Abstract: An outboard marine propulsion apparatus has a rotor unit axially displaceable between forward and reverse operating positions. In the forward position, concentric horizontal paths through the apparatus are provided for rearward discharge of the propulsive water stream and the engine exhaust gases; in the reverse position, paths are provided for discharging the propulsive water stream forwardly and the exhaust gases in radially outward directions ahead of the rotor unit. For reverse operation one of the concentric flow paths is closed by an automatically actuated butterfly valve responsive to the axial displacement of the rotor unit. A dashpot device controls the axial sliding speed of the rotor unit.