Ship system can be divided into four sub-systems: hull, propeller, main engine and operation system which severely affect the characteristics of a ship. In determining ship speed in waves, two factors are considered the involuntary speed loss due to added resistance caused by wind and waves, and the voluntary speed loss by command of operation system to prevent severe ship motions. In this paper, the main function of four sub-system is analyzed for input/output relations and propulsive coefficient and a useful method to predict involuntary speed loss of a ship is presented. Two calculated examples for a high speed container ship and a passenger ship with single screw and diesel engine are given.