Oblique wave testing at Davidson Laboratory

Note No. 538 July 1959

DAVIDSON LABORATORY

STEVENS INSTITUTE OF TECHNOLOGY HOBOKEN, NEW JERSEY

OBLIQUE WAVE TESTING

AT DAVIDSON LABORATORY

by

Edward V. Lewis and Edward Numata

FOR PRESENTATION AT TWELFTH MEETING OF AMERICAN TOW!NG TANK CONFERENCE

SESSION ON SEAGOING QUALITIES

AT UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA

A prov

n P. Breslin DIRECTOR

SUMMARY

A description is given of the apparatus being used at the

Davidson Laboratory for recording model motions at oblique headings to regular waves. Test techniques are described and problems

en-countered such as those of maintaining a constant course

-- are

discussed.

Some results 'obtained to date on 'a Series 60 model of 0.60 block coefficient are presented and show that a large leeway angle is required at low speeds in order to maintain a desired course oblique

to the waves. This effect appears to be more significant for ship

mo-tions and control than oscillatory yawing, which is of comparatively

small amplitude. It is noted that under conditions where the wave en-.

counter frequency is approximately double the natural rolling frequency, the rolling records sometimes show response to both frequencies.

Finally a comparison of pitching and heaving amplitudes at different wave headings shows a reduction in these motions as the direction; of wave approach changes from dead ahead to 300 off the bow.

Further research underway or planned is discussed, the

objective being to explain the above results and to determine the extent of applicability of the superposition principle to predicting motions in

irregular seas.

It is concluded that the problem here is basically

one of ship control rather than of 'motions per se, and that consequently the use of automatic steering is essential to obtaining steady conditions in model tests..

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--TABLE OF CONTENTS

Introduction L. A *A, 0" *

Motions Apparatus AD. ;in, )10 41,, Ipt 1J, IN) p p A

Test Techniques IP 0' *. A * 4

Test Results ** ',Sig iw*t * 1111 A. A 10,1 5

Research Plans it S. S * rai* A * It1, A 01 ,na r10,1 5 AI *. * 1 7

Concluding Relnarki It 6

References OOOOO .06 44 4iis, 4-;;'im

Figures N-538

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INTRODUCTION

A report in October 1957 by Numata, Spens and Muley]. described the facilities at the Davidson Laboratory for model tests at oblique

head-ings to waves. Descriptions were given of the square tank, the wavemakex extending along one side,, the beach on the opposite side, and the adjustable

bridge spanning the tank to carry the test carriage. (See Fig. 1 and 2) It

was mentioned that the evaluation of regular waves in this tank had shown them to be of good form, reflections from the beach being only 5 to '9% of

the wave height.

In another report by Nurnata2 some model tests for the measure-ment of bending momeasure-ments in oblique waves at the midship section of a T-2

-tanker were described. These tests were sponsored by the S-3 Panel,

SNAME. No attempt was made to measure model motions in these tests. However, a brief description was given of the apparatus then under

con-struction for recording all six components of model motion in waves. On. the basis of the above background, the objects of the present

paper are.:

to describe the apparatus .now being used for recording model motions at oblique headings to waves,

to describe test techniques ,adopted in cu rent ..programs,

to present some interim results which may

be -of interest,

to discuss future plans and research objectives.

The interim results reported here were obtained under a project -Sponsored by the Bureau of Ships under the technical cognizance of the David Taylor Model Basin (Contracts Nonr 263-19 and Nonr 263-26, Davidson. Laboratory Projects JZ 2063 and KS 2130.

References are indicated by superscripts and axe Listed at the end of' the paper.

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1.

1

-1-The total weight of apparatus supported directly by the model is four pounds including the gimbal unit and heave plunger. This weight must be included

when ballasting the model to correct displacement, C. G. , and longitudinal

radius of gyration.

In addition to the motion records, an additional transducer provides a record of rudder angle and a resistance wire gauge ahead of the model

provides a wave elevation record. The signals from six motion transducers

and the rudder angle and wave pickups are transmitted through cables to an eight-channel, direct-writing recorder on shore.

During the acceleration interval at the start of a run, and the de-celeration period at the end, the servocarriages and the yaw axis a.re

locked. This feature prevents the model from getting out of control during

these intervals of transient conditions. Only during the period when the

carriage reaches its constant forward speed are the servomotors activated and the yaw axis freed.

After initial adjustments, the motions apparatus functions

satisfacto-rily and produces excellent records. The following problems and difficulties

should be noted, however:

In the beginning failure of any one of the ten potentiometers or other essential components put the entire apparatus out of action until a complete set of spare parts was provided.

Although manual steering by means of Selsyn controls is reasonably satisfactory in head seas, it is sometimes difficult to control the model in

quartering seas. Therefore, an automatic

steering system is believed to be an essential

development.

The concentration of the apparatus weight along the longitudinal centerline of a model has caused difficulty in obtaining a desired transverse mass moment of inertia.

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3

-2..

TEST RESULTS

Present ship motion studies involve a comprehensive study of the motions of a 5-foot, Series 60 (0.60 block) model under the following

conditions:

Speeds, from 0 to 4.2 fps

6 wave lengths, from 0.5 to 2.0 x model length 5 model headings, from 180° (head seas) to

00 (following seas)

1 model gyradius and metacentric height.

Although complete results of the current tests will be published as

soon as they are completed, certain of the findings may be of interest here

to the researcher. One of the most interesting early results was the

sur-prisingly large leeway angles required to keep the model on its desired course parallel to the bridge across the tank, particularly at low speed in short waves. Some typical mean values obtained for the 5-foot, Series 60 model are as follows (for a heading of 120° and a speed of 1.0 fps):

In all of these cases the oscillatory yawing motion was on the order of only one degree double amplitude.

It was also noted that the mean rudder angle was often opposite to

the leeway angle. For example, with 1. 25L waves on the starboard bow

at a 150° heading, the mean leeway angle at a speed of 2.1 fps

was 0.7°

to starboard, but the mean rudder angle was 4.2° to port. At a 1200

heading a mean heel up to 5.0° was recorded, higher values occurring at

high speeds in short waves. The direction of the heel

was to port, i.e.,

in the general direction of wave travel.

In the planning stages it was thought that a steady condition of

roll-ing might build up slowly. This suggested that an arrangement might be

needed to start the model rolling at the beginning of the run. However, it

has been found that the rolling motion usually reaches a steady state quickly, and therefore this complication now appears unnecessary.

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-5-Leeway Angle Rudder Angle

2 -1/2 -foot wave s 16. 5° 1.40

3.75-foot waves 100 6. 80

RESEARCH PLANS

One of the main objectives of future research is to determine the extent of applicability of superposition theory for motions with six degrees

of freedom in irregular waves. (See Ref. 4) The peculiar behavior of the

Series 60 model in respect to roll suggests that simple superposition of

responses to the component waves may not be satisfactory. One of the

essential requirements for superposition is that there be a characteristic

sinusoidal response to each wave frequency and heading, i.e. , a definite

transfer function. It has been assumed by Marks5 and others that these

could be easily obtained by model tests at oblique headings. However, the present tests show that this is not as simple as it first appeared.

Korvin-Kroukovsky6 has warned also of the difficulties to be

ex-pected because of the non-linear character of rolling, and the even more serious effects which may arise out of couplings between the symmetrical

and unsymmetrical modes of motion. For example, pitching may be

ex-pected to introduce a periodic oscillation in the yawing characteristics and in directional stability. And rolling, in turn, will be influenced by yawing and by gyroscopic components of inertial forces. Tests to date indicate

that pitching is affected by both roll and yaw. For accurate application of

superposition theory, therefore, it may be found that the response amplitude operators for oblique waves are much more complicated functions than for the head sea case.

Finally, it has been established that superposition cannot be applied directly to all modes of motion, as for example surging, even if one can establish from tests in regular waves a unique value of amplitude for each

wave frequency. The trouble in the case of surging seems to be that there

are two components in the fore and aft motion of the model or ship:

I. drift, which can be reduced to zero in regular

waves if the correct towing force is applied, and

2. _{surge, which is oscillatory in the frequency of wave}

encounter.

When two or more waves are superimposed, the towing force required to obtain zero drift is not constant and is not simply the sum of the forces re-quired for towing in each of the component regular wave trains. For the towing force depends on the net wave energy which, in turn, depends on the

square of the resultant wave height. This varies continually with the phase reinforcement and cancellation of the component waves. Hence, the model not only oscillates in surge but drifts gradually forward or aft depending on

the local wave pattern. When the wave components combine to produce a

number of steep waves, the resistance increases and the model drifts aft;

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-7-Investigation of exact or approximate methods of determining yawing in irregular waves by superposition, taking into account the changing

leeway angle.

Investigation of other aspects of superposi-tion theory applied to complex mosuperposi-tions with

six degrees of freedom.

The experimental aspects of the research program include the

following:

The completion of the current project in which the motions of a Series 60 model are recorded at all headings to regular waves,. The remaining beam and quartering sea tests, making use of automatic rudder con-, .trol, will help to complete the general picture

of model behavior at oblique headings.

The conduct of tests of the Series 60 model in both regular and irregular beam seas in order to determine how best to allow for non-linearity in the superposition of rolling responses.

Initial tests will be at zero speed, after which the investigation will consider the effect of

forward speed,.

3, The investigation of model motions at oblique

headings to long-crested irregular waves for comparison with motions predicted by

super-position theory. Tests with the model restrained

in roll will be included in order to determine the influence of roll on the other modes of motion. The exploratory determination, of generalized response amplitude operators covering varia-tion in heading to the waves without change in course. This can be accomplished by apply-. ing known torques and/or side forces to the

model.,

Other research is planned or in progress aimed at a better under-. standing of the behavior of ships or models at oblique headings to regular waves, viz..,

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-CONCLUDING REMARKS

The work on oblique wave testing at the Davidson Laboratory is still in the early stages, and hence the results presented here have been

somewhat fragmentary items of experimental interest. The work to date

permits several general conclusions to be drawn, however.

First, it is

evident that there are many problems to be solved before it will be pos-sible to predict ship motions in all six modes in irregular short-crested

seas. Superposition theory still appears to be the best approach, but

various difficulties must be overcome and refinements introduced.

Another conclusion is that when research on ship behavior is ex-tended to oblique headings to waves, it becomes primarily research on

control rather than on motions. Pitching and heaving in head seas can be considered as problems in motions alone, but tests at oblique headings cannot be made without rudder control and rudder control affects all mo-tions of the model. Maintaining the required leeway angle seems to be

more important than control of oscillatory yawing. Furthermore, most

ships for which motions are a serious problem can be expected in the

future to be fitted with controllable fins for roll stabilization. Fixed fins

at the bow -- and perhaps controllable stern fins -- are also being con-sidered for reducing pitching amplitudes. Hence, the basic problem for research is the improving and coordinating of the various devices and systems for maintaining a steady course at any heading to irregular waves

with minimum deviations of any kind.

The assistance of Mr. Yasufumi Yamanouchi in carrying out most of the actual model tests referred to here, as well as analyzing the re-sults, is gratefully acknowledged.

FSYSWISVS'

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SERIES 60, 0.60 BLOCK MODEL NO, 1445

5.00 FT. MODEL L 120 COURSE ANGLE

0.75L X L/48 WAVES 2.41 FT./SEC.

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D131.... AMP, iNCHES DBL. AMPL. Dee: EFFECTIVE 1.25 L WAVES HEIGHT = L/48 HEAVE PITCH BOW SEAS 1500 COURSE (ACTUAL LENGTH. 1.0831..)

ENCOUNTER FREQUENCY, RAD./SEC.

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ENCOUNTER FREQUENCY, RAD./SEC.

HEAD SEAS 160° COURSE _{HEAD SEAS}

190° COURSE E, EFFECTIVE 1. 00 L WAVES HEIGHT = L/48 HEAVE FIGURE 6

COMPARISON OF MOTIONS IN EQUIVALENT WAVES

SERIES 60,

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