PURPOSE: To prevent the formation of a cavitation by a method wherein a rear end section of a crossing section between the bottom and a side wall of a shaft tunnel of the propeller lifting type small-sized ship is notched and connected by an incline, and the formation of a concave section on the surface of the water of an upper section of a propeller when turning is prevented.

CONSTITUTION: The crossing section between the bottom 5 and the side walls 6 of the shaft tunnel of the propeller lifting type small-sized ship is notched lest edges should be formed, and connected by the inclines 7. The inclines take the shapes of right-angle triangles, two sides thereof are the bottom 5 and the side walls 6 of the shaft tunnel, and the base (the bottom section side) is made the size of twice as large as the width of the shaft tunnel and height (the side wall side of the shaft tunnel) within a range of one tenth of the base. The concave section is not formed on the surface of the water of the upper section of the propeller when the hull is turned owing to the inclined notch sections 7, and the formation of the cavitation can be prevented.