Applying Results of Seakeeping Research 



Third and final, we must attempt to devise an approach which will discrimi- 

 nate between a family of ships all meeting the owner's requirements, and will 

 choose the one that exhibits the best capacity for sustaining a preassigned speed 

 in rough water. 



REPLY TO THE DISCUSSION 



E. V. Lewis 



Webb Institute of Naval Architecture 



Glen Cove, Long Island, New York 



Mr. Lackenby's comments are appreciated. In reply to his question, I would 

 expect to find linearity apply to merchant ships as well as to destroyers. He is 

 quite correct in pointing out that the larger angles are most important, when 

 water is shipped over the bow or slamming occurs. However, since we are in- 

 terested mainly in identifying when these non-linear events occur, rather than to 

 determine how deep the bow is immersed or how far out of the water it emerges, 

 we do not need to push the assumption of linearity too far. 



Mr. Goodrich suggests that uni-directional sea spectra are adequate for the 

 present. However, we have found in our calculations at Webb that short- 

 crestedness has a significant effect, and therefore even an approximate allow- 

 ance for it is better than none. Mr. Goodrich is quite right in pointing out that 

 to draw significant conclusions one must take into effect the combined effect of 

 different sea states based on their probabilities, and he has illustrated this fur- 

 ther step very clearly in his own paper before this Symposium. 



Professor Aertssen has called attention to particular features of the paper 

 and indicated their possible implications for ship design. His comments based 

 on his own wide experience in making measurements on ships at sea is greatly 

 appreciated. As for the speed of 12 knots for the 600 ft ship, this was simply 

 the lowest speed for which model test data were available, and I doubt very much 

 that it would be maintained in an extremely rough sea. 



Mr. Swaan has made a number of good points, and I concur with all of them. 

 It is certainly true that different numbers of cycles should be used for large and 

 small ships when comparing them over the same long period of time. However, 

 the difference in predicted stress will not be great. Although most ship motion 

 spectra seem to be narrow enough to make predictions of the highest expected 

 value from a Rayleigh distribution reasonable, working directly with the Gauss- 

 ian distribution of points in the record is a useful and simple approach. 



I feel that Mr. Vassilopoulos has given a little too much emphasis to follow- 

 ing strictly the mathematics of linear systems theory without recognizing the 

 peculiarities of the ship-wave problem. In particular, we must recognize that 



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