This paper deals with a simplified method of calculating normal force of a rudder whose height differs from propeller diameter. This paper limits the problem to the accuracy for the calculating method of the normal force for a case in which inflow velocity to the rudder is not uniform along the span. The values obtained by the simplified equations are compared with a solution of an integral equation.It is shown that as long as velocity increment due to propeller slipstream is not high and ratio of propeller diameter to rudder height is not remarkably small, the approximate equations in this paper provide the values permissibly similar to the value obtained by the integral equation. Further, it is indicated that an approximate equation based on an elliptical circulation distribution can explain the tendency of influence of propeller arrangement in the rudder height direction on normal force of the rudder.
