisition

and analysis cn be 'applied to realistic 'engineering

problems. A new 0.01% regulatiOn speed control

system is under contract. It will automaticallY control a

new 8channel

digital 'data acquisitiOn which is tied

directly to a 'time-shared computer'SySteflI. 1The data

acquIsition system conSistS of eight scalers controlled by

a single time base. The present analog devices for

statistical anaIysi'Of turbulence and acoustic data will

soon be joinedby

an on_l'inè,"real'time, signal processor. The facUlty is engaged in a major effort iti the

experi-mental statistical analysis of coherent signatures buried

in noise, especially _{as related to ASW.}

Itis important

that the future officers, _{who will be directing}

vast

research and development programs in these areas, have experience in an experimental labàratory

whe±'e the' complex ASW problems _{can be isolated into speciflO projects}

studies under controlled _{conditions. It is most effective}

to learn first hand of _{the limitations and frustrations}

of trying to solve _{a fleet problem through}

research and

engineering efforts involving _{simplifying, models. In} addition, the midshipmen and facul'y gain _{a creative}

out-let for personal involvement in the. design, _development,

test and evaluation of significant Navy-oriented

engineer-ing projects.

2. _{Present Limitations}

Even with the _{improvements in instrumentation,}

the existing towing tank

is subject to severe limitations.

These are largely imposed _{by its small size,}

with resulting

lack of ability to achieve _{realistic. Reynolds Numbers,}

to hold reasonable speeds for adequate steady state _periods,

or to test models of adequate size.

It becomes necessary both in research 'and

instruction to examine isolated and

individual portions of the ship _{resistance problem.}

Such examination, because of the _{difficulties of scaling and} the limitations of the physical law of similitude, _requires

testing of larger models at _{speeds more closely}

related to

??full scale."

Much of the problem of _{model scaling is} related to the modeling of a turbulent boundary layer _to

correlate with that found on a full scale ship. _{This is} impossible of course, Since Reynolds and Froude scaling

cannot be achieved simultaneously

experimental approximations may be obtained provided the

boundary layer remains turbulent, once stimulated.

The problems of achieving "natural transition"

in the boundary laye'r of ship models can best be analyzed

by looking at the influence of model size on the model

Reynolds Number at a given Froude Number.

Recalling that, for equal Froude Numbers

Rn

= V

_1'fl , and

V = V j

m

it can be seen that

Rn = Vs

(L)3/2

tmr

V

Thus, a tank cross section and length which will enable testing of 12 foot ship models, as compared

to 3 foot models in the 5 foot tank, will increase the

model Reynolds Number by a factor of for the same Froude Number. For most6tests, this means increasing Reynolds

Numbers to the 10 range where turbulence stimulation is

quite easy, as compared to the l0 range, where effective stimulation is more difficult.

3. Hydroacoustic Requirement

For submerged models, Reynolds Number must be

increased by at least a factor of 20 compared to the present tank. (Speed by a factor of 5 and model length by a

factor of 4.) This conclusion is based on futile efforts

to make meaningful flow noise measurements in the

5'

tank. Because submarines cannot effectively "see" what

lies ahead at speeds greater than 20 knots because of the self-generated "flow noise", this is an especially important

problem area for the Navy.

As Patrick Leehey (1) ad others have noted, no experimental facility exists anhere in the world

to study flow noisearound submerged bodies under controlled

environmental conditions. Most flow noise facilities investigate internal flow in a pipe, where the radiative field is quite different from that found in _external f low about a body. _{Buoyant. and gravity propelled}

submerged bodies have been used with _{some success to study} external flow in open lakes, but much remains to be

learned about the near-field "pseudo-sound" _{generated by}

the interaction of the turbulent boundary layer _{with a}

solid body. it is felt that a major cotitribution _to

greater understanding of this phenomena could be made by

testing small well-instrumented sonar domes and other

objects at nearly fill scale Reynolds Numbers, _{at speeds}

above 20 knots inthe controlled envIronment of _{a towing}

basin designed to acoustic. specifications. _{The ambient}

noise levels in all existing tanks preclude such _testing.

Special care must therefore be taken to inéure. that the desired signal from the "pseudo-sound" _{is greater than}

that of the acoustic radiation sources withinthe. new_=.

-tank. It must be clearly understood that anechoic, _noise free, conditions are not required.; rather an acceptable

signal-to-noise xust be achieved at the model so that meaningful hydrophone data may be obtained..

4. _{Proposed Experimental Techniques}

Qnce the, considerable technical problems of

achieving acceptably low noise and vibration levels in the tank are solved, several important new experimental

techniques can be used to study the problem of. "near-field

sound (as opposed to radiated sound, which will be .

Flush-mounted hot film anemometers, accelerometers,

wave-vector filters, and small hydrophones can be

mounted close together to try to separate the.influenCe

of the locally radiated sound, involving fluid momentum transfer on the microscopic scale, from "pseudo-sound",

which involves fluid momentum transfer on the macroscopic scale.

The wave-vector filter (2), under development at NSRDC, is sensitive to pressure fluctuations which

travel at the convection velocity of the turbulent eddies

in the boundary layer (this is what is meant by "pseudo-sound"). It is designed, however, to reject pressure fluctuations traveling at the local speed of sound in the

fluid (hence the term "filter"). Since acoustic

radiation is of the latter type, the wave-vector filter does not require anechoic conditions for successful flow noise measurements.

Because of the well established relationship between wall shear stress and convective heat transfer,

the hot film sensoris sensitive only to macroscoPiC,

momentum transfer, which in turn is related to the

instantaneous wall shear in the case of a flush-mounted

sensor. Acoustic radiation accompanied by no relative fluid motion produces no detectable signal in the

anemometer.

A. flush mounted hydrophOfle is sensitive to

both macrosCOPiC and microsCOPiC momentum transfer as well as to.vibratiOt1S in the structureto which it:is attached.. It is this complex sensitivity which makes hydrophone

measuremfltS difficult to interpret by themselves. Acce1eromete'S- are, of course, sensitive to model

By subtracting and analyzing _{the respective}

signals of the four sensors in the amplitude, frequency, and time domains, it is _{hoped that greater insight can} be gained into the. nature Of flow-noise. . By comparing

the results to similar experiments performed in air, it

could be determined whether _{the relative strengths of} the macroscopic and _{microscopic momentum fields vary with}

the medium according to similarity _{laws yet to be}

determined. _{Until such measurements can be made,}

significant differences of opinion will continue to exist as to the nature of the forcing function _{causing sonar}

done vibrations.

B. _{Investigation of Existing Facilities,}

In the summer and fall of _{1966, Professor Johnson} visited 26 towing tanks in _{the United States, Gerrnany,}

Denmark, Holland, England, and _{apan, gathering design} dataon basin construction, rail design, carriage

configuration and propulsion, _{instrumentation, and}

wave-making capabilities. _{In observing both the.good}

and bad features of towing tank design, _{the following innovations} stood out and have been, considered in the conceptual

design of the Naval Academy Tank.

1. _{Rail Systems} . . .. .

Many recently built

tanks.have.continuous.

welded rails, which eliminate _{the vibration and noise}

associated with rail joints. _{So long as the rails are} mounted directly on the sides, of the basin, however, _even.

the quietest carriage (found _{at Feltham, England) transmits} too much noise to the water for _{acoustic tests. The}

cantilevered rail systems found _{at the Osaka University} Tank, at one of the Wageningen _{tanks1 and at the NRc tank} in Ottawa offers several advantages in addition to the'

possibility of good acoustic isolation. The carriage

spans less of the. tank width, and can consequently be made lighter; and the tank water level can be changed without disturbing the rail alignment.

2. Carriage Configuration

Many tanks favor a rectangular carriage with four sets of drive wheels. The center section is quite open so that interchangeable dynamomel.er and

instrumen-tation modules may be easily mounted as needed. The new carriages at Feltham, England; West Berlin; and the Ship

Research Institute, and the University Tank, in Tokyo all feature large central openings. The University of

Tokyo carriage is specifically designed for photo studies

of ship generated waves.

The Feltham. and Ship Research Institute

carriages are of tubular construction of a more open design than those at the U. S. Naval Ship Research and

Development Center. The Fukuoka, Japan, carriage is of

box girder construction, with a large open platform.

Aircraft-inspired monocoque construction is featured in

the Meguro (Tokyo) and new Berlin carriages.

The Feltham carriage is exceptionally vibration free.. This can be attributed to the.use of individual

drive units which are vibration isolated from the

carriage through a three-point rubber suspension system. The drive units make use of two-wheel bogeys with a

wheel spacing designed.to minimize the effects of rail

deflection between supports.

Rubber tires are used as drive wheels in a unique way on the Berlin carriage. Each rubber tire has a pivoted steel wheel on either side of it, so that

the relative carriage weight supported by either set

of wheels can be varied by changing the tire _inflation. Precision seakeeping tests can be performed on the

steel wheels, and a softer ride can be utilized for high speeds.

3. Carriage Drive Systems

On Board Electric Motors

This is the most popular carriage _drive system, with improvements being made in speed control through various feedback systems. _{One of the best}

systems is found on the Feitham carriage, which _{has four}

tachometer generators, mounted one to each drive motor.

As the carriage speed exceeds the _{range of the lower} speed tachometers, they are disengaged _{through the use}

of magnetic clutches. _{This is an excellent technique to} give wide speed ranges. _{Another interesting speed}

control is found on the Fukuoka carriage: a fast

responding tachometer generator analog _{control for}

acceleration up to 99% of preset speed with _{takeover by} a slotted disk digital device for very accurate steady

state speed control.

On Board Hydraulic

This is a very smoothly accelerating

propulsion device, but it is quite noisy. _{Preset steady}

state speeds are difficult to attain if an accumulator

is used for acceleration. _{Better feedback controls for} hydraulic drives are still needed.

On Board Water Jet Sustaining Engine

This has been proposed for the Wageningen high speed carriage asa minimum cost, high _performance system, but its practicality is still tobe _proven.

External Continuous Loop Steel Cable Drive Thisis the most popular external drive

motor can be either, electric or hydraulic with the

advantages and disadvantages previously, listed. The

carriage is made fast to a loop of cable pulled externally

by a drive drum located at one en4 of the tank. T1is,

drive makes possible the use of lightweight carriages for high acceleration rates with no on board drive vibrations; but cable stretch, due to e1asiCitY and untwisting, can cause serious velocity fluátuatiOn

problems and sometimes results in the slack side of the cable loop dragging in the water. One: posib1e improvement

would be the use of preformed non-rotating stainless cable

developed for ocean technology applications by U. S. Steel. External Continuous Loop Pretensioned,

Tape Drive

This scheme, utilizing relatively inelastic steel tape, was originated at M.I.T., and seems to offer

the best combination of low weight, quiet operation and

lack of cable elasticity induced speed oscillations.

Linear Induction Motor Drive

This is a' fascinating idea in which the rail acts as the "rotor" while the carriage is the "stator".

It is frequently suggested by those who would like to see

someone else try it first. It would represent a sizeable investment in research and development and might present a magnetic field problem for many types of sensors and

5ocjated cabling. This technique is currently being. developed in France for the French "Aerotràifl" by The:

Merlin-Germ Company of Grenoble, and for rapid

transit by the compagnie d'EriergtiqUe

Lineaire; itmar

ultimately become popular.

Hydraulic or 'Pneumatic Catapult

This idea is under' 'development at both the

date. _{The problem of matching feedback control}

sytems

between the catapult and the electric _{sustaining motors}

requires a great amount of _{additional development.}

The

Berlin technique of using the arresting _{gear, to help} recharge the catapult is _{very clever but mechanically} complex.

h. Water Jet Catapult

This technique,in which a high energy jet impinges on a carriage _{mounted bucket,is used by}

the

Langley, Virginia high speed test facility and has been

proposed for the Wageningen _{high speed tank.}

This is a very economical system but it _{cannot maintain a constant}

speed, is very noisy, and _{gets everything rather wet.} It is spectacular _{to watch in operation.}

j. _{Acceleration and Deceleration}

Rails This novel concept is _{used on the small} Meguro tank. _{An extra set of rails is located}

outside the main rails in the _{acceleration and deceleration} sections of the basin. _{An automotive tire}

is mounted outboard of each of the _{carriage wheels to give greater}

wheel traction for _{acceleration and deceleration.}

The wheels are free spinning during the steady _{state portion}

of the run.

SECTION II

II. _{LABORATORY PLANS}

A. _{Hydromechanics Laboratory}

The proposed hydromechanics

laboratory (Figure 1)

will include: the 3O foot towing tank; an existing

16" x 16" section, _{75 hp, free surface circulating water} channel; an existing 19? _{x 23' x 4' deep steel tank,} currently used to study the static _{stability of large}

new towing tank; a new cavitation tunnel,, proposed to

have a 12" x 12" test section with no free surface but

the capability of testing various hydrodynamic bodies and propellers; and the existing small towing tank. The

small towing tank which will be extended to 120' length now measures

85' x 6'.x

3.51

deep and is. of steel

construction. The rail system, and perhaps the water

basin proper, will require replacement in the new building. The carriage, the speed control system, the wavemaker, the instrumentation, and the iodels from the small tank will be moved to the new building. It is felt that the utility

of a small tank, with easy and unobstructed model viewing, is a very great asset in the teachig of large groups of midshipmen. The tank has been especially useful in the

study of boundary layer additives which necessitate a complete change of tank water after their use, and in

testing small models such as yachts. If the water basin is replaced, it will be made a

1/3

scale model of the

30'

tank, thus creating a "geosim" for blockage and scale effect studies.

B. The 30' Towing Tank

The toiing tank will have a total usable run. of

30'; it will be 26' wide,

16'

deep, and will have

435'

of rails. Two towing carriages will be used: a

conventional carriage designed for resistance, seakeeping,

and other Froude-scaled testing in the one to fifteen

knot speed range; and a high performance carriage, designed

for flow noise, boundary layer, submerged model, and other Reynolds-scaled testing in the two to 30-knot speed range. The basin will be equipped with a wavemaker and an

1. Tank Length

In designing _{a towing basin for hydroacoustics} and turbulence research, _{'thestatisticalaccuracy which} can be obtained for a given data run is an important consideration, and must be viewed in the light of the

data analysis techniques which _{are tobe used. Turbulence}

and flow noise data are generally analyzed for spectral

content (amplitude vs. ±'requency),In an attempt to

establsh the energy. content of the _{ariOus eddy' scales} present in the turbulent boundary layer. _{(1) The most} elementary measure. of the eddy _{scales is. the.}

one-dimensional, wave number, k1, 'defined _{in. the. direction.of}

the mean motion as follows:

k1 = 2rrf

_{where f=frequency}

measured by the sensing device, and, mean convection velocity of the eddies past the sensor ' ..

The convection velocity, _{as measured through cross correlation} of several closely spaced _{sensors, or with th wave-vector}

filter, varies with theeddy sizes, but for purposes of

this discussion an average mean value may be taken 'as

approximately O.3 times the free stream velocity () (môdel

velocity in the _{case of towing tests), i.e.}

O.3 V.

Combining the above expressions _{it s seen that eddy scales}

of a given wave number, k1.,appearas _{higher irequencies} as the velocity past the sensing _{device is iicreased.}

Table 1 shows the range of wve..numbers generally found.

number will appear when sensed by a single hot film

probe at velocities of 10 and 50 feet per second.

Table 1. Equivalent frequencies corre-sponding to wave number ranges found in

boundary layer turbulence. -.

Analysis of the spectral energy content (turbulence or acoustic energy at a given wave number) is usually done..

on a power spectral density analyzer (analog or digital). Since the data sample is finite. (and quite short in towing

basin tests), one must be concerned about the statistical

accuracy of the data, relative to the infinite ensemble

of a stationary random process which is statistically

equivalent to the phenomoneflabeiflg measured.. If one assumes the turbulence to be a stationary random process

(time invariant mean values, correlation functions, etc

which are difficult to achieve without very precise ca'riagespêed control) and that: the data smples are ergodic (every firiite..data sample exhitsthe same.

statisidal prOperties as an infinite sample) , 'the following analysis parameters may he defined:

-15 -Frequency Corresponding to V .1Ofps Frequency Corresponding to. V 50 fps (ctn_l)

_(ft)

. ':z. , .

()

10_i 1' 10 100 0.5 305

.3050j

'

. if O 'J+O0 .4000 - '20 " 200 '2000 20000

Lr _{Length of the steady state tank run,}

Tr _{= Tirne.over which the sample is to be}

analyzed, the maximum _{possible time}

being dictated by the length of the

steady state run divided _{by the}

carriage velocity,

f _{= frequency of the elecr.onic}

signal ,p Hertz (Hz),

B _{= Bandwidth of the analyzer filter (Hz),}

V = Steady state carriage velocity,

Q = f/B ??quality??

or analysis resolution,

n _{= "degrees of freedom?? (See Appendix A)}

2/2,

= normalized mean square error (Appendix A). Now from Appendix A, the _{reliability of a statistical}

analysis may be related to the statistical degrees _of freedom, n, i.e.,

flmax2BTr2fTr21Lr

QV

but K1 and U

_O.3

_V,

thus _nmax

_2(O.3)UK1L

= KlLr

2rrTJQ .8Q

and rinaxQ _KlLr,.

3.8

which shows that for _{a given wave number, a new dimensionless}

parameter, _{max9' which is a measure of the statistical}

accuracy and resolution of a spectral _{analysis, may be defined} and is a function only of _{the length of the steady state}

Typical analyzer Q's are as follows:

Previous experience has shown that the

minimum value of n which can yield meaningful, turbulence

results is about 50, with a value between 100 and

400

being desirable for good statistical accuracy. At lower

values of n, Figure A-1 in Appendix A shows confidence levels diverging severely so that the measured power spectral density function may no longer approximate the true power spectral density function. Thus for one-third

octave analysis, which represents a minimum resolution figure for most work, and. for a minimum value of n,

= (50) (4.43) =

222

Comparing the 50 foot steady state run in the present tank with the

250

foot steady state run

(50

fps

for 5 seconds) in the proposed tank, one obtains, for the expected values of turbulence wave numbers:

Table

2.

Comparison of dimensionless

statistical parameter for wav number ranges found in boundary layer turbulence.

La1yz

1/3.

Octave 1/10 Octave 5% 1% Q

4.43

14.5

20

100

max max .

(cm)

(ft1)

Lr = 50 ft L = 250 ft

101

3.05

40

200 1

30.5

400

x lO

10

3.05

4 x

io3

2. x

100

1

3050

4 x lO

2 x l0

Thus Table 2 shows that the proposed new tank length

just meets. .the minimum criteria at. the lowest wave

numbers of interest in turbulence work.

From a different point of view, increasing

the steady state runIenth from

_{50 to250 feet allows} an increase of

flm by a factor of five. This allows

corresponding reduction of the _{uncertainty interval at 90%} confidence by a factor of approximately three. _Reference to Figure 1-A, Appendix A, _{shows that for n = 10, the} true mean 'square value lies between _{+lao% and -44% of the} measured mean square value. _{For .j = 50, 'the true mean}

square is between +40% and -27% of the _{measured mean}

square value. _{Thus with a 250 foot steady state run, the}

true power spectral density function may be estimated

with three times the _{accuracy at a 90% confidence level.} This is a significant _improvement.

Note that so long as the carriage speed is

steady'and repeatable, which requires precise speed control, all of the above results _{are independent of carriage}

speed. _{Speed only determines the}

frequency corresponding

to a given wave number. _{Changing the carriage}

speed will not allow use of a shorter tank. _{Even though statistically} significant. hot film _{measurements will be possible at}

frequencies as low as 20 Hz, it is still expected that _low

frequency hydrophone _{measurements win be meaningless.}

Low frequency acoustic absorbtion _{in water is not possible} with anechoicmaterjals of _{reasonable thickness. These} facts, and thegerieralnecessjty _{of high Reynolds numbers,}

requires high carriage speeds despite. the independence of velocity and

%ax

'

From a more traditional naval architectural

viewpoint, the' p'rOpose.d basin length _{is. 'd.esirable for}

the ability to run-high speed EHP tests

with a reasonable steady state data collection interval

and relatively mild accelerations,

the.ability to obtain a reasonable

period of steady state conditions in self-propelled model tests, and

c. the

long (and hence more without unreasonable It must

ing experiments in random seas involve extremely low.

frequencies, and thus require great run lengths to achieve good statistical accuracy. The proposed tank will use models ten feet long, and will operate with a maximum

wave lengthof

thirty feet. Therefore, by reasoning similar to that above, it is possible .to.calcu.latE.. a

minimum steady state run as. a function of spectral. analysis techniques. It was, shown above.that

flmax=2fLr.

QV

where in this case f is the frequency of- encounter with the longest wave of .interest, . .

f =V.+.Vw

E

Thus,

=

'y)

imaxw

Therefore for the.followiflg assumed conditions: model

speed, V = 7 ft/sec.; maximum wave length, L = 30 ft;

maximum wave velocity,.V =12.4 ft/sec; and

minimum

desirable

-

"max

= 222;' the required steady state run

length, Lr will be 1200:feét. If the model proceeds at

only L1. ft/see, the length of run required to achieve the ability to, install a relatively

efficient) wave absorbing beach, sacrifice in useful model ran.

seakeep-necessary run time, and thus the seakeep-necessary will

be reduced to 00 feet. _{Thus, in the case of seakeeping}

experiments, Lr is strongly speed _dependent.

Since a 1200+ foot tank is _{not practical,} seakeeping experiments must allow for the restricted

length. _{In the case sited, five}

consective runs at exactly the same speed _{or thrust will meet the minimum} criteria. This is obviously time _{consuming. In addition,}

multiple runs require truly random _{seas, and exactly}

repeatable carriage speeds. _{Violation of either of these}

requirements adds to the uncertainty _{of the results. Some} relief may be achieved by using _{smaller models, thus}

raising the frequency of the longest _{important wave, and}

lowering the model speed at a given Froude number.

However, several other factors make seakeeping

tests in random seas quite _{impractical. The uncertainty}

interval quoted above for _{n = 50 is very large in the}

context of steady state or low frequency _{naval architectural}

problems. Bringing n to a really desirable _{value, such as}

400, will require a steady state run of 9600 feet. _For

high speed vessels, such as planing vessels and _hydrofoils, the dependence of run length _{on speed will increase this}

length even further. _{In the following sea case, where} frequency approaches zero, the required length of run

will approach infinity. _{Therefore, except in cases of}

highly non-linear phenomena, _{testing in regular seas and}

employing linear superposition to predict random _response appears not only to be more practical, but often _more

accurate as well.

Thus ignoring the random _{sea problem, we see} the minimum 30 knot run length is _{250'. To this we must}

(1 second at 30 knots) to.allow starting transients to damp out. The total usable run over which the carriage

can actually tow a model must therefore measure

30'.

Any less run will compromise the intended mission of

the proposed facility.

In addition to the 30' of run, the tank

room will require auxiliary space at either end of the basin itself for a wavemaker, a drydock, and a rigging

area. Th drydock is essential when operating frequently

with submerged models. A large pump will transfer water very rapidly from the d.rydock to the tank proper. Thus adjustments, repairs, and inspections may be made on a submerged model quickly and efficiently without removing

the model from the carriage.

Since the basin will employ two carriages, described below, parking space must be available for the

unused carriage. The high speed carriage will be parked over the wavemaker when not in use, while the conventional carriage will be parked over a small "finger" drydock

behind the main dock. In this manner either carriage may

use the main drydock the high speed carriage may accelerate from within the drydock; the high speed carriage may be

stopped over the wavemaker in the event of braking system failure; and the conventional carriage may be rigged with surface models in the "finger" dock while the high speed

carriage is in operation.

The entire length of rail required includes the two 17' drydock lengths, 21' for the wavemaker and

emergeny braking, and a run of 30', for a total rail

length of 435 feet.. The tank is nominally referred to

2. _{Tank Depth and Ceiling Height}

The testing of self-propelled models and high speed surface and sub-surface craft impose strict restrictions on tank cross section. _{Since shallow water}

wave effects will inevitably be strongly _{present whenever} the model runs at the maximum _{design speed of 30 knots,}

a compromise depth must be specified. _{A depth of'16 feet,}

which seems maximum under _{architectural considerations,} allows a model speed of 9.5 knots without shallow water

effects (4). This represents a 10 foot model operating

at a speed/length ratio of 3.0, _{or a 5 foot model at a} speed/length ratio of 4.25. _{These model speeds are} adequate and 16 feet was chosen as the required depth.

Most models operating at higher speeds will be:ubmerged and Froude number scaling will _{be of less importance,}

thus shallow water waves will caus _{less difficulty.}

The ceiling height is also _{of critical concern.} As seen in Figure 3, the _{cantilevered rail system and}

large flat carriage deck _{necessitàtesa carriage which}

runs relatively high above the free surface. _{Nine foot}

clearance above the carriage deck is essential for head-room, hoist clearance, and safety in the _{vicinity of the} electrified trolley system. _{In addition to these}

considerations, Inui and Tagori recommend an unobstructed height over the free surface of 1.5 times the model length for purposes of stereo-photo _{analysis of ship mOdel}

wave patterns (5). _{This type of work wOuld normally be done} using 10' models; thus requiring an unobstructed 15'

above the model. Inlight of theseconsiderations, _{it is}

evident that a.total ceiling ôlearance _{of 16'-6" above}

3. Tank Width

Tank width was determined after consideration of self-propelled model tests. A six inch diameter

propeller was considered the minimum useful size. A

variety of ships, with differing displacement_length and

L/T ratios, were considered operating through arange of Froude numbers. The resulting blockage data was

weighed against architectural contraintS and a width of 26 feet was chosen. The tank's designed cross sectional area is therefore 416 ft2. The largest self-propelled

model considered would have a midship sectional area of 2.1 ft2, or about 0.5% of the tank area. The maximum

increase in local flow velocity seen by any model due to blockage will be less than 2% depending on the

interaction of wave and blockage effects (6). Similar considerations applied to the existing 5' tank limit model lengths, in some cases, to as little as 24", which

is uncomfortably small when scale effects are considered.

Because a significant amount of testing is to done at high speeds with a submerged model, the

traditional 2:1 ratio of depth to width was not employed. It should be noted that an increase in either depth or width would be welcome, but because this tank is to be

constructed as part of a larger building it does not appear architecturally feasible. The chosen dimensions

represent the minimum cross section consistent with the tank's pràposed mission.

Another feature effecting cross section

design is the roposed hydroacousticS capability. In order to minimize noise reflection from the tank walls

and bottom it is anticipated thatthe tank will be lined with a sound absorbing material. The tank dimensions

L1.. Architectural Features

The detail design of the following items

will be intimately related _{to the design of the new}

Engineering Department building, _{and thus will be the} responsibility of building architect.

a. Water Basin

The basin is to be _{constructed of any}

suitable material, probably _{reinforced concrete. If}

reinforced concrete is to be _{used several questions must}

be answered.. _{Is it possible that steel reinforcing will}

cause electrical: problems in the towing tank instrumentation,,

for example acting as an antenna, collecting and

reradiating radio energy? _{Or perhaps can the}

reinforcing

material be grounded and act as screened room, shielding

the tank from radio interference? _{Is it possible to use}

post-tensioning to alter the natural _{frequency of the}

basin itself, thereby improving its acoustic and vibrational

characteristics?

The basin walls must conform _{to dimensional}

tolerances of about +1", and _{must be as smooth as}

possible. Roughness elements,, such as exposed aggregate, seams,

non-flush fittings etc., will _{interact with waves generated}

in the tank and lead to _{undesirable secondary wave} patterns.

b. Basin Foundation

The water basin foundation must be

carefully considered. _{The strict acoustic specifications} will undoubta.bly effect the _{choice of foundation and fill} material. _{Sand, or other granular material, is a good}

base with good acoustical _{properties. The foundation of}

the water basin must be _{acoustically independent of all} surrounding foundations.

c. Observation and Photography

Observational consideratiotis are of utmost importance and fall into three categories: above the

free surface, at the free surface, and below the free surface. For observation above the free surface, a walk way should be constructed along both sides of the basin. Open central well carriage will allow good viewing from above, while photographic subcarriages, such as developed

by Inui and Tagori in Japan, might someday be employed for overhead photography of wave systems.

Observation at the free surface will be

accomplished by the installation of 2' high windows over the mid 50 feet of the basin length. These windows, flush mounted to the inside of the basin, will be centered at

the free surface and will be spaced as closely together

as is structurally sound. A platform will be constructed below these windows so that the free surface is at a

short male observer's eye level. Several underwater

flood lights will be installed below these windows, so as to illuminate the field of vision.

Far underwater viewing a vertical shaft and a horizontal tunnel to a point beyond the tank

center-line will be constructed at the tank midlength. A series of flush mounted windows or port lights will be placed

along the shaft and tunhel to allow observation, television, and conventional photography, and flood lighting of models.

These windows or ports should be as closely spaced as structural considerations will allow. The ports will accommodate only several persons. However, the building

is to have an educational television system, and it i anticipated that this system may be employed to view the

d. _Environment

The acoustic environment specification will be written as a result of acoustic studies, and will

be very stringent. _{Three other envi1onmenta1 features} must also be considered: air conditioning,, _water

treatment, and lighting.

(1) Air Conditioning

Air conditioning and humidity _control of the towing tank room may prove impractical, however

provisions must be made to insure _{a very constant ±oom}

temperature at some point between 7Q0 and _{O°F. In the} event the constant temperature and humidity which are to be maintained are not suitable for electronics _{and office} spaces, such spaces will be separately _{air conditioned.} The presence of 1-1/4 million gallons of _constant

temperature water will certainly assist in stabilizing conditions. _{Temperaturemustbe maintained high enough}

to allow effective turbulence stimulation, but _{must be}

lOw enough for comfort, for the electronic instrumentation, and for humidity control.

(2) Water Treatnient

The water itself must be very

care-fully considered. Water clarity is essential for high-spéedH

color motion picture and television photography _through

13 feet of water. A good filtering system and _{a surface}

skimmer are therefore required. _{But eén more than clarity,} bacterial cleanliness is essential. _{Bacteria and algae}

excrete substances which severely affect the frictional

drag of ship models. _{Thus organisms in the tank water}

must be strictly controlled through chlorination, the _control

of acidity (pH), and the contrOl Of sunlight _{in the tank}

room. The water must be maintained at the same very

heating and filtration

must be accomplished without

establishing convection currents and/or thermal gradients.

(3)

Illumination

The tank room need not

be brightly

lighted. .Lights suitable

for corridors or passages

should be sufficient, with

loca.l illumination

pr.ovided.

on the large

towing carriage and

in the work areas..

Sunlight (and some types

of artificial

light) promotes the

growth of algae and is thus not, desired.

In way of the

observation areas illumination for photography,

bçth

below and above the

surface will be possible.

e.

Mono-rail Hoist

A mono-rail hoist system

will be installed

to service the

380' towing tank.

A low overhead electric

hoist will run on an

I-beam monorail over,

the centerline

of the basin, the

model storage racks, the'model rigging

area, the

stability/ballaSt1n

tank, and the loading

dock.

f

Pumps' and Pi,ing

A pumping capacity of

1000 gal/mm

will

be required to drain the.water basin, and to shift water

from the drydock to

the water basin. .This

wili,r?qUie

a pump notor

of.abOut 50 'HP, which

is substantial, but

required to drain the dry docki' 15 minutes.

A pump of

this ,size will draii

the basin in 2l.hotrs.

,,,

Pumps .and piping will

also need be

supplied to operate the filtration, surface skimming, an

heating systems.

These.sySterflS should be co

cted tothe

hydrorneChafliCs and

oceanography laboratories, so ,that,

these laboratories

might use treated, filred

and.heated:

water.

Discharge.frOm these 1aborat0.rie5

riust not return

to the large tank;

therefore.all facilities

should have

independent drains to a sewer

or to the river.

Inlet water should be supplied to the

large tank at the maximum rate acceptable to the Naval Academy Public Works _{Department. It is estimated that} an inlet rate of 250,000 gal/day will _{be acceptable,}

filling the tank in five _{days. All inlet water should} pass through the heating, treating, and filtration system, so as to be completely compatible _{with water already in} the tank.

SECTION III

III. _{CONVENTIONAL EQUIP1VNT}

The conventional equipment _{planned for the new tank} will include _{a 15 knot resistance and seakeeping}

carriage,

a wavernaker and beach, model and _{support facilities,and} a rail system.

A. _{Conventional Carriage}

1 _{Configuration}

The conventional carriage (Figure _{4) is} envisioned as being quite similar _{to a number of existing} box-girder central-well _{carriages. Model visibility will} be good, since a box-girder _{offers no Obstructions.}

Thus

the open central well offers advantages as a teaching aid. The carriage will _{accommodate approximately 22}

nidshiprnen,

standing around the central well. The instructor may

stand on the catwalk within the _{well while explaining the} experiment underway. _{One side of the carriage is}

cantilevered away from the well _{to allow construction of}

an air-conditioned cab containing _{instrument racks and} a

desk, leaving the central well area unobstructed. _The carriage will be used both to teach the Naval Engineering curriculum and to demonstrate research which is not

formally curriculum _{connected. The entire facility hs} been designed with this in mind, and it is hoped that

Naval Architecture students _{will be shown all current} research.

The carriage structure must be quite stiff, so as to allow the forced oscillation of models at wave encounter frequencies (0 to 7 Hz) with no resonant

carriage response. This will require a carriage structure

natural frequency of at least 14 Hz. This stiffness should apply to all vibrational modes, but especially

to the vertical vibrations of the cross tank span. If a reinforcing truss is required, model visibility must be

retained from every side of the center well.

2. Speed Control

Controls 'of undetermined type will be

located on the carriage.. In one system considered, the

carriage would be propelled by four synchronous motors,

driven by what is essentially an adjustable frequency

power supply. Such solid state rectifier-inverter 'drives

are now commercially available in the 5 to 500 horsepower range. In these drives, ordinary a.c. current is

rectified by silicon diodes to d.c. current. The inverter then employs silicon controlled rectifiers to invert the d.c'. current back to a.c. current at a predetermined frequency. The output frequency is set by very stable

oscillators which may be digitally controlled for

repeat-ability. The speed regulation of such a system is, from no load to full load, essentially 0%. The most serious

disadvantage of this system is a relatively slow response

time when speed changes are called for, and a limited

range of motor speeds (15:1).

Another promising control system is the

digitally controlled d.c. drive. In this system, control is accomplished through a frequency based feedback loop. Excellent speed regulation may be achieved through the

by Cutler-Hammer, is presently being installed on the

3 H.P. carriage of the existing Naval Academy tank. It will offer a speed regulation of O.Ol%.of.t..speed over a100:l speed range. _{The primary.advantage. of this} system is the rapidspeed change which may be accomplished

through the use of regenerative 'braking and the d.c.

motor's inherently rapid response. The primary disadvantage of the.system is the complexity of control _{required. to.}

achieve the required .speed regulation.

The speed control finally chosen must: achieve a steady speed regulation of .0.01% of set speed

through .a spee.d range, of at least 1 to _{15 knots. . Equally}

important is repeatability. _{If statistical da.ta is}

being taken, either in _{waves or in the boundary .iayer,}

.

runs made at different speeds may not _{e compared} iith-out reduction of the level of confidence.. _{It is,.therefore}

necessary that the speed control be pre-set, and that

the system repeatedly match that pre-.set speed within.the tolerances of the speed regulation. _{Because 'of the}

inherent reproducibility of digital speed _{settings, they}

are to be preferred. _{Repeatable pre-set speeds are also} of very great convenience..in the stereo-photo analysis

of wave trains... . .

Other considerations in the design of a

speed control system are: .compatability with, or use. of. the high performance carriage .drive (eg.. sharing an external drive, or an M.G. set) electromagnet±c inter-, ference with the various .instrumentat-iôn; a tracking

capability for operation with self-propelled models; _data acquisition controls which 'will initiate data collection

upon the. .carriage':attaining steady speed; adjustable _.:

accelerationand braking in the 0.01 g to 0.2 g range

(4 ft/sec2 is an acceptable working acceleration for

steel wheels on steel rails);'and noise and vibratiOn

reduction. The new 3 H.P. system at Annapolis meets these requirements, as' they apply.

Wheels "

The recent world-wide increase in rapid' transit technology is a potential boon to towing tank

designers. An example of this is research being done to reduce wheel noise and vibration. Thè"Acoustic Flex"

railroad wheel, manufactured by the Standard Steel Division

of Baidwin-Lima-Hamilton, employs an aluminum center

bonded to a steel tire with an acoustically inhibiting

material. Such 'a wheel is. lightweight andis said to

substantially reduce iheel induced noise in rapid transit opera1ons; it is therefore under consideration for use.

on the proposed towing carriage. " V

Fast Return System

The faculty at the Naval Academy feels'that

a' system to allow fast return runs of the carriage would

greatly increase the efficiency of testing. . 'Such a

system should allow the carriage to proceed backwards at speeds greater than the speed at which the model should be backed downo ±f at the same time the residual waves in the tank could be "skimmed off," the tank should be ready for the next ,run shortly after the, carriage returns to the starting end. This might be accomplished, for' one-way tests, by a mechanically positioned plank towed beneath the carriage. The system must keep the model dry

during the return run, by either lifting it from the water

or by protecting it in some manner. The system used should put approximately the same amount of energy into

the water on the return _{run as the model put in on the}

ahead run. In this way surface currents will _{tend to}

be canceled. The fast return apparatus should _also

maintain an evenly distributed high level _{of background}

turbulence conducive to turbulence stimulation. For

calm water tests, a turn-table mounted _dynamometer,

allowing precisely l00 of rotation, would make data runs

in both directions possible. _{It might also be'used for} yawed model tests, as with yachts.

B. Seakeeping Capabilities 1. Wavemaker

A wavernaking capability is envisioned for

the new facility for the investigation _{of wave induced ship}

motions and hull structural loadings. _{Regular, long-crested,} waves _{will undoubtedly be the most highly used wave}

configuration. The resulting _{responses will be used in}

conjunction with the principle of linear _{superposition to} predict vessel responses to irregular seas. _{The wavemaker} will be able to generate _{regular waves from 2 to 30 feet} in length, with wave heights of 2.0 _{to 36.0 inches}

(i.e. 0.3< Lw = 3.0, and a maximum wave slope of 0.144

for wave lengths of 21 feet). _{For generation of the very}

high waves, the tank water depth will _{be reduced.}

The large wairemaker will _{allow testing of}

reasonably large models of vessels and _{structures in}

surf conditions. Coupled with the variable profile

beach (paragraph 2, below), this will allow _{the tank to}

conveniently test a variety of. problems _{important in the} design of amphibious and rescue craft, assault ships, and ocean structures. _{Few tanks exist today with this} capability.

In addition to the regular long-crested

wave capability, the ability to generate irregular

long-crested waves, using a preselected irregular. signal on

a magnetic tape, is desired for investigations involving

such variables as slamming frequency, deck wetness,

forefoot emergence, or to simply demonstrate to students the principle of linear superposition, Fourier analysis techniques (generalized and periodic),etc. It is hoped

that real-time spectrum analysis can be made during

preparation of the magnetic tape such that, by filtering and component frequency amplitude control, a desired

spectrum shape and energy can be "sculpted" and presented

to the wavernaker control system. The wavemaker will be

controlled both from ashore, and from the conventional

carriage.

While vessel response to waves coming from directions other than directly ahead or astern are

important and are desirable to investigate, the problems

of wave reflection, refraction, orbital velocity

interaction, and consequently the unknown wave characteristic seen by a ship model as it traverses a long narrow basin

seem at this time to rule out the installation of a "snake-type" wave generator.

It appears at this point that a pneumatic

wavemaker, similar to the NSRDC installation (and the present USNA wavemaker), will be designed and built for the proposed. tank. However,, in the event a hydraulically

actuated plunger proves significantly less expensive than

the pneumatic device, it will be used. The.pneumatic,

although large and complex, will have fewer problems due to inertia, plunger buoyancy, natural frequency, and

mechanical alignment. The dome of such a wavemaker will

be adjusted vertically to accommodate changes in water level. Programming of the butterfly valves will allow

regular and irregular wave generation. It is also

conceivable that nodal wave patterns could be generated

by incorporating contiguous but independent valve-dome units across the tankwidth.

2. Wave-damping Beach

The generation of well formed waves of desired length and amp1itude is only part of the problem

encountered in a seakeeping test facility. In order to

know what waves the model "sees" and to reduce the time

interval between test runs, an effective wave absorbing beach (Figure 2) must be installed at the tank end

opposite the wavemaker. Tank sidewalls must be smooth and vertical to avoid diagonal wave generation. Two

beach types are being considered: a curtain of air bubbles, and a variable profile rigid beach. The bubble curtain

has reportedly been successful in Europe, and is presently

being tried at the Naval Academy.

The rigid beachwil]. be utilized in the simulation of near shore ship motions, amphibious craft

situations and surf structure.problems. Therefore, the

ability to roughly simulate a shelf or reef coastal

geometry is necessary. To accomplish this, a segmented,

adjustable beach is envisioned, constructed of six, or more, eight foot concrete segments, positioned with hoists

and held by chains. The beach must have great

rigidity, to maximize wave energy absorbtion, and a

roughened surface, to break up the waves and cause viscous

energy dissipation. Bubble curtains along the tank wall, in way of the beach, may allow sustained _zero Froude Number surf testing, damping any model generated

transverse waves.

In regular seakeeping work, the beach

could be configured ma

roughly parabolic shape with a low (about 50) slope at the waterline fOr maximum wave energy absorption. It ma.y also be that for some tests a short Hammil-tYPe beach will be required; this could

be rigged using existing beach segments. If the bubble

curtain beach is successful, the rigid beach will not be required for a majority of experiments.

In any case, the beach will be lowered

during cairn water tests to allow model passage into the drydock_outfitting area.

C. Model Handling and Technical Support Instrument Modules

The use of two carriages has inspired a

modular instrumentation concept. All dynamometers,

sub-carriages, and instrumentation packages will be rigged to fit the 11' well on both carriages. The well will have a set of machined ways on their fore and aft sides upon which modules up to 11' in width may rest. It will also

be possibleto run

models 5' off center, in order to allow clear photography and observation. The modules will be rigged in the shop area and transported to the towing tank for mounting and calibration, all while the second carriage is engaged in testing.

Drydock

The proper layout of the drydock (Figure 2)

will be crucial to this system. It must be possible to attach a floating or immersed model to the carriage. Thus,

the drydock must allow the installation of floating models

on the conventional carriage, and the installation of submerged models on the high performance carriage. A drydock with a deep dock adjacent to the tank, and a

shallow finger dock extending behind should be satisfactory. Submarine models must be rigged

_{in thedeep dock}

available to either carriage. _{Surface models could.be} rigged in the finger dock, _{available only to the}

conventional carriage. _{Thus the conventional carriage}

will be placed on the rails aft _{of the high speed}

carriage.

3. Technical Support

The new Engineering Department building is to house not only the proposed hydromechanics laboratory, but aerospace, mechanical, _{marine, materials, nuclear,}

weapons, and systems engineering _{laboratories. What} would normally be considered _{a school of engineering,}

along with its laboratories and _{shops, will all be under}

one roof. _{As such, the new building will contain}

approximately 20,000 ft2 of technical _{support facilities}

including: a machine shop, _{a model and pattern shop, an} electronics shop, a mechanical instrument shop, a

c'alibration and _{standards laboratory, a photography}

laboratory, welding, foundry, _{electrical and paint shops.}

Approximately 35 to 40 men will be _{employed in support of}

the research teaching laboratories, _{including model} makers, a mechanical instrument maker, and electronic technicians. _{The hydromechanics laboratory will employ} several technicians to set up, run, and maintain the

equipment.

D. _{Rail System} 1. Rails

The rail system is a critical component in any towing tank, and must reflect the _{tank's overall}

mission. _{Traditionally, the primary consideration in the} design of rail systems is a.àcurate installation. Accuracy

is required both in maintaining a constant vertical 36

eight above the free surface, and in providing at

Least one rail with a very straight horizontal alignment.

kaii insta11aion tolerances should be between 0.001" nd 0.005", since they represent the reference plane ror ship motio.n measurements. and for straightline motion

ssential to most testing. it is interesting to note

hat alignment errors of as little as 0.001" in the bwo-mile length. of the Stanford .Linear Accelerator h.ave

een measured using gas lasers and special detection equipment, (7)

In recent years more emphasis has been laced on smooth rails, for the elimination of carriage

ibration. Thus, as mentioned above, several recent

tanks have utilized continuous welded and machined rails.

3ince the study of flow noise is a primary mission of he proposed tank, such rails are absolutely essential.

2. Rail Supports

The primary reason that flow noise studies are impossible in existing towing tanks is the presence of carriage generated, noise in the water. To. avoid this,

and to gain other previously mentioned advantages, a

pantilevered rail system has been chosen. This cantilevered

ystem will be structurally independent of the water basin,

thus preventing structure borne carriage noise from 3ntering the water.

The cantilevered rail system (Figure 3) is patterned after the Osaka University tank in Japan and the National Research Council tank in Ottawa. Reinforced concrete cantilevers will extend over the tank, steel

r concrete beams will be supported by the cantilevers and

ill run the length of the tank, supporting the actual ails. The deflection of reinforced concrete beams can