PURPOSE: To prevent occurrence of negative pressure and/or a turbulent flow in order to improve propulsion efficiency by forming a fluid introducing passage in a casing and providing, at the central part of a propeller, a fluid outlet port, which is connected to the introducing passage and opens backward.

CONSTITUTION: When an outboard motor 10 is on the advancing travel, air introduced into an air introducing passage 25 from the air intake port 26, located above the water surface, of a casing 13, flows out backward from the air outlet port 29 of a propeller 19 because of negative pressure produced behind the propeller 19. This forms a stream-lined, torpedo-like air zone behind the propeller 19 as shown by double-dot-and-dash lines in the figure, thereby increasing the boat's speed. When the boat is going astern, positive pressure is produced behind the propeller 19, and no air flows backward from the fluid outlet port 29 of the propeller 19. Therefore, there is neither diffusion of air bubbles nor disturbance in a water flow, around the propeller blades, thereby increasing the backward propulsion force.