the aft of the vessel. Ref. [29], calculated the hydrodynamic forces and the 

 ship motions approximating the section contour by the three-r-parameter family 

 and the comparisons betv7£eri. the results thus obtained and those by Lewis form 

 ai^proximation are shown in Figs. 3, 4 and 5. Although the approximations by 

 the three-parameter family is significantly better than those by Le\vif.-form, 

 there appears to be no large differences in the calculated values of the 

 damping force and of the added mass except for the regions in which the fre- 

 quency is large or the motion is small. Thus the Lewis-form approximation is 

 considered adequate except for special hull forms. 

 Forward Speed Effect 



In a strictly theoretical sense, strip theory (except the version of 

 [23] cannot be used for a ship with a steady forward motion. This is because 

 the linearized free-surface condition cannot be satisfied on transverse strips. 



Theoretical considerations suggest that the strip theory is valid if the 

 forward velocity is small and the f requency of encounter is high . 



In practice, of this has led to the formation of a number of different 

 versions of the strip theory. Some representative versions of the fluid 

 reactive terms in the equations of motion for the longitudinal and transverse 

 motions are given in Tables 1 and 2. Although the differences may seem to be 

 significant at first sight, recent computations by [6 ] and [39] for the 

 longitudinal motions show that the real differences for the speed and the 

 frequency ranges of interest are insignificant except for the coupling terms 

 between heave and pitch, where the results of [23] show a different trend. 

 The evaluation of the integral term of [23] is difficult and therefore this 

 method is impractical. On the other hand the expressions of [37] and [26] 

 give a better prediction than the others. Fig. 6 shows the comparison between 

 the three approaches [2 ]. 



61 



