To have reliable results by applying the modern methods of nonlinear dynamics, a realistic mathematical model of ship motions is needed. In this paper we discuss, on an experimental basis, the large amplitude roll motion modelling, an issue of great interest in the studies of ship capsizing. In particular, the coupling of roll motion with the other motions in the transversal plane, heave and sway, is analised by means of a parameter identification technique used to discuss the goodness-of-fit of different mathematical models to the experimental data and to obtain values for the coefficients. The results relative to a series of test conducted in beam waves on the scale model of a small RoRo in different initial stability conditions indicate that:
- the results in terms of motion amplitudes do not depend on the mooring arrangement, provided this does not fix a particular roll axis;
- the coupling of roll into sway and heave is needed to account for observed motions;
- the coupling of heave and sway into roll is not so important nor for roll motion simulation nor for the estimate of the roll motion coefficients;
- the effective wave slope coefficient estimated is sensibly smaller than IMO prescription.
