nondimensionalized frequency number becomes zero and the computed hydrodynamic 

 coefficients are far smaller than those predicted by strip theory. Therefore, the 

 motion amplitudes are magnified at this wavelength ratio. If we compare the results 

 of the strip theory and the mixed method, this magnification effect is easily seen 

 in the motion results. In the mixed method, the hydrodynamic coefficients are 

 computed by the slender-body theory, and the exciting forces and moments are computed 

 by strip theory. 



The results for SWATH 6D at 20 knots in following seas are plotted in Figures 

 19-22. When the nondimensionalized frequency number is 0.65 (ooU/g=0.25) , there is a 

 discontinuity in the curve of the hydrodynamic coefficients similar to that observed 

 in the calculations for SWATH 6A. These coefficients are found to be smaller at low 

 frequencies and larger at high frequencies than those computed through strip theory. 

 While the heave exciting forces are overpredicted when compared with those of the 

 strip theory and the experiments, the pitch moments are underpredicted for both 

 results. All computed heave motions are in close agreement at low wavelength ratios, 

 but for high wavelength ratios, discrepancies between experiment and computations 

 are quite large. Pitch motions are in good agreement with the experimental results, 

 except near a wavelength ratio of 1.0 where the encounter frequency becomes zero. 

 The pitch moments computed by the strip theory are less than those obtained experi- 

 mentally, but the pitch motions are larger. Generally, the motion results are 

 affected by the inertia, hydrodynamic, and hydrostatic forces. Therefore, it is 

 very difficult to determine the exact cause of these discrepancies. 



Although there is a peak response shown in the motion of SWATH 6A in Figure 18, 

 SWATH 6D (Figure 22) does not show such a peak value. This difference can be 

 analyzed through the motion equations. If the coupling effect between heave and 

 pitch motion is neglected, the solution of the pitch motion becomes 



^R " {F R [C 55 -(I + A 55 )a) 2 ]-F I 0)B 55 }/D 



h " (F I [C 55 -(I+A 55 )a ) 2 ] + F R a,B 55 }/D 



where 



2 

 D = [C 55 -(I+A 55 )oj 2 ] + OB 55 ) 2 



34 



