PURPOSE: To allow the rotational energy of turning flow to be converted into thrust by catching the turning flow specially along the inner peripheral surface of a duct, and guiding it backward regarding a marine propeller with the duct increasing thrust by covering a screw propeller with the duct.

CONSTITUTION: The inner peripheral surface of a duct 3 extended backward after covering a propeller 1 loosely is provided with plural current plates 6 extended in the longitudinal direction of the duct 3. The current plates 6 are fitted being inclined retrorsely to the turning direction of turning flow 7 so as to be able to catch the turning flow 7 along the inner peripheral surface of the duct 3. The current plates 6 thereby catch the turning flow 7 accurately and guides it backward so as to bring the increase of thrust.