In this paper, the existence of subharmonic oscillations in the rolling motion is investigated. To this end, the roll motion is described by a mathematical model nonlinear both in the restoring and in the damping. Particular attention is payd to the subharmonics originated by the coupling between the forced and the natural solution, having thus order three. It is shown that these subharmonics are subject to a threshold phenomenon strongly influenced by damping. The obtained results appear very important in the frame of ship safety from capsizing.
