PURPOSE: To uniform streams flowing into a main propeller to prevent cavitation, by arranging wake accelerating nozzle propellers on both port and starboard sides in the vicinity of the top end of rotating circular periphery of the main propeller.

CONSTITUTION: The cross-sectioned shape of the stern section of a hull in the front of a main propeller 2 is formed such that it is constricted above a main propeller shaft 5 and is bulged in the vicinity of and below the shaft 5. Wake accelerating nozzle propellers 7 are disposed on both sides of the hull in the constricted sections thereof in the front of and the vicinity of the top end of the rotating circle 2a of the main propeller 2. With this arrangement a strong wake in the vicinity of the hull 1 above the main propeller 2 is accelerated by the nozzle propellers 7 and is introduced into the main propeller 2 so that the stream distribution in the main propeller rotating circle 2a is made uniform, thereby cavitation is prevented from taking place.