Abstract: To reduce engine exhaust gas pressure behind a motorboat propeller formed with a boss for educing engine exhaust gas therethrough, without forming a trumpet-shaped boss end and without allowing exhaust gas to flow upstream along the boss outer surface, a single or plural small subblades are provided between two adjacent large main blades at such a position that a positive pressure domain produced by the subblade reduces a negative pressure domain produced by the main blade. Since engine exhaust gas can be effectively educed, engine power increases and therefore motorboat acceleration performance can be improved.