Exhaust system for marine propulsion unit

An outboard motor comprised of an internal combustion having at least one exhaust port for discharging exhaust gases and an outer housing assembly comprising a driveshaft housing and a protective cowling surrounding said internal combustion engine, said driveshaft housing containing a driveshaft for driving a propulsion device for propelling an associated watercraft, an exhaust system for said engine comprised of an inlet for receiving exhaust gases from said exhaust port, high speed underwater exhaust discharge to which exhaust gases are delivered from said engine through said driveshaft housing and an above the water exhaust gas discharge formed in said outer housing assembly at a point above the water level even when the outboard and associated watercraft are stationary communicating with said exhaust system for discharging exhaust gases directly to the atmosphere, and an exhaust treatment device detachably affixed to said outer housing on the exterior thereof and having an inlet opening communicating with said above the water exhaust gas discharge and an outlet opening communicating directly with the atmosphere externally of said outer housing.