Large amplitude roll motion of a ship in heavy beam seas has been investigated experimentally, and a nonlinear feature of ship motions in high waves was demonstrated. It was pointed out that the drift motion induced by high waves changes the encounter frequency and shifts roll resonance to higher frequency in heavy beam seas. This causes that large roll motion appears in wide frequency range that is higher than the roll natural frequency. In addition, the experiments showed that a jump phenomenon of roll motion appears. In this paper, characteristics of drift motion that influence the roll motion of a ship in beam seas are experimentally investigated. The drift force acting on the hull in regular beam seas is measured by using a three-dimensional model ship. The experimental results demonstrates that drift force has remarkable two peaks in the frequency region of roll and heave resonance. The obtained results indicate that the drift resistance coefficient depends on Keulegan-Carpenter number. Using the measured drift force and the drift resistance coefficient, drifting velocity can be calculated and multiple steady drifting velocities in the same incident waves are confirmed. An approximate prediction method of roll amplitude and drifting velocity using a strip method is deduced.
Large amplitude roll motion has been researched by many researchers because the ship in this condition is regarded as having a high capsizing risk. Kuroda et al. (2003) reported that, in heavy beam seas, roll resonance shifts to higher frequency, and large amplitude roll motion occurs not only in the frequency region of the roll resonance but also in the wide frequency region higher than the resonance frequency. In addition, a kind of jump phenomenon of roll motion appears.