SHIP MOTIONS 



213 



behavior on the basis of both visual interpretations and 

 spectral charts. Once the spectral charts are completed, 

 it is no longer necessary to retain the original recording 

 tapes and they can be used over and o\-er again. Most 

 of the results of the voyage then will be available im- 

 mediately upon return to the home port. 



Experience has shown that a large bulk of the recorded 

 data is of little direct interest. Continuous recordings 

 of an hour's duration are necessary, howe\'er, in order to 

 derive the spectrum. Only relatively short portions of 

 some of the records must be kept and transcribed into 

 motion curves for visual examination. These may in- 

 clude, for instance, some large motions during which 

 water is shipped or shunming occurs. 



7.3 Summary. Ship motions were considered in 

 Chapter 3 from three points of view : 



1 Theory of motions. 



2 Model testing. 



3 Observations at sea. 



Each of the subjects was considered in detail and sug- 

 gestions for further research activity were made. 



It was shown that, while theoretical research on heav- 

 ing-pitching motions has been progressing satisfactorily, 

 the treatment of rolling and yawing is in a deplorable 

 state. The linear theory of ship heaving and pitching 

 has reached a state of practical usefulness. Its further 

 improvement will consist of more reliable evaluations 

 of coefficients of the differential equations of motions. 

 This formed the subject of Chapter 2. 



In model tests the emphasis has been placed on ex- 

 periments in oblique irregular waves. Tests in oblique 

 regular waves are belie\'ed to lie misleading in predicting 

 roUing and yawing motions. Attentiou has been calleil to 

 the apparent lack of planning and definition of methods 

 and objecti\'es in model testing in oblique irregular wa\'es. 

 The basic theory of such testing also needs further de- 

 velopment. The theory of superposition has been proved 

 true (within practical limits) for heaving-pitching mo- 

 tions in which the simple cause-effect relationship is 

 evident. Its application is not clear when several 

 causes (due to se\'eral modes of motion) exist, and when 

 a ship's response depends on dynamic (involving prod- 

 ucts of velocities) as well as hydrodynamic character- 

 istics. 



The introduction of irregular seas and oblique waves 

 into towing tanks leads to greatly increased reliance on 

 theoretical considerations. Steps will have to be taken 

 to provide the necessary training for personnel. 



The object of observations on ships at sea are : 



(a) To indicate the conditions occurring in service. 

 These will be shown by the spectra of sea and ship mo- 

 tions. 



(b) To derive the hydrodynamic properties of ships. 

 These will be obtained from cross-spectral analyses of 

 waves and ship motions." 



(c) To obtain detailed information on ship and sea 

 motions at certain short intervals of time; for instance, 

 at the time of slamming or of maximum bending mo- 

 ment. The principal use of this intV)rmation will be in 

 structural analysis. 



(d) To collect information on sea conditions. 



It is recommended that in observations on ships all 

 six components of ship motion and also rudder motions 

 be recorded. (Jb.servations must take cognizance of the 

 properties of irregular seas and ship motions. A ship- 

 borne wave recorder must be used, and it is recommended 

 that recordings be of an hour's duration. A minimum 

 number of maneuvers and test runs should 1k' specified 

 to permit long observation runs. 



Effort should be made to use one type of pick-up, 

 amplifier, and recorder. All pick-ups should be in dup- 

 licate, and circuits should be so arranged that spare 

 amplifiers and recorders can be switched on instantly. 



It is presvmied that magnetic-tape recordings will be 

 used. Portable analyzers, based on the principle of 

 frequency filtering, should be provided aboard instru- 

 mented observation ships. The analysis should be 

 made immediately after recording. Observers should 

 make visual appraisals of sea and ship conditions aided 

 by analyzed data which will be presented in the form of 

 spectra. Only records of certain specific interesting 

 events need be preserved (jn the original tapes so that 

 they can be transcribed into curves of motions. 



Only ship motions have been discussed in this chapter. 

 The effect of motions on speed and powering will be the 

 sul)ject of Chapter 4. The effect of .sea and ship motions 

 on stresses will be discussed in Chapter 5. 



8 Condensed List of Suggested Researcli Topics'' 



1 A Step-by-Step Integration is suggested of the 

 coupled heave and pitch differential ecjuations of motions. 

 Section 2, with variable coefficients. The coefficients 

 are to be e\'uluated using the actual sectional wetted 

 beams and drafts at a given instant. This project is 

 needed in order to evaluate the error in\-olved in the 

 usual assumption of constant coefficients. A small error 

 is expected in the ca.se of normal cargo ship forms. This 

 error is expected to increa.se with increasetl inclination of 

 ship sides, and, as it now appears, invalidates the appli- 

 cation of theory of small motions to sailing yacht forms. 

 The error is also expected to be large in shallow-draft 

 craft, Section 2.13. 



2 Step-by-Step Integration, as outlined in the fore- 

 going, is suggested with the particular objective of 

 evaluating the effect of nonlinear damping described. 

 Section 2-3.22. 



3 Summary of Data should lie prepared for use in 

 evaluating ship motions liy means of eciuations (2). The 

 collection of reference data and the computation of 



'' This will be possililo onl>' under certain favorable conditions. 

 It does not appear that it will be possible to derive hydrodynamic 

 characteristics in the general case of a shi]) moving in short- 

 crested irregular seas. 



■'* Apjiroximately in the order of presentation in the l)ody of the 

 Monograph. These suggestions are confined to ship niolions. 

 Suggestions for the evaluation of the coefficients of the equations 

 of motion are given at the end of Chapter 2. 



