Effects of half breadth to draught ratio of hull under water surface on bilge-keel roll damping component

Abstract

Roll amplitudes of parametric roll in regular head waves are sometimes underestimated by motion calculations including Ikeda's roll damping prediction method, especially for the case of the large roll amplitude caused by high wave height. In this case, generally the ship has significant relative draught changing because of its heave and pitch. It may be said that the effects of the change in relative draught on roll damping is significant. However, in Ikeda's prediction method, the bilge-keel component which is the largest component of total roll damping does not take the effects of draught. As the effects of draught, there are at least two related significant effects on the bilge-keel roll damping component not fully considers for developing Ikeda's prediction method. They are the effects of underwater hull aspects (: half breadth to draught ratio) and the free surface effects. In this study, as a first attempt, the effects of draught on the bilge-keel component are investigated experimentally and numerically by using a 2-dimensional model. And a prediction model of the effects of draught on bilge-keel component to collect Ikeda's prediction method is proposed. Moreover, the calculated results by the proposed method are compared with some measured data including the effects of draught and its validity is confirmed.