The nonlinear modelling of parametric rolling of a ship advancing in longitudinal waves is studied on an experimental basis. Different descriptions of the righting arm in waves are compared with experimental results using a perturbative analytical solution obtained by means of the averaging method. It appears that an isolated roll motion equation provides a very reliable simulation of experimental results including both threshold and roll amplitude above threshold, but the interaction between righting arm nonlinearity and time dependence induced by wave and vertical motions requires further studies.
