Abstract: A marine propulsion device comprising an engine including an exhaust outlet, a lower unit adapted to be mounted on a boat for pivotal movement relative thereto about a generally vertical steering axis, the lower unit including a rotatably mounted propeller driven by the engine, an annular chamber having a circumferential length and including a lower portion filled with water, an exhaust gas inlet located below the level of the water and communicating with the exhaust outlet, and an exhaust gas outlet located above the level of the water, and a seal for requiring exhaust gases flowing from the chamber inlet to the chamber outlet to flow around the chamber through a distance substantially greater than 50% of the circumferential length of the chamber.