A marine propulsion system for a cruising boat or the like which utilizes two engines, one of the engines having substantially more power than the other. The propulsion system provides for twin propellers and propeller shafts and either engine (but not both) can be made to drive the twin shafts. The engines are positioned in line with one another on the fore and aft center line of the boat and the drive shafts of the two engines extend into a common transmission assembly. Clutches in the transmission assembly permit either one of the engine drive shafts to be drivingly connected to an athwartships shaft the ends of which are respectively connected through transmission gear case units to the propeller shafts. The propeller shafts are laterally spaced from one another and are parallel to the center line of the boat. With this arrangement, the more powerful engine can be used for high speed cruising or when severe weather conditions dictate and the less powerful engine can be used for slow speeds and economical long distance cruising.