fin

fin, organ of locomotion characteristic of fish and consisting of thin tissue supported by cartilaginous or bony rays. In some fish, e.g., the eel, a single fin extends from the back, around the tail, and along the ventral surface. In the majority of fishes, however, there are one, two, or three dorsal fins, a distinct tail fin, and an anal fin. These are called median, or unpaired, fins. In addition to these unpaired fins, most fish also have paired fins. These are the pectoral fins, placed just back of the gills, and the pelvic, or ventral, fins, variable in position and sometimes lacking entirely. The tail is an important organ of locomotion and the paired fins are used for steering, checking speed, balancing, and for slow movements. An adipose fin (fatty tissue without support) is found behind the dorsal fin in some fish, e.g., the salmon and the catfish. See climbing perch; flying fish.

or finback whale or razorback whale or common rorqual

Swift, slender-bodied baleen whale (Balaenoptera physalus) named for the ridge on its back. It is 59–89 ft (18–27 m) long, with a triangular dorsal fin, short baleen, and several dozen grooves along its throat and chest. It is gray, with white on the underparts and on the right side of the lower jaw. It is found in oceans worldwide, in groups of a few to several hundred. It lives in polar waters in summer, feeding on crustaceans and small fishes, and moves to warmer waters in winter to breed. Once commercially valuable, it has been substantially reduced in numbers by overhunting and is now listed as an endangered species.

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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.
• the fin of a rocket uses pressure on both sides of the fin to create a more stable flight in a rocket

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