A Rational Treatment of Viscous Effect on Roll and Sway Coupled Motion

Abstract

In this paper, the authors investigate a rational treatment of the viscous effect on a coupled motion of a ship in waves. In the linear theory, the non-linear viscous effects are taken into account by using equivalent linear forces obtained by linearizing the non-linear ones which are usually assumed to be proportional to the square of the each velocity component. Exactly speaking, however, the viscous forces are proportional to the square of the resultant local relative velocity. Then, it is more rational to use the equivalent forces obtained by linearizing the square of the resultant velocity. From this standpoint, a prediction method of the bilge keel effect on roll damping of a ship in sway-roll coupled motion is presented. The results of the present method show fairly good agreement with experimental ones which are obtained by sway-roll coupled forced motion tests.