Boat propulsion unit with a propulsion propeller arranged under the boat's bottom

In a boat propulsion device having a propeller mounted under the bottom of the boat, and a sleeve into which said propeller is retractable beyond the plane of said bottom, said bottom being essentially flat in the area of the propulsion device, the improvement wherein said sleeve is surrounded at its lower edge by an annular groove, said propulsion device comprising a sealing means disposed in said groove for sealing said sleeve when said sealing means is coplanar with the bottom of the boat, and said propulsion device comprises means for enclosing said sleeve when the propulsion device is withdrawn into it, said enclosing means comprising a plate acting as a cover in the plane of the bottom of said boat, the plate being mounted on the propulsion device in fixed disposition with respect to the axis of rotation of the propeller so that the plate and the propeller can be vertically raised and lowered together, the propeller being rotatable relative to the plate, and further comprising pump means for emptying the sleeve of water after the plate encloses the sleeve.