PURPOSE:To improve propulsion performance, in a propeller for high speed boat having an exhaust passage in a cylindrical boss, by providing, between blades on the outer surface of the boss rear end part, an auxiliary blade having an area smaller than the blade area and the mounting pitch angle nearly same as that of the blade. CONSTITUTION:On the boss surface close to the rear edge 2b of a boss 2, two auxiliary blades 10 are mounted nearly in the middle with the rear edge root parts of adjacent blades 3 at nearly the same pitch angle theta as that of the blades 3 to form a propeller 1. In this constitution, resistance of water is small, the back flow of an exhaust air is prevented, and the exhaust pressure can be reduced. Thus, the output of an engine can be increased to improve propulsion force.
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