III.

Ship Propellers

A ship propeller operates in much the same way as the airplane propeller. In the ship propeller, however, each blade is very broad (from leading to trailing edge) and very thin. The blades are usually built of copper alloys to resist corrosion. The speed of sound in water is much higher than the speed in air, and because of the high frictional resistance of water, the top speed never approaches the speed of sound. Although efficiencies as high as 77 percent have been achieved with experimental propellers, most ship propellers operate at efficiencies of about 56 percent. Clearance is also less of a problem on ship propellers, although the diameter and position of the propeller are limited by the loss in efficiency if the propeller blades come anywhere near the surface of the water. The principal problem of ship-propeller design and operation is cavitation, the formation of a vacuum along parts of the propeller blade, which leads to excessive slip, loss of efficiency, and pitting of the blades. It also causes excessive underwater noise, a serious disadvantage on submarines.

See also Fluid Mechanics.