DAVIDSON LABORATORY STEVENS INSTITUTE OF TECHNOLOGY HOBOKEN, NEW JERSEY OBLIQUE WAVE TESTING AT DAVIDSON LABORATORY. Edward V. Lewis.

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1 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 Note No. 538 July 1959 A prov n P. Breslin DIRECTOR

2 N-53, 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 encountered -- 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 motions 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..

3 N-538 ii TABLE OF CONTENTS Introduction L.. 1 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, Research Plans it S. S * rai* A * It 1, A 01,na r10,1 5 AI *. * 1 7 Concluding Relnarki It 6 References OOOOO i is, 4-;;'im Figures 11 12

4 N INTRODUCTION A report in October 1957 by Numata, Spens and Muley]. described the facilities at the Davidson Laboratory for model tests at oblique headings 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 measurement of bending moments 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 construction for recording all six components of model motion in waves. On. the basis of the above background, the objects of the present paper are.: 1. 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 and Nonr , Davidson. Laboratory Projects JZ 2063 and KS References are indicated by superscripts and axe Listed at the end of' the paper.

5 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 deceleration 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 satisfactorily 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. N

6 N 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 surprisingly 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): Leeway Angle Rudder Angle 2-1/2 -foot wave s foot waves foot waves ,40 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 rolling 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.

7 N 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 expected 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 expected 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 required 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;

8 ,N.538 9, - 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 superposition theory applied to complex motions 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 superposition 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 variation 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..,

9 N-538 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 possible 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 extended 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 motions 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 considered 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 results, is gratefully acknowledged.

10 BRIDGE FSYSWISVS' AA/it/it/AAA Iv Ii CARRIAGE RAIL PLASTIC FOAM PLUNGER BALANCING CAM-- CRANK WOOD SLATS PARABOL/C BEACH PNEUMATIC TIRE CONNECTING ROD /* e.. V D a, I>, I ADJUSTABLE SPEED MOTOR ADJUSTABLE ECCENTRIC 10 BALANCING SPRING FIGURE SEAKEEPING INSTALLATION TANK NO2 0 SCALE OF FEET, I

11 N-538 II 0 f BRIDGE WHEEL PATH / / / 0 ""\,b0 P/ I/7 BRIDGE 180 STABILIZING -4. ARM ROTATING ARM WAVE ABSORBER FIGURE 2 PLAN OF DIAGRAMMATIC SEAKEEPING INSTALLATION TANK NO. 2 SHOWING BRIDGE POSITIONS

12 N-538 t_ongitudinal SUB- cagstiage TRACK (FIXED TO MAIN CARR/AGE) FI.4N PARALLELOGRAM LI NKAGE RIGID CAGE HEAVE PLUNGER,... t ROTATIONAL RESTRAINT FOR HEAVE PLUNGER HEAVE BEARING PITCH BEARING v- -> YAW BEARING..-N /1.- ROLL BEARING ATTACHMENT TO MODEL FIGURE 3 SCHEMATIC DIAGRAM OF APPARATUS FOR MEASURING MOTIONS OF A MODEL WITH SIX DEGREES OF FREEDOM

13 MME wie111111r1; FIGURE 4 MODEL TEST AT 150 COURSE ANGLE

14 UP as HEAVE, IN. DOWN 05 DOWN 4- PITCH,DEG UP 4- PORT 4 ROLL, DEG., STBD 4 STBD 4 YAW, DEG. PORT 4 - III IIIIIIIIMMONNAMMOVIAMAIMMMO arm Erma im ii taismilvis I pot V I a 1 A lgidiiiii IIV 1.1, mill I d 11.11!il.,,. I I A m4101 -i 7.-1 IIIIIIIII disnadiraigil; II; N-538 PORT 20- RUDDER, DEG. STBD noiREIMMII =111 Im STEM 10- SWAY, IN. PORT MI ---..u.mumminiminiiimininiiimii FWD 20 SURGE, IN, AFT 20 11/11/11 mmimminimmumeminnalli WAVE CREST TROUGH )1*I SEC..4 TIME., SEC. FIGURE 5 SAMPLE RECORD SERIES 60, 0.60 BLOCK MODEL NO, FT. MODEL L 120 COURSE ANGLE 0.75L X L/48 WAVES 2.41 FT./SEC.

15 8 4 D AMP, inches DBL. AMPL. Dee: EFFECTIVE 1.25 L WAVES HEIGHT = L/48 HEAVE HEAD SEAS 160 COURSE EFFECTIVE L WAVES HEIGHT = L/48 HEAVE DBL. AMPL. INCHES HEAD SEAS 180 COURSE BOW SEAS 1500 COURSE (ACTUAL LENGTH ) 9 10 BOW SEA'S.-- * COURSE (ACTUAL LENGTH.0.8Ek,L) 5 10 ENCOUNTER FREQUENCY, RAD./SEC. ENCOUNTER FREQUENCY, RAD./ SEC. PITCH PITCH HEAD SEAS 190 COURSE E, 6 DBL. AMPL. DEG. HEAD SEAS 180 COURSE BOW SEAS 150 COURSE BOW SEAS 150 COURSE a ENCOUNTER FREQUENCY, RAD./SEC. FIGURE 6 COMPARISON OF MOTIONS IN EQUIVALENT WAVES SERIES 60, 0.60 BLOCK MODEL ENCOUNTER FREQUENCY, RAM/SEC. 1