PURPOSE: To contrive the improvement of propulsive efficiency by forming a rudder surface, not receiving a propeller back wash, in a right-left asymmetrical section bending a sectional shape of the rudder surface by a degree half the inflow angle so as to face the flow in the vicinity of a rudder and a rudder surface, receiving the back wash, in a symmetrical section.

CONSTITUTION: A rudder surface 2, not receiving an acceleration flow of a propeller (b), is provided with a camber by bending a front end 1a of a rudder 1 about α/2 in a direction toward a flow (c) from a ship side. Accordingly, the rudder surface 2 is formed in a right-left asymmetrical section. A rudder surface 3, receiving the acceleration flow of the propeller (b), is formed in an symmetrical blade section as in the past. Accordingly, a shape of the rudder surfaces 2, 3 is discontinuously changed, but a rudder trailing edge 1b is not changed. By the constitution thus obtained, a rudder, partly providing an asymmetrical surface, increases a lift for a flow (e) from the ship side further obtaining a smooth flow, consequently a drag is decreased, enabling resistance to be reduced of the rudder 1 improving propulsive efficiency.