2) Frequencies are high 



3) Forward speed (or Froude number) is low. 



If these conditions are not satisfied, experimentally-determined transfer 

 functions should be used for the prediction of ship motions in an irregular 

 seaway. 



The accuracy of the strip method is to be investigated with regard to 

 longitudinal ship motions, namely, heaving and pitching. The items which 

 should be studied are listed as follows: 



1) Three-dimensional effect 



2) Non-linear effect 



3) Approximation by Lewis form 



4) Viscous effect 



5) Approximate calculation of wave forces 



6) Displacement effect 



Thus, from the mathematical point of view the limitations and inaccuracies 

 of the classical methods for the ship wave problems are for the following as- 

 sumptions : 



1) Viscous/wave interactions 



The interactions between the viscous effects and the gravity waves 

 are assumed to be small so that potential flow theory can be used for pre- 

 dicting ship motions. 



In case of longitudinal motions (i,e. heaving and pitching) the level 

 of wavemaking is higher than in case of rolling motion and it can be justify- 

 ably said that the viscous effect caused by the bilge-keel etc. occur seldom 

 except for the region in which the frequency is high [33] . 



Considering viscous effect another important effect is that of dynamic 

 lift [30], [40]. 



72 



