Since wave-piercing catamarans were developed based on the concept to reduce wave-induced ship motions in high-speed, in Australia, the ships have special features, like two slender demi-hulls with sharp bow to reduce reserve buoyancy above the water surface, a center bow to provide reserve buoyancy in heavy waves and so on. Therefore, it is easily imagined that these features may generate nonlinearity or irregularity of ship motions in rough seas. In order to investigate the nonlinearity and the irregularity in ship motions of the ship, measurements of ship motions in various wave directions, heights and periods by using a model of the ship were carried out. The experimental results demonstrate that nonlinear features to the change in wave height appear in ship motion amplitudes in regular waves and suggest that linear ship motion theories widely used in a design stage of ship building do not work well. In shorter waves, irregular ship motions and twice period ship motions appear even in regular waves. Since the nonlinearity of ship motions is not so strong, the effect of the nonlinearity on the operation of such ships has not been pointed out as a serious problem.To investigate the cause by which the irregular ship motions appear in regular waves, ship motions were calculated by a nonlinear strip method. The results indicate that irregular ship motions of the ship were caused by drastic change of restoring force which acting on the cross deck structure of the ship.
