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Fin facts for kids

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Lampanyctodes hectoris (Hector's lanternfish)2
The anatomy of Lampanyctodes hectoris
1. operculum (gill cover) 2 & 5. lateral line 3. dorsal fin 4 fat fin 6. caudal fin 7. anal fin 8. photophores 9. pelvic fins paired 10. pectoral fins (paired)
64 Cadillac fins
Tail fins on a Cadillac car

A fin is a surface used to produce lift and thrust or to steer while traveling in water, air, or other fluid media.

The word was first used for the limbs of fish, but it is now also used when describing other animal limbs and man-made devices.

The fins of fish are called:

  • Dorsal fin on the back. Function: anti-roll stability.
  • Caudal or tail fin: propulsion
  • Pectoral fins (paired): steering
  • Pelvic fins (paired): manoeuvering
  • Anal fin: stability

Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling. Fins are also used to increase surface areas for heat transfer purposes, or simply as ornamentation.

Generating thrust

Foil shaped fins generate thrust when moved, the lift of the fin sets water or air in motion and pushes the fin in the opposite direction. Aquatic animals get significant thrust by moving fins back and forth in water. Often the tail fin is used, but some aquatic animals generate thrust from pectoral fins. Fins can also generate thrust if they are rotated in air or water.

Turbines and propellers (and sometimes fans and pumps) use a number of rotating fins, also called foils, wings, arms or blades. Propellers use the fins to translate torquing force to lateral thrust, thus propelling an aircraft or ship. Turbines work in reverse, using the lift of the blades to generate torque and power from moving gases or water.

Moving fins can provide thrust
Fish get thrust moving vertical tail fins from side to side
Cetaceans get thrust moving horizontal tail fins up and down
Stingrays get thrust from large pectoral fins
Ship propeller
Airplane propeller
Compressor fins (blades)
Cavitation damage is evident on this propeller
Drawing by Dr Tony Ayling
Finlets may influence the way a vortex develops around the tail fin.

Cavitation can be a problem with high power applications, resulting in damage to propellers or turbines, as well as noise and loss of power. Cavitation occurs when negative pressure causes bubbles (cavities) to form in a liquid, which then promptly and violently collapse. It can cause significant damage and wear. Cavitation damage can also occur to the tail fins of powerful swimming marine animals, such as dolphins and tuna. Cavitation is more likely to occur near the surface of the ocean, where the ambient water pressure is relatively low. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.

Scombrid fishes (tuna, mackerel and bonito) are particularly high-performance swimmers. Along the margin at the rear of their bodies is a line of small rayless, non-retractable fins, known as finlets. There has been much speculation about the function of these finlets. Research done in 2000 and 2001 by Nauen and Lauder indicated that "the finlets have a hydrodynamic effect on local flow during steady swimming" and that "the most posterior finlet is oriented to redirect flow into the developing tail vortex, which may increase thrust produced by the tail of swimming mackerel".

Fish use multiple fins, so it is possible that a given fin can have a hydrodynamic interaction with another fin. In particular, the fins immediately upstream of the caudal (tail) fin may be proximate fins that can directly affect the flow dynamics at the caudal fin. In 2011, researchers using volumetric imaging techniques were able to generate "the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes". They found that "continuous tail beats resulted in the formation of a linked chain of vortex rings" and that "the dorsal and anal fin wakes are rapidly entrained by the caudal fin wake, approximately within the timeframe of a subsequent tail beat".

Controlling motion

Orca porpoising
Fins are used by aquatic animals, such as this killer whale, to generate thrust and control the subsequent motion

Once motion has been established, the motion itself can be controlled with the use of other fins. Boats control direction (yaw) with fin-like rudders, and roll with stabilizer fins and keel fins. Airplanes achieve similar results with small specialised fins that change the shape of their wings and tail fins.

A rudder corrects yaw
A fin keel limits roll and sideways drift
Ship stabilising fins reduce roll

Stabilising fins are used as fletching on arrows and some darts, and at the rear of some bombs, missiles, rockets, and self-propelled torpedoes. These are typically planar and shaped like small wings, although grid fins are sometimes used. Static fins have also been used for one satellite, GOCE.

Static tail fins are used as stabilizers
Fletching on an arrow
Asymmetric stabilizing fins impart spin to this Soviet artillery rocket
Conventional "planar" fins on a RIM-7 Sea Sparrow missile

Regulating temperature

Engineering fins are also used as heat transfer fins to regulate temperature in heat sinks or fin radiators.

Fins can regulate temperature
Motorbikes use fins to cool the engine
Oil heaters convect with fins
Sailfish raise their dorsal fin to cool down or to herd schooling fish

Ornamentation and other uses

In biology, fins can have an adaptive significance as sexual ornaments. During courtship, the female cichlid, Pelvicachromis taeniatus, displays a large and visually arresting purple pelvic fin. "The researchers found that males clearly preferred females with a larger pelvic fin and that pelvic fins grew in a more disproportionate way than other fins on female fish."

During courtship, the female cichlid, Pelvicachromis taeniatus, displays her visually arresting purple pelvic fin
The huge theropod dinosaur Spinosaurus may have used its sail or dorsal fin to regulate its temperature, or as a courtship display.
Car tail fins in the 1950s were largely decorative

Reshaping human feet with swim fins, rather like the tail fin of a fish, add thrust and efficiency to the kicks of a swimmer or underwater diver Surfboard fins provide surfers with means to maneuver and control their boards. Contemporary surfboards often have a centre fin and two cambered side fins.

The bodies of reef fishes are often shaped differently from open water fishes. Open water fishes are usually built for speed, streamlined like torpedoes to minimise friction as they move through the water. Reef fish operate in the relatively confined spaces and complex underwater landscapes of coral reefs. For this manoeuvrability is more important than straight line speed, so coral reef fish have developed bodies which optimize their ability to dart and change direction. They outwit predators by dodging into fissures in the reef or playing hide and seek around coral heads. The pectoral and pelvic fins of many reef fish, such as butterflyfish, damselfish and angelfish, have evolved so they can act as brakes and allow complex maneuvers. Many reef fish, such as butterflyfish, damselfish and angelfish, have evolved bodies which are deep and laterally compressed like a pancake, and will fit into fissures in rocks. Their pelvic and pectoral fins are designed differently, so they act together with the flattened body to optimise maneuverability. Some fishes, such as puffer fish, filefish and trunkfish, rely on pectoral fins for swimming and hardly use tail fins at all.

Other uses
Swim fins add thrust to the kicks of a human swimmer
Surfboard fins allow surfers to maneuver their boards
In some Asian countries shark fins are a culinary delicacy
In recent years, car fins have evolved into highly functional spoilers and wings
Many reef fish have pectoral and pelvic fins optimised for flattened bodies
Frog fish use their pectoral and pelvic fins to walk along the ocean bottom
Flying fish use enlarged pectoral fins to glide above the surface of the water

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