PURPOSE: To increase the rotative flow energy recovery ratio and lessen the rudder resistance by determining specially the distribution of the amount of twist of a rudder plate in relation to a propeller shaft line.

CONSTITUTION: A reaction rudder 1 is symmetrical in shape which has no twist at its three points: the extension (a) of the propeller shaft line and the top and bottom ends of the rudder. The leading edge of the rudder plate in way of an upper section (b) above the extension of the propeller shaft line and that in way of a lower section (c) below the extension of the propeller shaft line are twisted conversely, or twisted toward the port side in the former (b), while the starboard side in the latter (b), and the amount of twist varies continuously with respect to a vertical direction. This constitution gives a better accord with the stream.