Gray Marine Motor Company was an important marine engine builder. It built engines ranging from one to six cylinders in both gas and later diesel layouts which were used in pleasure and work boats.
Gray was an American firm, based in Detroit from 1901 until 1967. Many fishing boats, lobsterboats, tugs and pleasure craft used Gray engines. These boats were usually of small size, ranging from 12 to 32 feet. Many of their engines were marinized automotive engines from Hercules, Studebaker, Pontiac, Continental, American Motors or General Motors Diesel Division.
Gray also produced a line of outboard motors without the typical ninety-degree gearbox. A curved housing connected motor to propeller. The lower-unit housing contained a flexible inner rotating shaft.
Gray Marine started as the Michigan Yacht and Power Company by O. Mulford in 1892 and was a dealer for the Sintz gas engine, one of the first marine engines. It bought out Sintz in 1902. Michigan ventured into automobile engines and was re-branded Gray Motor Company about 1910 by O. Mulford and his partners Paul and David Gray. Charles King, one of the investors, used a Gray engine in King automobiles. Within a few years, the Gray Motor Company was building 7000 engines a year. Some would go into World War I lifeboats, and others would be used for pump units for Western Front trench service. These were called Victory motors. Victory engines were also used by Traffic Motor Truck Corporation of St Louis, Kohler Truck of New Jersey, Panhard truck, and the Crow-Elkhart car. Prairie Queen tractors also used this engine in 1922.
In 1921 Frank L. Klingensmith, William Blackburn (from Cadillac) and Frank F. Beall (from Packard) took over the Gray Motor Company, renaming it Gray Motor Corporation with $4 million capital, with the intent of competing with Ford. Two models of Gray car were made using the Beall developed Z motor. The Z engine was a 12-18 horsepower, 4 cylinder, L-head design that was said to resemble the model T Ford engine.
By 1924 the company was in poor financial condition and Mulford managed to buy back the marine engine division, re-establishing Gray Marine Motor Company. Gray Motor Corporation ceased producing cars by 1926.
The marine engine division continued operations for over forty years, and is most known for converting automotive engines for fishboats, cruisers and World War II landing craft, such as the Canadian Ramped Cargo Lighter and the famed Higgins Boats. Gray built their own engines up to 1924, but converted automotive engines from about April 1924 on. During WWII, Gray Marine built 100 marinized GM style 71 series diesels a day using GM cylinder blocks. 
On June 14, 1944, the company was purchased by Continental Motors Company for $2.6 million. John W. Mulford, the son of O. Mulford, was made general manager of Gray. Gray continued to make marine engines in the post-war period until its closure by Continental in about 1967.