In this paper, a synthetic method for predicting the speed loss of a ship in waves by taking account of the main engine characteristics is shown. In determining the ship speed in waves, two factors are considered : the nominal speed loss due to added resistance caused by wind and waves and the deliberate speed loss by the captain's command in order to prevent severe ship motions. An example of the calculations by this method for 175m length single screw high speed container ship equipped with diesel engine is also presented.