Evaluation of Motions of Marine Craft in Irregular Seas 



mass by means of proper geometric phase shift. The total excitation force in- 

 duced by the wave was then obtained by computing the kinematic acceleration, 

 velocity and displacement of the fluid particle at the mass center of the body 

 and then multiplying by suitable coefficients determined post-priori. Is it cor- 

 rect for us to assume that the crux of the whole approach lies in the adjustment 

 of the coefficients a', b', c' and A', B', C appearing in the forcing functions 

 specified so as to be compatible with the measured response data ? 



It is observed that the wave height information or fluid particle displace- 

 ment is strictly a point function at a given instant of time as measured by the 

 wave probe. Its subsequent introduction into the forcing functions specified by 

 Eqs. (11) and (12) leaves one with some apprehension as to how the total flow 

 field and hence total force over the entire body is generated. What is needed is 

 some spatial integration of the total field effects of each individual particle ac- 

 tion over the entire body surface. It is only for very long-crested waves in 

 comparison to body length or beam seas encounter that the approximation will 

 meet with success. 



The third issue is perhaps more philosophical in nature. While the surge 

 degree-of-freedom in the system equations has generally been neglected in 

 practice, there are evidence that indicate that such a neglect may not be justi- 

 fied. This happens especially in certain following- seas condition where the 

 craft may be running in a "surfboarding" condition. In severe instances, this 

 can lead to broaching. With the modern computers available today, computa- 

 tional drudgery should pose no problem. Has there been any attempts to try a 

 similar three degree-of-freedom (with and without coupling) response problem 

 to justify the neglect of surge or to define the bounds for its neglect because of 

 second-order effects ? 



Finally, I would like to state a dream for the future to come in the ship mo- 

 tion studies. While the major brunt of the work so far has been concentrated in 

 the realm of analysis, i.e., to predict the motion given the ship and the seaway, 

 the naval architect in the design office is still looking for a rational approach to 

 design his ships for seaworthiness and minimum motion. It is opportune for 

 someone to undertake the problem of synthesis and the ultimate optimization 

 problem. This undertaking implies an understanding of pole-zero synthesis, 

 defining a meaningful criterion for optimization, plus an identification of the 

 parameters of ship form, loading, hydrodynamic characterization, and seastate 

 environment so as to tie in the relation to the poles and zeroes of the transfer 

 function. Such poles and zeroes, if properly characterized, serve to define the 

 behavior of the ship. At this stage, it is not important to get unnecessarily in- 

 volved into fine details and accuracy but to look at the broad approach and to 

 arrive at definitive concepts and trends. Such an approach is still awaiting as 

 the eventual goal of ship motion studies to tie in with a rational design practice. 

 This is useful even if only to improve our present go, no-go methods. 



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