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Propeller efficiency is used to define how well a propeller transmits its rotational force or energy into thrust. The propeller, whether it be used to power a boat or an airplane, must turn rotational energy into forward thrust or reverse thrust when used on a airplane or boat. The amount of energy it takes to rotate the propeller is almost always greater than the thrust from the propeller. Reducing this loss is the goal of propeller efficiency.
The amount of thrust generated by a propeller is controlled by the angle at which its blades attack the air or water it is spinning in. Propeller efficiency lies in these same blade angles. By producing a blade with the correct angle and attaching it to a properly sized hub, the propeller efficiency can be drastically altered. Therefore, it is the design and shape of a propeller that defines its efficiency more than the speed at which it is turning.
In a jet engine, the efficiency of the engine is measured as a fraction of the potential heat energy of the propellant fuel as converted to thrust energy. With a propeller-driven aircraft, propeller efficiency is measured as horsepower and not thrust. This relates to the horsepower of the engine along with its ability to make power to drive the airplane.
One of the most efficient propeller driven aircraft was the Wright R-3350 turbo-compound radial engine. This piston-driven aircraft engine was able to capture some of its exhaust energy due to having three turbo-chargers coupled to its drive shaft. This allowed the engine to reach an overall propulsive efficiency of about 32 percent at Mach 0.5. This number is significant due to the wind resistance as well as the thermodynamics of pushing a propeller driven aircraft through the wind.
Poor propeller efficiency is due, in part, to the propeller's struggle to go through the wind. The propeller not only fights its way through the wind straight ahead of the aircraft, but each propeller blade has to fight its way through the air in front of each propeller blade as it makes its revolution around the crankshaft. This double-drag coefficient takes its toll on propeller efficiency.
Whether it be water or wind, the propeller efficiency of any given craft suffers from the drag of the environment it travels through. The resistance of friction and drag causes the propeller to consume more energy than it produces. Evolutions in propeller design and materials have increased the efficiency of these propellers; however, they will never have the efficiency of more modern jet engines and hydro-propelled water craft engines.