Another point of interest regarding the effect of forward motion is 

 about the value of the parameter y — 



If Y> 0.25, then the waves generated by the ship oscillation travel 

 more slowly than the vessel and hence are confined in the sector behind 

 the ship, whereas if y< 0.25 the waves travel faster than and ahead of 

 the vessel. As indicated by [36] and [37] and recently [38] this feature is 

 not a theoretical anomaly because experimental measurements around the para- 

 meter value Y =0.25 show some irregularities and scatter. 



The computation of the wave excitation is carried out either a) from 

 the Froude-Krylov theory by using the defined relative motion between the 

 ship and the wave, as given in [ 8 ], [15], [ 1 ] or 2) from the diffraction 

 theory by using Has kind -Newman relationship [23], [37], [26], 



Consideration of the forward speed in the coefficients of equations of 

 motion is another source of difference between various strip theories. 

 Wave Exciting Forces and Moments 



There are at present two methods for the calculation of wave exciting 

 forces and moments, namely: 



1) Korvin-Kroukovsky type of approach 



2) Use of Haskind-Newman formulae 



The first method makes use of the relative motion concept and in a 

 way employs Froude-Krylov theory combined with this relative motion definition. 

 Consequently, this approximate method is valid only for the medium range of 

 frequencies; for the short waves the Froude-Krylov hypothesis is not valid 

 whereas for the very long waves the strip theory fails. 



Recently [18] experimentally obtained the magnitude and the distribution 

 of wave exciting forces on a segmented tanker model and showed this difference 

 between the theoretical predictions of this type and the experimental measure- 

 ments for the short wavelengths as illustrated in Figs. 7 and 8. The good 



66 



