On the Effect of Viscous Forces on the Motion of High Speed Hulls

For fast slender hullforms, the damping forces due to the generated waves are so small that the damping forces relative to viscosity cannot be neglected in evaluating the vertical motions. In this paper the numerical and experimental assessment of the vertical motions in head sea for three mono-hulls are presented. The considered hulls are fast slender displacement type with transom stern and L/B ratios 14, 12, 8. The most slender hull is a trimaran main hull, the second one is a catamaran demihull and the last one is the Model 5 (Blok and Beukelman, 1984). The calculations were performed using 2 ½ D high speed theory by Faltinsen and Zhao (1991). To obtain potential flow theory results the viscosity correction from the cross flow was added. The effect of the viscosity from cross flow was considered as reported by Lee (1977), Chan (1992), Centeno (2000). At the Trieste towing tank experimental program was conducted for the catamaran demihull and the trimaran main hull for four different speeds while for the Model 5 experimental data were collected from the literature. The effect of cross flow coefficients is evaluated and discussed, and empirical coefficients are set. Furthermore, the influence of slenderness ratio and speed is discussed and finally some conclusions are given.