To reduce blow holes, improve durability and reliability from the viewpoint of strength, suppress blow hole distortion and improve parts accuracy by forming a thinning part at the root of each blade from the inside of an outer cylinder in a propulsion hydrofoil provided with a plurality of blades on the outer surface of the outer cylinder.

In a propulsion device mounted on a hull of a water vehicle, a propeller 10 serving as a propulsion hydrofoil is arranged in a housing provided with a nozzle, fitted with a deflector rotatably, at the lower reaches end, and the propeller 10 is rotated by an engine through an output shaft. The propeller 10 is formed by connecting an inner cylinder 11 and an outer cylinder 12 by three ribs 13 radially extended at intervals of 120°, and three blades 14 are integrally provided on the outer surface of the outer cylinder 12. Space parts 20 are formed between the outer cylinder 12 and inner cylinder 11, in positions corresponding to the roots 15 of the respective blades 14, and a thinning part 15a is formed at the root 15 of each blade 14, along the shape of the root 15 from the inside of the outer cylinder 12.