The first procedure described above is essentially already being 

 used in your programs developed by Dr. Webster except, as 1 understand, 

 the wave frequency dependence of the forces is not included. 



2. The method of bispectral analysis was suggested as a promising means 

 of obtaining the slow drift forces and other nonlinear effects from 

 experimental data either on model scale or full size. I would certainly 

 agree that this method should be explored and its full potential evaluated, 

 but I have one serious reservation. This concerns our ability to measure 

 the input (i.e., the waves with the required precision). In full-scale 

 measurements especially, but also in model scale, the presence of the 

 ship significantly distorts the input waves by diffraction. It is, 

 therefore, not possible to measure the "pure" input signal, although the 

 output or ship response can be measured quite precisely. In the labo- 

 ratory, we can approach this pure signal by first recording the waves 

 with the model removed and then replaying exactly the same control 

 signal through our wave generator with the model in place. There is 

 still a source of error due to imperfect synchronization and other 

 imperfections in the apparatus. In full scale at sea we can only measure 

 waves in the near vicinity of the ship and accept the distortion. I 

 think, therefore, that these problems must be addressed simultaneously 

 with further exploration of the bispectral approach in order to fully 

 evaluate the accuracy of any results which might be obtained in that 

 way. 



172 



