The characteristic features of subharmonic components in the roll motion of a ship are investigated. The mathematical model used for this purpose is a second order roll motion equation nonlinear both in the restoring and in the damping. The restoring nonlinearity is cubic in the angle, whereas the damping is nonlinear both in the amplitude and in the velocity. Particular attention is devoted to the possibility of subharmonics for a ship with upright equilibrium. It has been proved that a subharmonic oscillation of order 3 is possible in this case but it is subject to a threshold for the oscillation amplitude. The phenomenon can by particularly dangerous for ships having a strongly nonlinear righting moment, like modern containerships.
