PURPOSE: To prevent generation of cone vortex cavitation by providing a sliding control mechanism capable of controlling sliding of a movable boss.

CONSTITUTION: Axial sliding of a movable boss 4 is controlled by means of a sliding control mechanism formed by a hydraulic cylinder 5, the movable boss 4 is allowed to project toward the rear of a propeller main body 1 by appropriate length. Accordingly, receiving the flow of wing base vortex along the surface of a propeller main body 1 by the surface of the movable boss 4 and weakening of said flow by means of frictional resistance become possible, and cone vortex cavitation can be thus prevented. Further, flow of the wing base vortex is more efficiently weakened by rotating the movable boss 4 in the direction reverse to that of the propeller main body 1.