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.
