PURPOSE: To maintain a high thrust efficiency and prevent cavitation by making the sectional form of side surface of a duct symmetrical with respect to an axis of a propeller shaft, by splitting the duct into front and rear halves and by integrating the rear half and the propeller.

CONSTITUTION: A duct 2 is longitudinally splitted into front half 22 and rear half 21, with the front half 21 fixed to a boat hull (not shown) and the external extremities of a propeller 1 fixed to the inner periphery of the rear half 22. The sectional form of side surface of the duct 2 is made symmetrical with respect to an axis of a rotary shaft of the propeller 1. A high thrust efficiency may be thereby maintained and cavitation prevented.