method is the result of the combination of the strip theory and slender-body theory. 

 In the mixed method the added-mass and damping coefficients are computed through 

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

 strip theory. The results obtained with the mixed method are generally the same as 

 those of the strip theory. This means that the effect on motion amplitude of the 

 added-mass and damping coefficients computed with the slender-body theory is 

 negligible in this example and that the oscillatory results in the pitch motion are 

 caused by the pitch moments. 



The results of SWATH 6D at 28 knots in head seas are shown in Figures 11-14. 

 As previously noted for the SWATH 6A, the added-mass and damping coefficients of the 

 slender-body theory do not show any difference from those of the strip theory, except 

 in the low-frequency range where the slender-body theory values for the hydrodynamic 

 coefficients are lower than those of the strip theory. In the (nondimensional) fre- 

 quency range between 1.5 and 4.0, the slender-body heave forces F show the same 

 tendency as those of the strip theory, and the pitch moments F,. display oscillatory 

 behavior at low wavelength ratios (Figure 13). This phenomenon is also observed in 

 the results of the SWATH 6A. 



In the previous report, the results for SWATH 6A and 6D at 20 knots in follow- 

 ing seas have been compared with those of the strip theory and experiment. Since the 

 numerical procedure has been corrected in the computation of the kernel function, the 

 computations of the following seas have been repeated in this report. These results 

 for SWATH 6A are plotted in Figures 15-18. When the nondimensionalized frequency is 

 0.56, there exists a discontinuity in the curves of the hydrodynamic coefficients. 

 As mentioned in the previous report, when GjU/g is 0.25, G_ of Equation (31) is 

 singular and the numerical results become unstable. The damping coefficients are 

 nearly the same as those obtained through strip theory, except where this dis- 

 continuity occurs. Compared with the previous results, the damping coefficients do 

 not show any change; however, the added-mass coefficients are quite different. While 

 the previous results are uniformly greater than those of the strip theory, the 

 present ones are less at low frequency and greater at high frequency. The results 

 of the exciting forces and moments computed by the slender-body theory and by the 

 strip theory are similar, except there is a peak value in the pitch moment computed 

 by the slender-body theory for a wavelength ratio of approximately 2.0. The motion 

 results are also similar to those computed by strip theory, except at the peak value 

 for a wavelength ratio of approximately 1.5. When the wavelength ratio is 1.3, the 



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