Experiments and Computations of non linear effects on the motions of Catamaran Hulls

Aim of this paper is to analyze the influence of wave steepness on nonlinear ship motions, as well as the capability of a weakly nonlinear seakeeping methodology to predict the related nonlinear effects. Experimental tests and numerical simulations have been performed for two catamaran vessels: one with round bilge and the other with a hard chine hull form. Regular head waves of different wave length combined with two wave slopes have been considered. In the numerical analyses, the nonlinear forces are evaluated in the time domain and the equations of motion are solved in the frequency domain. This procedure iteratively passes from the frequency to the time domain and allows to avoid the initial transient phase, resulting faster than the classical approach based on the time domain only. For solving the relevant boundary value problems in the frequency domain a three-dimensional Rankine panel method has been employed which has the capability to deal with the complex free surface flow pattern between the hulls of multi-hull vessels. Examining the experimental data and the related numerical results, conclusions are drawn about the influence of the wave slope and about the capability of the numerical method to consider such influence.