PURPOSE: To embody the structure which facilitates the rudder operation in the high speed navigation and maintains the rudder effect since a rudder plate is suspended and the whole is under the water level in the low speed navigation, by forming the shape with which almost all the area of the rudder plate which sinks under the water level is directly under the rudder axis center line or in the vicinity when the rudder plate is turned rearward and put into a tilt state by the water stream in the high speed navigation.

CONSTITUTION: In the state where a rudder plate tilts rearward in the high speed navigation and a part of the rudder plate is inundated, the center of gravity point of the area of a rudder plate 13 below the rear extension line K-K of a ship bottom is set at the position close to the rudder axis center line Z-Z in comparison with the middle point between the rear edge of a rudder plate 10 and the rudder axis center line Z-Z. Accordingly, the rudder operation can be facilitated by slightly turning the rudder plate rightward in the rearward operation.