Tests of a model at low speeds in regular head seas

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ARCHEF

STEVENS INSTITUTE OF TECHNOLOGY

EXPERIMENTAL TOWING TANK

HOBOKEN. NEW JERSEY

TES OF A MODEL AT LOW SPEEDS

IN REGUlAR }AD SEAS

by

E. Numata

November 1957

ETT Note No.

)48

Lab. y.

Schek

Technische Fesc

Dellt

(2)

'ESTS OF A MODEL AT LOW SPEEDS

IN IEGULAR AD SEAS

Introduction

Numerous

experimenters, Gerritsma (1)

most

recently, have observed and commented on the erratic fluctuation of motion amplitudes of a model

as it proceeds at low speed into relatively short regular waves. The

erratic behavior is due to waves generated by the oscillating model which

travel

directly ahead to distort the oncoming waves, and

outward to reflect back to the

model from

the tank sides.

It can be argued that the first effect represente

a realistic condiüon

as long as the

model run is stopped before the model-generated waves are

reflected from the wavemaker back to

the

model. However, side wall

re-flection is obviously unrealistic and leads to a distorted prediction of

ship behavior at low speeds. At higher speeds side reflection is

m-important because the model outruns the reflected waves, even in a narrow

tank. While

an

unreasonably large basin would be needed to eliminate the

undesirable effects of reflection, model tests in a wider than

normal

tank

should be subject to less severe effects than tests in a conventional

narrow tank, and should permit valid testing at low speeds.

Accordingly, a -f t. model of the Series 60, 0.60 block hull was

tested in regular

head

waves in the recently conleted seakeeping

instal-lation in the 7-f t. square Tank No. 2. Pitching and heaving amplitudes

were recorded at speeds in the regions of synchronism with waves O.7S and

1.00 times the model length. Comparisons were made with resulte obtained

using the saine model under similar wave and speed conditions in the 100-ft.

by 9-ft. Tank No. L

This work was performed under Bureau of Ships Contract N6onr-2L70,

technically administered by the David Taylor Model Basin.

Model and Test Apparatus

The Series 60, 0.60 block s-ft. wood model (ETT No.

llii5)

was ballasted

to a gyradius of O. 2L at the designed draft. The gyradius was determined

by a compound pendulum method. The model was self-propelled by a 2L.-volt

(3)

rheostat control on shore. No rudder was fitted

Since the tests in Tank No. i with the same

model had been conducted

with the motions apparatus described

by Lewis and Numata (2), the same

apparatus was used in the present tests.

Signals from the pitch, heave,

surge and wave height pickups were amplified and recorded on a Sanborn

oscillograph unit.

The wave height pickup was mounted on the

carriage

ahead of trie model.

The Tank No. 2 bridge was positioned as shown in

Fig. 1.

The

distance between the model and the nearer

tank side was 20 ft.

Test Program and Procedure

The O. 7L and l.00L wave lengths were chosen

because the peak pitching

and heaving arlitudes for these lengths occur at the

relatively low speeds

where model-generated waves are most troublesome.

A uniform height of

i/18

the model length was used, duplicating the conditions

of the Tank No0 i

tes ts.

Earlier exploratory tests of the Tank No. 2 wave generator,

reported

by Spens (3), showed that at these two wave

lengths there was a cyclic

variation of wave height with position in the tank

amounting to - 10% of

the mean height.

In the current tests the fluctuating amplitude was

re-corded by traversing the tank with a slowly moving wave height pickup.

The resulting wave record, (a) of Fig. 2, was averaged and

both a static

and a dynamic calibration applied to obtain a mean wave height.

The

dyna-mic calibration was obtained by taking a tape record with the wave height

pickup at a fixed point, together with a simultaneous visual height

obser-vation abreast of the same point with a hook gage.

This procedure of

determining a mean wave height is illustrated in Fig. 2.

Closely spaced model speeds were chosen starting at 03

ft/sec.

(Fraude no.

0.L2), the lowest carriage speed available, and extending

past the speeds for peak amplitudes;

zero speed rims were included.

Test Results

Model motions amplitudes were deternined from tape records, using a

model travel distance of about 20 ft0 as the interval for averaging

pur-poses.

Figures

3

and

1

show double amplitudes of pitch and heave on a

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-2-base of model speed. The corresponding ilts obtained from tests in Tank Lo. i (Ref. 2) are included in these figures.

Discussion of Results

Brard (1f) has developed the following relation to detennine the

critical model speed below which model-generated waves will move ahead

of a model:

2iiv

1

gTe

=E

where y is the model speed, ft/sec.

Te is period of encounter with waves, sec.

The critical speeds for the two wave lengths used in this investigation

are indicated on Figures 3 and

1.

Near or below these speeds,

model-generated waves may be reflected from the tank sides and affect the motions

of the model.

In 0.7L waves the model in the narrow Tank No. i yielded an odd. trend

of pitch and heave amplitudes at speeds near or below the Brard critical

speed, as shown in Fig.

3.

By contrast corresponding results in Tank No. 2,

given on the same figure, show a smoother and more reasonable trend. At higher sDeeds the Tank No. i and Tank No. 2 results show excellent agreement.

At model speeds, rar or below the Brard critical speed of 1.O ft/sec.

ir. 1. DOL waves the differences between the Tank No0 i and Tank No. 2 results

are as shovm in FIg. Lj. Single data points at 1.2 ft/sec. for pitch and

0.6 ft/sec. for heave appeared to distort the Tank No. I results. Above

2 ft/sec. the Tank No. 2 heave amplitude results are higher but Abkowitz

() pointed out th.3t the Tank No. 1 heave amplitudes were definitely low in this region compared with results from the tanks at M.I.T., D. T.M.B.,

and U. of California.

The differences between the Tank No0 2 and Tank No. 1 results at low speeds are apparently due to reduced wave reflection effects in the wider

tank. The distance between the model and the nearer side wall in Tank No. 2

is about four and a half times the corresponding distance in Tank No. 1. Consequently in Tank No. 2 model-generated waves must travel four and a half times as far as in Tank No. i before being reflected back to the mode1

N-U,8

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-3-This means that in the wider tank a larger proportion of the reflected waves

will fall astern of the model and not affect its motions at low speeds.

Although the model was self-propelled in Tank No. 2 instead of being

towed as in Tank No. 1, it is believed that this difference in conditions

has little if any effect on the motions comparison.

A comparison between

motion aaplithdes obtained with the sar

model self-propelled and then

towed in Tank No. 1 (unpublished) showed negligible differences between

the two sets of results.

This finding was confirmed in similar tests at

the David Taylor Model Basin as reported by Szebehely (6).

REFEPEN CES

L Gerritsma, J., "Seaworthiness Tests with Three Geometrically Similar

Ship Models," Proceedings of Symposium on the Behavior of Ships in a

Seaway, Wageningen, Holland, Sept. 1957.

Lewis, E.V. and Nuinata, E., "Ship Model Tests in Regular and Irregular

Seas," E.T.T. Report 567.

Spens, P. G., "Wave Measurements in Tank No. 2," E.T.T0Not.e No. iL5.

14.

Brard, R., "Introduction a l'Etude Theorique du Tangage en Marche,"

Association Technique Maritime et Aeronautique, 19148.

Abkowitz, M. A., "Correlation of Model Teste in Waves -- Report of

Panel," Transactions, Eleventh General Meeting, American Towing Tank

Conference, Washington, D. C., Sept.

196 Szebehely, V. G., Stefun, G. arid Bledsoe, M. D.,, "Scale Effects in

Seaworthiness," Transactions, Eleventh General Meeting, American

Towing Tank Conference, Washington, D.C., Sept. 1956.

N-14148

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Diagraimnatic Plan View of Tank No. 2 Set-up for

Regular Head Wave Tests

IL

t

-

2O'-O

T

Bridge

o

Stabilizing

=L

I

Rotating A

I

dave

Absorr

T

I

T

(7)

M

2S ft. Travel

Zero

Wave Height Record with Slowly Moving Wavewire Between

Points M

d N

in Tank No. 2.

t

b1 b2

(mean)

Wavewire Record Hook Gage

Dynamic Calibration by Simultaneous Measurements at

a fixed Point with Wavewire

_and

Hook Gage.

1.0" Crest

1.0' Trough

(e) Static Calibration of Wavewire

¡

Mean Wave Height

(a) (b2)

Between M and N

(b1J

N -LLL8

Fig. 2

where (a) and (b,)

are

each multiplied y

the

static

calibration

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EXPERIMENTAL TOWING TANK

STEVENS NSTTTI.JTE OF TECHNOLOGY

HOBOKEN. NEW JERSEY

N-li8

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