propeller, device consisting of a hub with one or more blades that propels a craft to which it is attached by rotating its blades in a fluid such as air or water. In the latter part of the 1830s the Swedish-American engineer John Ericsson and the English inventor Sir Francis P. Smith independently patented screw propellers. Screw propellers have almost entirely replaced paddle wheels and a variety of other devices that were designed to propel waterborne vessels. In a single-screw ship the propeller is mounted on the end of a shaft immediately in front of the rudder; the shaft is connected to a transmission or directly to an engine, which turns it and the propeller. The thrust generated by the propeller is transmitted to the hull of the ship by a thrust bearing attached to the shaft. Twin-screw vessels were first introduced c.1860 in England. Located on either side of the rudder, the two propellers may be used to assist in steering; if one breaks down, the other can still propel the vessel. The introduction of steam turbines has brought about the use of four propellers on large ships. Screw propellers are made of cast iron, cast steel, or manganese bronze, the last being noted for its resistance to corrosion. Propellers on airplanes generally have from two to six blades. These are usually made of wood, aluminum alloy, steel, or composite materials. At first, all were of fixed pitch, i.e., the angle of the blades was not variable. Later, advantages in speed and power brought variable-pitch propellers into general use; their blades are set into sockets in the hub with gear arrangements capable of altering the pitch in flight. The development of automatic equipment to alter the pitch as needed for maintaining a predetermined speed produced the constant-speed propeller. Variable-pitch propellers generally take the name of the pitch-controlling device; the principal types are hydraulic, mechanical, automatic, and electric. With modifications they can also act as air brakes. When the number of blades was increased from two to three, then from three to six, to achieve greater thrust or propulsion or to keep the blade size down, new stress problems arose. These were met by the development of contrarotating propellers, in which the blades were arranged as two separate three-bladed units rotating in opposite directions.