Results are given of a recently broadened investigation into the stability of ships in following seas, based on a theoretical study and model tests on Series 60 hull forms. The analysis is derived from perturbations to the steady yaw, sway, surge and automatic control equations. At low frequencies of encounter, corresponding to high ship speeds, it is found that significant destabilizing effects arise from the large surge forces generated by the waves, leading to broaching: stability is greatly improved by appropriate characteristics of the control system. At higher encounter frequencies, typical of lower speed ships, oscillating yaw and sway motions are induced and the rudder has little influence. The study clarifies the influences of wave encounter frequency, encounter angle, control system constants and ship configuration.
