slender-body heave amplitudes shown in Figure 6 are greater than those of both the 

 strip theory and the three-dimensional theory. The pitch amplitudes agree quite 

 well even though the pitch moments are computed differently. For the twin ellipsoi- 

 dal hulls, the damping coefficients are almost the same, the added-mass coefficients 

 are computed as slightly less, and the exciting moments are higher than those pre- 

 dicted by the three-dimensional theory. In the case of pitch amplitude, it appears 

 that the underprediction of the added-mass coefficients and the overprediction for 

 the exciting forces cancel out in the motion results. 



In a previous report this author presented an explanation for the rather large 

 discrepancies in the added-mass coefficients computed through Equation (37) employing 

 the singularity distribution a„. Since that publication, the author has computed 

 the singularity distribution 0_ directly through the multipole expansion method for 

 the semicircular section and has compared this result with Equation (37); the two 

 computations agree quite well. Unfortunately, for the twin-hull section, the 

 direct computation C~ through Equation (14) is apparently impossible. Therefore, o„ 

 has been computed for all models through Equation (37) . The large discrepancies 

 mentioned above have been identified as the result of a different numerical handling 

 of the singularities of the kernel function. As shown later, the prediction of the 

 added-mass coefficients in following seas has the same tendency — lower at low fre- 

 quencies and higher at high frequencies than those of the strip theory. 



The results for SWATH 6A at 28 knots in head seas are shown in Figures 7-10. 

 The added-mass and damping coefficients do not show any difference when compared 

 with those of the strip theory. While our interest range is in wavelength ratios 

 between 1 and 6, the corresponding nondimensional frequency number varies from 2 to 

 6. These frequency numbers are fairly high, and, therefore, the slender-body theory 

 does not improve the prediction of the added-mass and damping coefficients. However, 

 the exciting forces and moments show large discrepancies when compared with those 

 computed through theory. This is due to the fact that whereas the two-dimensional 

 source strength a., for the diffraction potential has the factor exp(iK x cos 3) in 

 Equation (23), O-, in Equation (36) varies harmonically along the ship's length. This 

 sinusoidal change of 0-, affects the solution of q 7 in Equation (34) and also the so- 

 lution change of C ? in Equation (33) . The oscillatory behavior of the exciting 

 forces and moments is caused by this solution for C . The heave and pitch ampli- 

 tudes show some discrepancies between experiment and the strip theory. The mixed 



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