As told by John Sheldon
Former OMC rotary engineer

After the successful introduction of the 35 HP and 45 HP air cool/charged cooled snowmobile engine, OMC started working on water cooled/charged cooled variants of the same engine. Basically all the parts were the same except the housings, which were water cooled. Contrary to previous OMC, Curtis Wright and Mazda water cooled engines which had the water flowing axially; parallel to the crankshaft; these engines were partially circumferentially cooled. Water entered before the spark plug, ran around the rotor housing and exited around the exhaust port. The side housings picked up water at the entrance point for the rotor housing, travel across the hot section of the housing and exited at a low pressure point after the exhaust port. Both single and dual rotor engines had been prototype and development work had begun. The single rotor produced 50/55 HP and the 2 rotor produced 110/120 HP. The increase in HP was due to the improved volumetric efficiency (increased air flow) due to the lower temperatures resulting from water-cooling. Both these engines were configured for outboard use and were coupled to current lower units. At one board of directors meeting the twin rotor engine was mounted to a boat for demo rides. The tachometer was disconnected and the engine was left running. Every one that got into the boat turned the key to start the engine. There was no noise, no motion, no vibration. It was impressive.