The History of the OMC Wankel Rotary Program
by John Sheldon
former OMC project engineer on rotary engines

Part 1; Oil Cooled Rotors

In 1958, Curtis Wright signed an exclusive agreement with Wankel GMBH giving them exclusive manufacturing, marketing and sublicensing rights for North and South America as well as a portion of the same worldwide. In 1966, OMC signed a sublicense with Curtis Wright and Wankel GMBH to develop, manufacture and sell Wankel engines for the recreational market worldwide. The original Wankel engine had a stationary crankshaft with the housings rotating around their own centers. This allowed for dynamic balance without the use of counter weights, but had the added complexity of a moving sparkplug and a complicated induction system thru the crankshaft. Max Bentle of Curtis Wright is credited with the kinematic inversion of the engine allowing the housing to remain stationary with a rotating crankshaft. This simplified the engine immensely, but required the addition of counter weights on each end of the crank to achieve dynamic balance. Curtis Wright developed single and twin 60 ci per rotor engines. These engines were both water and air cooled externally and oil cooled internally. The pressurized oil system allowed the use of conventional babit bearings and with the addition of an oil cooler to maintained internal temperatures adequately. I believe Curtis Wight was more interested in selling licenses than producing engines. Curtis Wright went on to develop several different engines from a small lawnmower size to enormous oil field engines.

OMC started their development work at their Research Center in Milwaukee. They took the basic Curtis Wright 60ci engine and started development work on trochoid coatings and apex seals. Research changed the original CW engine by reducing the displacement to 50ci/rotor and using 2 main bearings instead of the 3 used by CW. The target was 80 HP using side porting. Curtis had use “D-Gun” tungsten carbide from Linde as their coating, which was very effective, but extremely expensive. OMC hired 2 Curtis Wright Engineers, Harry Ward and Mike Griffith. Harry was a former Chevy engine engineer, having worked on the Corvair engine and Mike’s background was in the ag business. Less Foster was named manager of the rotary engine group. OMC also hired Ralph Treadway from GM. Less’s group started a production design of a 2 rotor 100ci engine in hopes the research guys could develop a suitable coating/seal combination before production started. The engine was initially designed as an outboard with it’s own new midsection, gear case and motor cover. The engine was designed to produce 200hp at 6500 RPM. It had water-cooled housings and oil cooled rotors similar to the Curtis Wright engine. It had an oil to water heat exchanger, was peripheral and side ported, 2 oil pumps and used the midsection as the oil reservoir. The engine produced 210 hp. The one difference to the Curtis Wright engine was the oil seal. Curtis had a highly loaded scraper type oil seal and OMC followed Mazda with double oil seal with wave springs and o-rings. This reduced friction hp, provided lower temperatures for the O-rings and was more compliant to side housing distortion.

I joined OMC in the summer of 1968 and was put in the OMC 1 year new hire training program. At that time OMC was totally integrated, from melting their own aluminum, die casting their own parts, carburetion, electrical, gears, painting and machining. In fact OMC was the largest aluminum captive die caster in the world, casting 250,000-300,000 lbs of aluminum a day. The training program assigned me to every department in the plant for 6 months and than 3 months each in 2 of the engineering departments. My second stint was with the rotary group where Ralph Treadway too me under his wing. I was assigned to the oil seals, as what we had at the time didn’t work very well; very high oil consumption. Ralph was also assigned to lay the engine down for a stern drive. Without the midsection for the oil reserve, the housings had to be modified to include an oil pan ala automotive along with a high rise exhaust. The engine turned out really neat as it was so small a complete rear seat could be built in the boat eliminating the big box common to stern drives with automotive engines. Being dynamically balanced, there was virtually no vibration felt in the boat. It was also hundreds of pounds lighter than the V-8 being used and as such would out accelerate them even with 100 less hp. My seal work went on with some successes and some not so successful. The seals were being made in Japan, so the iteration time between design/development changes took some time. Oil consumption finally was under control and both the outboard and stern drive versions performed very well. Remember at the time, 125 hp was the largest engines available. The outboard version pushed a 23’ Wellcraft cuddy cabin at 52 MPH. Quite an accomplishment for that period. OMC was in the boat business, so the stern drive version was in an OMC boat, known for its heavy weight and lack of performance. The rotary allowed a full rear seat and performed well against the V-8 in the same boat. Both versions meet the design requirement of 100 hrs @ WOT. Less Foster kept pushing to release both units to production, but the powers to be insisted on a true manufacturing costing vs Less’s estimates. The result was disastrous! The true manufactured cost was twice what Less estimated; opps! Less left the company and George Miller took over as head of the group. Mike Griffith and I were assigned to a major cost reduction design of the oil cooled engine program, while Harry Ward started work on a snowmobile engine. The original oil cooled engine was made from all sand cast parts from an outside source. The trochoid coating was still “D-Gun”, as research hadn’t developed a lower cost suitable coating yet. The first step was to redesign all the housings as high-pressure die-castings. We even explored die cast aluminum rotors and 2 piece center housings glued together. After several months of work, it became apparent an oil-cooled engine could not be made cost competitive with a 2 cycle and work on oil cooled engines stopped. I believe stopping work on the stern drive version was a mistake, as it was lower cost than the Detroit V-8 being used, offered performance advantages and allowed the removal of the “dog house” in the back of the boat. It was about this time Ralf Treadway left OMC and returned to GM to work on their rotary program.