The marine propulsion device includes a lower unit having a gear case normally submerged in water and arranged to discharge engine exhaust gas rearwardly from the rear end into the water exteriorly of the propeller hub. A ring member or thrust ring including a hub mounted on the propeller shaft for common therewith, an annular collar and a plurality of circumferentially-spaced, radially extending spokes or ribs interconnecting the annular collar and the hub serves as a fishline and weed cutter. The annular collar includes an annular outer surface which extends closely adjacent an annular inner surface in the interior of the gear case adjacent the trailing edge of the gear case. In one embodiment, forward migration of the engine exhaust gases along the outer surface of the gear case, which can cause unstable handling of the lower unit, is prevented by an anti-ventilation means including an annular section on the ring member collar located rearwardly of the annular outer surface and rearwardly of the trailing edge of the gear case and having an outer surface tapering radially outwardly from the outer surface of the gear case toward the propeller.