Lewis 



I would suggest that for the merchant ship the loss in speed to be expected 

 is of prime importance; other factors such as accelerations, bow wetness and 

 slamming, are others that will influence the captain in reducing power and hence 

 producing a further reduction in ships speed. 



It is not enough however to consider such criteria for single sea states. 

 Predictions must be made on a long term basis, for after all, the more severe 

 sea states occur at low probability and the consequences of high seas may be 

 negligible in relation to the all year round operation of the ship. 



In conclusion I think the method of analysis and presentation in terms of 

 log &j used by the author is useful for visualizing what is happening to the re- 

 sponses of the ship as various parameters are varied. 



DISCUSSION 



H. Lackenby 



British Ship Research Association 



London, England 



The subject of Professor Lewis' paper is of particular interest to me, 

 namely the application of results of seakeeping research. A considerable 

 amount of work has been carried out on this subject over the past few years, but 

 it has not always been very clear as to the design applications in many instances. 

 A contributory factor in this has doubtless been the apparent complexity of the 

 subject. Against this bacl^round Professor Lewis' paper is very timely, and I 

 would just like to raise a point of principle which was touched on this morning. 



As I understand it, the essence of the theory and analysis is based on the 

 principle of superposition and the principle of linearity, that is, relatively small 

 angular displacements, wall-sidedness of the model or ship within the range of 

 the motions and so on. From the practical point of view, however, I think it is 

 the larger angles and the question of whether or not water breaks over the decks 

 which are the more important. This state of affairs appears to be well outside 

 the linear range, but from the discussion this morning it seems that the princi- 

 ple of linearity applies beyond the range that one would expect. The instances 

 quoted however have referred particularly to model tests on a destroyer form 

 and I should be glad if Professor Lewis would care to comment on this aspect, 

 more particularly as far as the fuller merchant ship is concerned. In other 

 words, to what extent can we use — or perhaps one should say abuse — the linear 

 principle and get away with it for practical design purposes ? 



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