On Roll Damping Force of Ship : Effects of Friction of Hull and Normal Force of Bilge Keels

Abstract

The authors present a method for calculating frictional roll damping force using three-dimentional unsteady turbulent boundary layer theory. The results of the calculation show fairly good agreement with experiments and also with the Kato-Tamiya's formula. Furthermore, a formula for roll damping due to the normal force of bilge keels is proposed on the basis of the recent studies on the drag coefficient of oscillating bluff bodies. It is found that the drag coefficient of the normal force of bilge keels depends only on the roll amplitude, or so called period parameter, and that the nomal force causes not only a non-linear roll damping but also a linear one. And the effect of advance speed on the normal force of bilge keels seems to be very small.