Large amplitude roll motion of a ship without bilge keels in heavy regular beam seas is experimentally investigated, and nonlinear features of ship motions in high waves are revealed. The model in beam seas is completely free to drift motion, and this leads to change of the encounter frequency. Usually, the drifting velocity is much lower than the wave velocity in small beam waves, and this effect can be considered to be negligible. The present experiments, however, show that the drift motion induced by high waves changes the encounter frequency and shifts roll resonance to higher frequency. As a result, large roll motion appears in wide frequency range that is higher than roll natural frequency. In addition, the experiments show that a jump phenomenon of roll, heave and drifting velocity, and nonlinear features of heave amplitude appear in high waves.
Large amplitude roll motion has been investigated by many researchers over the last forty years. Most of the material contains theoretical studies on the nonlinear features of roll in beam seas, and the many model experiments have been conducted. The purposes of this study are to investigate nonlinear features in large amplitudes of ship motions experimentally and to clarify the mechanism of these features. Firstly, the measurements of large amplitude ship motion in heavy beam seas are carried out. The experiments shows that, in heavy beam seas, 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 resonant frequency, and that a kind of jump phenomena of roll motion appears. It is confirmed that the cause of these phenomena is that drift motion induced by high waves changes encounter frequency and shifts roll resonance to higher frequency.
