PT Boat - Engines

Engines

All U.S. PT boats were powered by three 12-cylinder gasoline-fueled engines. These engines were built by the Packard Motor Car Company, and were a modified design of the 3A-2500 V-12 liquid-cooled aircraft engine. The 3A-2500 was an improved version of the 2A engine used on the Huff-Daland XB-1 Liberty bomber of World War I vintage. Packard modified them for marine use in PTs, hence the "M" designation instead of "A". (i.e., 3A-2500 then 3M-2500). The three successive versions of these engines were designated as 3M-2500, 4M-2500, and 5M-2500, each of which had slight improvements over the previous version. Their aircraft roots gave them many features of aircraft engines, such as superchargers, intercoolers, dual magnetos, two spark plugs per cylinder, and so on. Packard built the Rolls-Royce Merlin aero engine under license alongside the 4M-2500, but with the exception of the PT-9 prototype boat brought from England for Elco to examine and copy, the Merlin was never used in PTs. The 4M-2500s initially generated 1200 hp (895 kW) each, together roughly the same power as a Boeing B-17 bomber. They were subsequently upgraded in stages to 1500-hp (1,150 kW) each, for a designed speed of 41 knots (76 km/h (47 mph)). The final engine version, the Packard 5M-2500, (late 1945) had a larger supercharger, aftercooler, and power output of 1850 hp. This much power could push the fully loaded boats at 45 to 50 knots. However, using the older 4M-2500 engines, increases in the weight of the boats due to more weaponry offset the potential increase in top speed. Fuel consumption of these engines was phenomenal; a PT boat carried 3,000 gallons (11,360 liters) of 100 octane avgas. A normal patrol for these boats would last a maximum of 12 hours. The consumption rate for each engine at a cruising speed of 23 knots was about 66 gallons (250 l) per hour (200 gallons per hour for all 3 engines). However, at top speed, consumption increased to 166 gallons (628 l) per hour per engine (or 500 gallons per hour for all 3 engines). Navy acceptance trials for every boat required it be able to demonstrate ability to achieve design speed of 41+ knots. Going at this speed, the 3,000 gallons of fuel would be used in only about 6 hours. Wartime conditions such as hull fouling and engine wear could sometimes cause the boats top speed to be degraded until maintenance could be performed.