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A 'fin' is a surface used to produce lift and thrust or to steer while traveling in water, air, or other fluid media. The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices.
The foremost use of fins is to ensure the directional stability of an object moving through a fluid such as water or air and may be seen in the use of fletching on arrows and fins at the rear of some missiles, rockets, self-propelled torpedoes, and kinetic energy penetrators. They are typically "planar" (shaped like small wings), although grid fins are sometimes used in specialized cases.
Moving fins may be used to propel an object through lateral thrust (see mechanics).
Examples of fin use:

★ Propellers usually have a number of fins that work to translate torquing force to lateral thrust, thus propelling a ship. These are also called blades. In the case of high power application it is important to avoid cavitation, caused by excessive negative pressure, as this can cause noise, a loss of power, and damage to the propeller.

★ For scuba divers' fins, see swimfin.

★ In surfing, a skeg is a stabilizing fin located at the rear of the surfboard. A skeg has the effect of keeping the board moving forward in a controlled manner. The surfboard fin has undergone numerous phases of development.

★ Constructions of the same purpose as fins (producing thrust, but working in gaseous media) instead are usually called wings or stabilizers with aerodynamics as the governing science. The exception to this is the vertical surface of an aircraft to which the rudder is attached - this is still usually called a fin but is (more formally) called a vertical stabilizer.