Most US Navy PT boats of World War II had three Packard V-12 marine gasoline engines, from the Packard 4M-2500 family. They were supercharged, liquid-cooled, and ran on high-octane aviation gasoline. Each engine drove its own propeller shaft, which is why PT boats have three propellers. This applied to both main wartime types, the Elco and the Higgins.
The Packard 4M-2500
The 4M-2500 was a marine engine derived from an earlier Packard aircraft engine design. It displaced around 2,500 cubic inches, hence the “2500” in the name. Sources generally give output of around 1,200 hp per engine on early wartime versions, rising to roughly 1,500 hp on later ones, and some late or postwar variants are quoted higher still. Exact figures vary between sources and versions, so treat any single number with care.
With three engines, a PT boat had several thousand horsepower in a wooden hull of about 78–80 ft. That gave top speeds commonly quoted at around 40 knots in good condition, though a heavily loaded boat in service was usually slower.
How three engines fit into the hull
The engines sat in a compartment in the middle of the boat. Commonly described arrangement:
- The center engine drove its shaft directly.
- The two wing engines were mounted facing the other way and drove their shafts through V-drive gearboxes, which turned the drive back toward the stern.
This layout kept the engine room compact and let all three shafts exit the hull at a reasonable angle.
Not a Merlin, not a Liberty
Two engines often get mixed up with the PT boat’s:
- Rolls-Royce Merlin: Packard also built Merlins under license for aircraft such as the P-51 Mustang, so people sometimes assume PT boats used them. They didn’t. The 4M-2500 was a separate Packard design for marine use.
- Liberty L-12: The WWI-era Liberty aircraft engine is sometimes named as the PT boat engine, but the standard wartime boats used the Packard 2500-series.
Fuel, noise and mufflers
High-octane gasoline in a wooden hull was a real hazard, and fire was a serious risk when boats were hit. The engines were also very loud. Boats were fitted with mufflers, which crews could use to run more quietly, for example when approaching a target at night.
Exceptions
- PT-9, the British-designed Scott-Paine boat bought by Elco as a pattern, is generally reported to have used Rolls-Royce Merlin engines.
- Postwar experimental boats tried different engine setups, but they were few and are rarely modeled.
What this means for your model
Even if your kit has no engine room, the engine layout shows up on the outside:
- Three propellers, three shafts, three struts. Check that the outer shafts are angled correctly and match the center shaft’s height.
- Engine room hatches and ventilators on deck sit above the engine compartment.
- Mufflers at the transom are a visible detail on many boats. Aftermarket photo-etch sets for 1/72 Elco kits often include muffler baffles and linkages.
- Metal shafts are worth considering as an upgrade. Kit plastic shafts can be soft and easily bent.
For kit options at different scales, see our guide to PT boat model kits, or our motor torpedo boat plans for a scratch-built hull. If you want to see the real engines, some surviving PT boats have been restored to running condition, and our article on how PT boats were built covers the hulls that carried them.