We have been trying to synthesize the seakeeping qualities for obtaining the speed loss of ships in waves. However, the deliberate speed loss is not so investigated in detail as the nominal speed loss. In this paper, the detail of the prediction method taking the characteristics of the main engine into consideration is presented. It is fact that the method of the experiment about the deliberate speed loss is not established. Therefore, an experiment about the deliberate speed loss are carried out with a model of single screw high speed container ship in order to ensure the justification of the predicted calculation.