In another thread somewhere there was a short discussion of the engine used in the US PT boats of WWII. I was referencing some information today about this engine and thought I'd stick this up here for anyone who's interested.
The photo is from my copy of the operations and service manual for the Packard 4M-2500 marine engine.
While it is not an aircraft engine per se, its development has its foundation in an aviation engine, the famous Liberty V-12 of WWI. The Liberty was morphed by Packard into a series of engines for racing powerboats and hydroplanes during the 1920s and 30s. The engine used in the PT boats, the Packard 4M-2500, was a direct derivative of the latest of the racing boat engines.
But it's aviation heritage remained to a degree. It is an individual, wet-sleeve cylinder engine. When the cooling jackets welded around each cylinder would develop pinholes the PT motor macs would wrap a piece of rubber around the jacket and secure it tightly with big hose clamps.
The engine had a single magneto that fed power to two distributors. Each cylinder had two spark plugs, one powered by each distributor.
Horsepower was 1,200 @ 2,400 rpm. In an emergency the engine could be run at 2,500 rpm.
Here is a footnote from the manual that might be of interest to the folks who like to debate power, prop size, etc. This was Packard's take on it at the time:
"Horsepower at any engine speed is a function of propeller size and boat load and is indicated by manifold pressure. Propellers should be selected to permit the engine to turn up to 2,500 rpm with the manifold pressures shown in the above table when the boat is properly trimmed and carrying maximum service load including full fuel tanks."
Full fuel in this case was 3,000 gallons.
Engine power was set using manifold pressure and the manual has charts for the various combinations of manifold pressure and rpm.
The three, three-bladed propellers all turned the same way with left-hand pitch.
The photo is from my copy of the operations and service manual for the Packard 4M-2500 marine engine.
While it is not an aircraft engine per se, its development has its foundation in an aviation engine, the famous Liberty V-12 of WWI. The Liberty was morphed by Packard into a series of engines for racing powerboats and hydroplanes during the 1920s and 30s. The engine used in the PT boats, the Packard 4M-2500, was a direct derivative of the latest of the racing boat engines.
But it's aviation heritage remained to a degree. It is an individual, wet-sleeve cylinder engine. When the cooling jackets welded around each cylinder would develop pinholes the PT motor macs would wrap a piece of rubber around the jacket and secure it tightly with big hose clamps.
The engine had a single magneto that fed power to two distributors. Each cylinder had two spark plugs, one powered by each distributor.
Horsepower was 1,200 @ 2,400 rpm. In an emergency the engine could be run at 2,500 rpm.
Here is a footnote from the manual that might be of interest to the folks who like to debate power, prop size, etc. This was Packard's take on it at the time:
"Horsepower at any engine speed is a function of propeller size and boat load and is indicated by manifold pressure. Propellers should be selected to permit the engine to turn up to 2,500 rpm with the manifold pressures shown in the above table when the boat is properly trimmed and carrying maximum service load including full fuel tanks."
Full fuel in this case was 3,000 gallons.
Engine power was set using manifold pressure and the manual has charts for the various combinations of manifold pressure and rpm.
The three, three-bladed propellers all turned the same way with left-hand pitch.