I 



Understanding and Prediction of Ship Motions 



It must be appreciated that Fig. 4 represents an extraordinarily severe con- 

 dition. In fact, one must really stretch his imagination to conceive of a destroyer 

 captain maintaining a speed such that F = 0.37 in a high state 7 sea. The fact 

 that our hypothesis about superposition is not so good in this case should not 

 cause us too much unhappiness. At moderate speed (f = 0.18) it is still rather 

 good for the high-7 sea state, and the breakdown occurs only as speed increases 

 beyond this. Five years ago, the most sanguine investigators did not dare to 

 hope that the hypothesis could ever be pushed as far as Dalzell has done. 



Similar results can be observed for heave and bending moment f.r. func- 

 tions. The respective figures of Dalzell will not be reproduced here. For 



30 



< 



- 330 < 



- 300 



270 



1.0 



2.0 

 FREQUENCY, fi g 



3.0 



Fig. 4 - Dalzell' s pitch frequency response function, Model DD 

 Froude number = 0.37 (from Dalzell (1962b)) 



692 



15 



