In this paper, the rolling motion of a ship is considered in connection with the possibility of having roll oscillations subharmonic to the waves. Three different phenomena have been individuated as origin of this feature: the nonlinearity of the righting arm, the harmonic composition of sea waves and the parametric excitation due to the coupling among different motions in longitudinal waves. The more appropriate analytical methods to study the different mechanisms and the possible presence of a threshold are discussed in detail.
