The Packard V-12 PT Boat Engine

The PT boats made by Elko, Higgins and Huckins were lightning fast and all powered by Packard V12 engines of similar types. Even before the renowned Plywood Derby nailed down the final boat designs, the engines had already been developed and were on order for the Navy. All PT boats used in World War 2 were powered by these beefy V12 engines, about which there is a fair amount of misunderstanding. Let's take a look into what they were and were not. There is a common misconception that these were the Packard-built versions of the Rolls-Royce Merlin aero-engine known as the V1650. This is incorrect. The Packard Merlin did not come online until 1941 and at that time all of them were pre-ordered for urgent aircraft production. No surplus units were available until 1943. The other common misconception is that they were the earlier Packard Liberty L12 engines, which is also incorrect. This error is compounded by the existence of an inverted version of the Liberty engine being given the same V1650 designation, though some years earlier. Let's first have a look at the Liberty L12, since this is the grandfather of the PT boat engine. When America declared war on Germany in May 1917, there was an immediate need for aircraft and a robust, powerful and lightweight engine with which to power them. The head of the aircraft production board summoned two top designers, one Jesse G. Vincent from Packard Motor Company and Albert J. Hall of the Hall-Scott Motor Company, to the Willard Hotel in Washington DC and basically locked them in there until they came up with a suitable design. The engine was needed for over a dozen projected aircraft from the Airco DH-4 through to the XHB-1 Heavy Bomber. I guess they missed family life because in just five days they had come up with the drawings for a 45 degree V12 of 1640 cubic inches displacement with overhead cams, two valves per cylinder, and liquid cooling. Packard built a prototype that ran well and made the projected 400 horsepower, and that was approved in August. Come late fall, the War Department placed orders for 22,500 engines, now designated the L-12 or Liberty 12, with Packard, Buick, Ford, Lincoln, Cadillac and Marmon. Hall-Scott did not have any production capacity to get an order. Close to 20,000 engines had been delivered when the end of the war signaled the end of production, with Packard finishing engine number 6,500 on June the 4th, 1919. Although this engine had a displacement similar to the Merlin, the other similarities are quite minor. One major difference between the L-12 and the Merlin is that the cylinders on the Merlin are cast into upper and lower cylinder blocks, while the L-12 has 12 individual cylinders with integrated heads versus the Merlin's one-piece cylinder head castings. The L-12 had a 45 degree V angle versus a more conventional 60 degree in the Merlin. The 45 degree V angle was widened to the more common and better balanced 60 degrees. Where the Liberty engine looks a bit like six Harleys strapped together, the new engine looks more like a modern automotive V-12. Stroke was reduced from 7 inches to 6 1⁄2 inches, while the bore was increased from 5 inches to 6 3⁄8 inches, resulting in a swept volume of 2,500 cubic inches. Do we start to see where the Packard model numbers come from now? Where the L-12 had one-piece cylinders and heads, the new unit had forged steel cylinders with the combustion chamber integral with the cylinder, but used an aluminum valve cover and port unit that, like a cylinder head, had the intake and exhaust ports for the four valves, valve guides, water jackets, and the camshaft bearing pedestal all integrated. All of this being bolted to the top of the closed cylinder. One advantage with this idea is that there is no cylinder head gasket to blow out or leak from exposure to combustion pressure or coolant. With this change, all of the valve train was now enclosed. The bevel gears and shaft drive for the camshafts were retained, but the points and coils ignition the L-12 cams drove was ditched in favor of two magnetos and 12-cylinder distributors. Earlier units have the timing advance control slave to the throttle lever, while later ones are fully manual. Lubrication is handled with a twin-circuit oil pump with a three-gear rotor set to scavenge oil to the external oil tank, and two to provide pressure to the bearings and feed oil squirters. The first generation 1.8 5A-2500 of 1924 produced around 600 horsepower. This engine was a commercial flop in comparison to the L-12, with only a few hundred being built. The unit was developed little by little, powering aircraft like the Martin T3M and Boeing TB. The final 5A-2500 of 1930 was supercharged and produced 1,500 horsepower at the emergency power setting, which was impressive for the time. Still, with the war over and aircraft design moving on, sales did not really take off, with total production numbers in the mere hundreds. For reference, the most popular version, the 3A with 800 horsepower, sold for about $20,000. During this time, the Army Air Corps had shown little interest in this engine or the aircraft it powered, so obviously when the Navy came along and asked for a marine version, Packard was only too happy to help. If you ever wondered why Liberty engines are too a penny, and Packard 2500 series units are so scarce, well now you know. The marinised version started with, you guessed it, the 1M-2500. The 3M-2500 was the model fitted to the early Elko 77ft PT boats, later being updated to the 4M and 5M models, though not many 5M units made it into service before the end of the war. As a marine unit, the exhaust manifolds must be water-cooled or risk melting at higher loads. The manifolds are welded fabrications that have flanges at both ends, one for the exhaust pipe to be fitted and the other one to drain coolant from the manifold. They can be switched one way or the other, which allows the exhaust to exit from either end of the engine. A 5-inch exhaust system was specified for each cylinder bank, and with those long drain pipes fitted, they make a most impressive row. Remember that in the 77ft Elko boats, the two outer engines drive their propellers via a V-drive, while the center engine is mounted further forward and has direct drive. Most engines featured both a freshwater and raw water pump, though you could get them without a water pump if required. The crate engines from Packard would have the complete ignition system, carburetor, starter, generator, reverse gear unit, fuel pump, oil pumps, and oil tank. They did not come with a heat exchanger for the cooling system, a fueling system, instrumentation, or much else. Despite the marine application, the M-Series carried on with its two spark plugs per cylinder. All engines were supplied with 24 volt starter motors and DC generators, with quite primitive voltage regulators set to output 28 volts. The regulators being electromechanical, they have complex service needs, and like many of the other systems, require frequent tuning to get them to work correctly. Not only did voltage output have to be set on each unit, but all three in a PT boat needed to be balanced. Now throw in the fact that regulator output will rise as the unit reaches working temperature, and you can see how a mechanic could be chasing his own tail trying to do this. You can see from these pages in the service manual just how many tasks had to be performed at various intervals, some of which could be performed in situ, and some requiring the engines to be removed. These engines, being designed as aero engines from the outset, were built quite lightly. They are tested to be reliable with the scheme of maintenance that is shown in the manual, but are going to get fussy on poor fuel or when service work is skipped. The guys you see here swapping engines are not doing it because the gym is closed. Like many aero engines, they have an overhaul life set at 750 hours. At that point, the whole unit is supposed to go to be completely reconditioned. The 4M engine still had a few variations between the models, like the cooling arrangements and spark advance controls, but most importantly with the later versions getting an improved supercharger, which was more efficient and allowed higher boost pressure, emergency power rating jumping from 1350 horsepower to 1500. Boost limits however depend on fuel octane, and the engine is designed to run on 100 octane avgas. They can be run on 91 octane fuel, but with the penalty of maximum boost pressure being reduced. Timing however can be left alone. To run the engine on 87 octane gas, both timing and boost have to be reduced, which obviously reduces power output by quite a bit. Total production was somewhere over 12,000 units, so it was a lot more profitable for Packard than the A-series had been. The final 5M unit has an aftercooler integrated into the manifold, though I'm not sure if that was exploited for more power or as an improvement to reliability. The last running examples of these engines are installed in the Higgins-built PT658, located in Portland, Oregon.