The stability of upright ship position in longitudinal waves against the onset of parametric rolling has been investigated by a number of authors. The main aim was the analysis of threshold for parametric roll excitation in terms of wave height, ship stability and course. Some researchers investigated also the problem connected with the definition of the threshold in a stochastic environment, but the problem is still partly open. In this paper some shortcomings connected with the linear approach are highlighted, together with a preliminary analysis of the possibility of a nonlinear description based on concentrated parameters nonlinear description of the roll motion amplitude above threshold, a feature that can only be approached taking into account model nonlinearities.
