SHIP MOTIONS 



181 



flosely to typical sea waves in Nature. Thesi' arc 

 invariably irregular and short-crested. The hrst re- 

 quirement is to have available quantitative data on 

 natural seas; it has been .shown in Chajiter 1 that such 

 data are meager and uncertain. The directional wave 

 spectra are required.''' There is evidently no such thing 

 as a single typical sea. A towing tank must possess "a 

 library" of typical seas. A convention as to the charac- 

 teristics of these lalioratory "seas" must be established 

 to make possible a comparison of test results of different 

 laboratories. It appc'ars that such data on seas as are 

 available at present have not been given sufficient atten- 

 tion in the construction of towing tanks. Several tanks 

 have provided facilities for generation of waves in two 

 nuitually perpenilicular directions, but have not con- 

 sidered such directional wa\e spectra as have been 

 measured at sea to date. 



Short-crestedness and irregularity of long-crested 

 waves are both ]5articularly important in determining 

 a ship's rolling and yawing characteristics. It is well 

 known that due to low damping in roll the ship's re- 

 sponse has a sharp peak. In other words a ship acts as 

 a narrow-pass filter in regard to its response to excita- 

 tions in roll. In an irregular sea a ship resi^onds pri- 

 marily to the excitation near its natural frcquenc.v, 

 and rolling has a nari-ow normal period distribution 

 around the natural period of a ship. This is u.sually 

 C}uite different from the period of pitch. In model 

 tests made in regular ol)lique wa^■es, an artificial con- 

 dition is imposed on the model: i.e., it is forced to roll 

 and pitch regularly with the same period. Because of 

 the strong coupling influ(>nces on rolling of other modes 

 of motion, there is apparently no practical way of 

 estimating a ship's behavior in irregular short-crested 

 seas from tests made in long-crested regular ones. 

 Certain theoretical methods, to be described in the next 

 .section, are too laborious for practical u.se and do not 

 give sufficiently comijlete information. 



In connection with the foregoing remarks, the ob- 

 jectives of work in obhque seas should be stated. There 

 is little doubt that irregular head seas pi-oduce the most 

 critical conditions for .ship behavior connected with 

 pitching and heaving; i.e., vertical accelerations, ship- 

 ping water, slamming and \'ertical bending moments. 

 These can be determined generally in conventional nar- 

 row tanks. Wide tanks will provide more reliable data 

 for below-synchronism speeds, but need no further dis- 

 cussion in this connection. It appears to the author 

 that the research in oblique wa\'es must be oriented 

 primarily to an investigation of the rolling and steering 

 characteristics of ships, and, connected with it, lateral 

 bending moments. These motions are strongly in- 

 fluenced by both the cross couplings and the short- 

 crestedness of the sea. Cross couplings define the 

 dangerous pitch-yaw-roll motion in near-broaching condi- 

 tion, and short-crestedness, probably in conjunction with 

 the yaw-roll coupling, defines a ship's rolling in bow seas. 



4.42 Tests connected with theory development and 

 application. The dcxclopment of a theory for multi- 

 mode ship motions is necessary : 



(a) To permit reliable interpretation of test data. 



(6) To permit generalization of the test data to 

 ship forms and sea conditions other than those covered 

 l)y laboratory tests. 



A comparison of two ship models in identii^al wave con- 

 ditions is one of the most useful types of practical tests. 

 Any action taken to improve the inferior model is, how- 

 ever, based on intuition. Equations (12) show that a 

 .ship's behavior is governed by 108 derivatives, plus a 

 complicated mass distribution effect, plus the exciting 

 forces. Even if it were assumed that only a few of tliese 

 discrete characteristics are significant, the effects of the 

 significant ones are still too numerous for the human mind 

 to trace empirically. The judgment of a naval architect 

 would be facilitated and made more reliable were a sam- 

 ple of theoretically computed motion available as a 

 guide. Provision of such a sample generally re((uires 

 the theoretical ('alculations and their \'eri(ication by a 

 towing tank test. Test data on a typical ship-" in long- 

 crested obliciue seas would be useful here. Tests should 

 be made in regular and irregular seas. Additional tests 

 in short-crested irregular seas are ilesirabic in order 

 to bring out the different effects of these three wave 

 conditions. By using a linear theory, Rydill (1959) 

 indicated that two most critical conditions as far as the 

 rudder control is concerned occur with waves proi)agating 

 towards a ship at GO deg from the bow antl 45 deg from 

 the stern. The author suspects that a much smaller 

 angle from the stern may prove to be most critical if the 

 bow submersion were taken into account. The set of 

 tests, just outlined, will provide most useful experimental 

 material to stimulate theoretical activity and to verify 

 its conclusions. In this particular test series, it is not 

 important how typical of an actual sea the wave con- 

 ditions are; the primary requirement is that towing- 

 tank wave characteristics be completely and correctly 

 reported. 



Evaluation of the second, quasi-rational, theoretical 

 approach suggested in Section 2.33(a) also appears to be 

 desirable. This will reciuire an additional series of tests 

 in which a model will be restrained from rolling but left 

 free in all other respects. Restraint in surging does not 

 appear to be possible without interference with other 

 motion modes. While rolling motion is restricted, the 

 rolling moments can be measured. This will provide 

 data for the design of stabilization devices and also will 

 provide the verification of a theoretical evaluation of 

 rolling moments with all coupling effects taken into ac- 

 count in the second project of Section 2.33(o). 



(o) Measuranentfs of hijdrodynamic farces mid mo- 

 ments. The exciting forces and moments exerted by 

 waves, as well as the derivatives in equations (12), (13) 

 and (14), can in principle be evaluated by a series of 



" Various methods of mpasiiring directional spectra of sea waves 

 were di.scussed in Chapter 1, and these are also applieable to tow- 

 ing tanks. The author eonsiders Harlier's method to he the most 

 practical. 



™ A .series of tiO, 0.(10 l)l(ick coefficient model has recently been 

 investigated witli regard to heaving and pit<-hing in head seas. 

 It would be desiralile to continue with the invi'sligatiori of its 

 I)roperties in obH(|ue waves. 



