Gray Marine 6-71 Diesel Engine

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The 6-71 Gray Marine Diesel Engine Training Engine aboard the Training Ship Golden Bear

The 6-71 Gray Marine Diesel Engine is a marine diesel engine, a marinized version of the General Motors Detroit Diesel 6-71 engine produced by the Gray Marine Motor Company. It was used in landing craft during World War II and is used today in private boats and training facilities.

The 6-71 is a six cylinder engine, and is "inline" or with all cylinders arranged in a straight line.[1] This type of engine is commonly referred to as an "inline six" or an "I-6." The 71 refers to the displacement per cylinder which is 71 cubic inches.[1] The firing order of the engine, or the order combustion occurs in successive cylinder, is 1-5-3-6-2-4.[1] The engine's compression ratio is 18.7:1 with a 4.250 inch bore and a 5.00 inch stroke.[1] The engine weighs 2,185 lb (991 kg) and is 54 inches long, 29 inches wide and 41 inches tall.[1] At 2,100 revolutions per minute the engine is capable of producing 230 horse power (172 kilowatts).[1] V-type versions of the 71 series would not be developed until 1957.

The 6-71 is a two stroke diesel engine.[2] As the engine will not naturally aspirate, air is provided to the engine via a roots-type supercharger.[3] However, on the 6-71T models, both a turbocharger and a supercharger are utilized.[2] Fuel is provided by Unit Injectors, one per cylinder.[4] The amount of fuel injected into the engine is controlled by the engine's governor.[1] The engine cooling is via liquid in a water jacket. In a boat, external cool water is pumped through the boat and passed through a heat exchanger mounted on or nearby the engine in a "double loop" configuration.[4]

Side cutaway view of the Gray Marine 6-71


The Gray Marine 6-71 Marine Diesel was developed from the General Motors 6-71 diesel engine that was in use for on-road applications, farm machinery, and other off-road applications. The engine was installed as a pair in one variant of the M4 Sherman tank.[5] Gray picked up the contract to convert the engine over for marine purposes.[6] iuiu6-71 was used for Higgins LCVP, a landing craft used in larger numbers during World War II.[5] During the war years, about 100,000 (including the Gray-Marine variant) GMC 6-71 were built, serviced, and operated.[3]

Use in D-Day landing craft[edit]

When Andrew Higgins saw the suggested model for the D-Day landing craft, as suggested by the world's top boat designers, Higgins said, as a newspaper quoted "something to the extent of 'This boat is lousy' and 'this boat stinks'.[5] The oddly shaped boat which emerged from Higgins mind that could not only beach, but then pack up and head back out to the fleet was dubbed the LCVP and then the most powerful[citation needed] compact engines in the marine world were placed inside. The founder of Gray on June 6, 1944 commented to a local newspaper that his greatest dream was achieved that day, that when the crafts carried US soldiers into Normandy for the invasion, they were propelled by Gray Marine diesel engines.[5]


  1. ^ a b c d e f g Barrington Diesel club. "Detroit Diesel 6-71 engine specs at Barrington Diesel Club". Retrieved 11 April 2012. 
  2. ^ a b Tugboat Enthusiasts Society of the Americas. "DETROIT DIESEL INLINE 71 SERIES". Retrieved 11 April 2012. 
  3. ^ a b Caprio, Dennis. ""Jimmy" Rules: Detroit Diesel 6-71". Yachting Magazine. Retrieved 18 April 2012. 
  4. ^ a b G.M. Grey Marine Model 64 HN9. Seattle, Washington: Hatch and Kirk INC. 
  5. ^ a b c d Grayson, Stan (1999). Engines Afloat. Devereux Books. ISBN 0-9640070-7-X. 
  6. ^ Homfield, Max. "A Brief History of Gray Marine Engines". Gas Engine Magazine. Retrieved 18 April 2012.