Evaluation of Motions of Marine Craft in Irregular Seas 



Fig. 4 - Pitch and heave motions for destroyer in head seas 

 predicted by impulsive response technique 



seen that the response functions are physically realizable, i.e., K(t) = for 

 t < . If the surface wave probe were located at the LCG of the model, Fancev 

 shows that the resultant response function would have values for t < and 

 hence be classified as physically nonrealizable. The physical explanation for 

 this is that when the ship is long, relative to the wave length, the ship responds 

 to the wave crest even before it reaches the bow and before this wave is re- 

 corded by a wave probe located at the LCG; hence, in this case, the ship motion 

 would always precede the arrival of the wave crest at the LCG. 



The heave and pitch time histories were computed on an IBM 1620 by eval- 

 uating the convolution integrals [Eq. (36)], 



X(t) 



00 



(t) ri(t - t) dr 



0(t) 



K.(T) -nCt-r) dr 



479 



