Parametric excitation and the stability of a ship subjected to a steady heeling moment

The stability of a ship, rolling in regular beam waves at zero speed while under the influence of an additional constant heeling moment, is considered. Effects which produce parametric excitations are investigated theoretically; their inclusion in a study of the roll equation implies that the degree of instability depends on the direction of the additional moment relative to the wave source. This conclusion is tested using the data of model experiments in which this behaviour was clearly observed.