Propellers

A propeller is a device which transmits power by converting it into thrust for propulsion of a vehicle such as an airplane, ship, or submarine though a fluid such as water or air, by rotating two or more twisted blades about a central shaft, in a manner analogous to rotating a screw through a solid. However, the blades of a propeller don't push directly on the fluid, but rather act as rotating wings, and produce force via Bernoulli's principle, generating a difference in pressure from the forward and rear surfaces of the airfoil-shaped blades.

History

The principle employed in using a screw propeller is used in single oar sculling, a practice with a history of unknown length. It is part of the skill of propelling a Venetian gondola but was used in a less refined way in other parts of Europe and probably elsewhere. For example, propelling a Canadian canoe with a single paddle, involves a related but not identical technique.

In sculling, a single blade is moved through an arc, from side to side taking care to keep presenting the blade to the water at the effective angle. The innovation introduced with the screw propeller was the extension of that arc through more than 360° by attaching the blade to a rotating shaft. In practice, there has to be more than one blade so as to balance the forces involved.

The first screw propeller to be added to an engine was installed by James Watt in Birmingham, England. He used it with his steam engine although the screw propeller's origins can be traced to Egyptians by way of Leonardo da Vinci. The third century BCE Greek philosopher, Archimedes developed a related concept with a view to moving water and European crusaders found windmills in the Middle East in the 1090s CE. The first screw propeller to be powered by a petrol engine, fitted to a small boat (now known as a powerboat) was installed by Frederick Lanchester, also from Birmingham. This was tested in Oxford.

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