The propeller load varying test method, which has been developed in the S. R. I. to investigate into the propulsive performance of ships, has broad applications in the field of ship's performance problems. The basic idea of it is to study the propulsive performance in the wide range of the propeller load from T =0 to the thrust far beyond ship point. The self-propulsion factors are given as the functions of propeller thrust. In this paper the author mentions how the data from the method can be utilized in the estimation of the propulsive performance of a ship affected by the external force such as wave exiting drag and wind thrust or drag. He also shows the effectiveness of the method in the investigation of the effect of reaction rudder.