Understanding and Prediction of Ship Motions 



RESPONSE TO COMMENTS BY 

 WILLARD J. PIERSON, JR. 



T. Francis Ogilvie 



David Taylor Model Basin 



Washington, D.C. 



I appreciate Professor Pierson's comments very much. Since he was co- 

 author of one of the most important papers ever written on the subject of ship 

 motions, any worker in our field should listen carefully when he enters the 

 discussion. 



It is rather difficult to reply to his formal discussion, since his comments 

 generally refer to what I did not say. My paper was too long as it was, and so 

 a large amount of oceanographic data and statistical theory were omitted. In 

 fact, Figs. 2-4 of my report, which I took from Dalzell's work, were modified to 

 the extent that I cut out Dalzell's reported results on coherencies, since I wished 

 to avoid detailed arguments about such matters. Perhaps this was wrong. Nolo 

 contendere . 



Furthermore, I have been very close to this whole subject for several 

 years, and I have come to accept certain statements as being so obvious that 

 one need no longer state them. For example, I would have been quite surprised 

 if anyone were to suggest that the coherency between roll and wave height in 

 head seas were not extremely small. However, if Professor Pierson considers 

 that such facts should still be restated in 1964, I may have again committed a 

 sin of omission. 



The comments in the section, "Coherency and Resolvability of Spectral and 

 Cross Spectral Shapes," do not seem to be relevant to my paper and so I shall 

 not offer any response to them. 



Professor Pierson's comments on nonlinear problems are relevant and I 

 welcome them. It is very encouraging and stimulating to observe recent prog- 

 ress in the probabilistic treatment of nonlinear physical problems. It appears 

 that the oceanographers and statisticians have in fact stepped far out ahead of 

 the hydrodynamicists. 



Unfortunately, there is much more to the prediction of ship motions than 

 the establishment of statistical laws. Eventually, we would hope to be able to 

 start with geometric and dynamic descriptions of the ship, add to this an ade- 

 quate description of the seaway, and then predict any desired motions-related 

 quantity. Professor Pierson's comments almost imply that all of this can now 

 be done, because the oceanographers have supplied the tools. Actually, we can- 

 not make very good predictions of heave and pitch in long-crested head seas, 

 where the simplest concepts are most nearly valid. We are still lacking in 

 basic methods for treating the hydrodynamics of such problems, and under such 

 circumstances the statisticians' impressive accomplishments are of limited 



127 



