Gray Marine 6-71 General Motors Series

The Gray Marine 6-71 in the Higgins Boat

When we talk about a great or historic engine, sometimes we need to talk more about what it powered, because that’s what made it famous. It’s beyond argument that the Gray Marine 6-71 series, the marinized version of the General Motors  71 Series, deserves a high spot on the list of the world’s most influential diesels. Few of it’s exploits are more memorable, or meaningful, than when it powered the World War II LCVP, Landing Craft Vehicle Personnel, the legendary Higgins Boat.

The P-Boat

Known to troops and sailors alternatively as the P-Boat, Peter-Boat, Vee-Pee or  Veep, the 36-foot landing craft had it’s origins in an early-1930s design called the Wonderboat. Developed by Andrew Jackson Higgins, it was not a landing craft, but a shallow-draft work boat. That design evolved into the Eureka, which was completed in 1937. It was a beachable, shallow draft workboat that drew the attention of the United States Navy and Marine Corps when they were looking to upgrade their amphibious operations.

In 1940, the Navy conducted trails of an assortment of landing craft and the Eureka came out on top due to it’s combination of speed, simplicity, low cost and an almost uncanny ability to get itself on and off a beach. This early design was considerably different than the later LCVP in that it didn’t have the now well-known bow ramp. The production model emerged in 1941 and was called the LCP(L) (Landing Craft Personnel, Large) and disembarking troops were required to leap off the sides. The “large” came from the fact that the original design was 30 feet in length and the updated production unit was 36.

In 1941, the LCP(L) evolved into LCP(R) (Land Craft Personnel, Ramp), which had a narrow bow ramp to disembark troops more easily and a pair of machine gun tubs. Later in 1941, the more familiar full-sized bow ramp was developed and went into production as the LCV (Landing Craft Vehicle). It didn’t take long to realize the LCV setup could lend itself to all tasks and so in 1942, with a few other tweaks, it became the LCV(P), the parenthesis later deleted.

The LCVP was 35 feet, 9 inches long 10 feet, 6.25 inches wide and had a 3 feet, 9 inch draft fully loaded. The hull bottom used double planked mahogany on oak frames. The sides were 1-inch marine plywood with 1/4-inch armor plating  to protect from small caliber weapons The ramp was made from 1/4-inch armor plate. It carried 36 fully equipped troops, plus three (sometimes four) crew, mixed cargo or a small vehicle such as a jeep or a light truck. Maximum cargo weight was 8,100 pounds. The empty boat weighed 18,500 pounds.

LCVP Power

Higgins discovered early that in order to get on and off a beach with a heavy load, a lot of horsepower and a big prop were needed. His military designs were initially powered by 250 horsepower Hall-Scott gas engines. Navy planners balked at the fuel consumption but Higgins demonstrated time and time again, the power was necessary. Many of the earliest Navy Higgins boats were Hall-Scott powered, as well as a version sold to the British. The Brits were nervous enough of gasoline that they ordered their boats with self-sealing fuel tanks. Enter Gray Marine.

Gray had been in business since 1910, first as the Gray Motor Company building cars and car engines. That venture was less than successful, so they became Gray Marine in 1924 and switched to converting a variety of automotive and industrial engines to marine use. They were very big by the late 1930s and scored the exclusive contract to marinize the new 71 Series General Motors diesels when they first rolled off the line in 1937. General Motors already had a relationship with the Navy via the Cleveland Engine Division (formerly Winton) and Electro-Motive Division (EMD), but these were all large diesels. The Gray Marine 6-71 was about the same weight and size as the Hall-Scott and had nearly the same maximum output, with fuel economy 20-30 percent better. Plus, diesel fuel was much safer in a boat that was being shot at.

Those first generation 6-71s had some weak links compared to the later ones we are more familiar with. Maximum intermittent power was rated at 225 horsepower at 2100 rpm and when used it trucks, that was the typical rating. The continuous horsepower rating was 165 at 1800 for stationary and marine engines. Using the 6-71 at it’s continuous rating resulted in a service life in excess of 2000 hours. Used continuously at the max rating, it was less than half that. Generally you bought a 6-71 with one of two injectors; the GM60 delivering your continuous 165 ponies at 1800 and the GM90 giving 225 horses at 2100. To give the best of both worlds in a military environment, a special governor was designed with two settings, “Normal” and “Battle,” to be combined with the GM90 injectors. For training and everyday use, “Normal” was selected. When bullets were flying, “Battle” was used.

In the Battle setting, the Gray Marine 64HN9 could power a fully loaded P-Boat to 9 knots (10.3 mph) for 110 miles until the 180 gallon fuel tank ran dry. The engine was backed up by a Twin-Disc gearbox with a 1.5:1 reduction and generally swung a 22×20 propeller (22 diameter, 20 inch pitch)… pretty big for a 36 foot boat. It delivered a lot of thrust but not a lot of speed. The LCVP could do 12 knots (13.8 mph) empty.

LCVP Evolution

The LCVP variant proved versatile enough to be used in most applications. The LCV(L) and LCV(R) remained in use as command, repair, rescue and special duty boats. In all, 23,398 Higgins boats of all types were built by eight different manufacturers, including Higgins, reportedly 22,492 of them being the LCVP type.

The LCVP did not fade away after the war. While the wartime wooden boats had a relatively short working life, LCVPs were still needed and the Navy had an elaborate rebuilding program. Starting in 1946, the began Navy experimenting with materials less maintenance-intense than wood, and fiberglass emerged the winner. Fiberglass P-Boat production began in 1950 and gradually ramped up through the ‘50s and ‘60s, finally ending around 1971. United Boatbuilders of Bellingham, Washington, constructed most of them. While they had evolved somewhat from the WWII design, they were largely the same and still powered by a Gray 64NH9. A major feature change was the lack of the twin .30 caliber machine gun tubs.

The LCVP has been characterized by historians, generals and presidents as one of  a handful tools in the Allied arsenal vital to winning World War II. It was the perfect blend of performance, price and utility. They were quite literally throw-away boats with an engine so highly stressed that its life was given in the hundreds of hours. But in those few hundred hours, the cheap-ass LCVPs made history.

Sources

D-Day Conneaut
www.ddayohio.us

Naval  History  and  Heritage Command
www.history.navy.mi