A fin is a thin appendage or component attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fins are also used to increase surface areas for heat transfer purposes, or simply as ornamentation.Fin Oxford dictionary. Retrieved 24 November 2012.Fin Merriam-Webster dictionary. Retrieved 24 November 2012.
Fins first evolved on fish as a means of locomotion. Fish fins are used to generate thrust and control the subsequent motion. Fish and other aquatic animals, such as cetaceans, actively propel and steer themselves with pectoral and tail fins. As they swim, they use other fins, such as dorsal and anal fins, to achieve stability and refine their maneuvering.Helfman G, Collette BB, Facey DE and Bowen BW (2009) "Functional morphology of locomotion and feeding" Chapter 8, pp. 101–116. In:The Diversity of Fishes: Biology, John Wiley & Sons. .
The fins on the tails of cetaceans, ichthyosaurs, metriorhynchids, mosasaurs and plesiosaurs are called flukes.
Thrust generation 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.Carlton, John (2007) Marine Propellers and Propulsion Pages 1–28, Butterworth-Heinemann. . Turbines work in reverse, using the lift of the blades to generate torque and power from moving gases or water.Soares, Claire (2008) Gas Turbines: A Handbook of Air, Land, and Sea Applications Pages 1–23, Butterworth-Heinemann. .
Cavitation can be a problem with high power applications, resulting in damage to propellers or turbines, as well as noise and loss of power.Franc, Jean-Pierre and Michel, Jean-Marie (2004) Fundamentals of Cavitation Springer. . 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".
Motion control
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 and keel fins.Perez, Tristan (2005) Ship Motion Control: Course Keeping and Roll Stabilisation Using Rudder and Fins Springer. . Airplanes achieve similar results with small specialised fins that change the shape of their wings and tail fins.McClamroch, N Harris (2011) Steady Aircraft Flight and Performance Page 2–3, Princeton University Press. .
Stabilising fins are used as fletching on arrows and some darts,Vujic, Dragan (2007) Bow Hunting Whitetails Page 17, iUniverse. . and at the rear of some bombs, missiles, rockets and self-propelled torpedoes.Hobbs, Marvin (2010) Basics of Missile Guidance and Space Techniques Page 24, Wildside Press LLC. .Compon-Hall, Richard (2004) Submarines at War 1939–1945 Page 50, Periscope Publishing. . These are typically planar and shaped like small wings, although grid fins are sometimes used.Khalid M, Sun Y and Xu H (1998) "Computation of Flows Past Grid Fin Missiles" AVT Symposium on Missile Aerodynamics, Sorrento, Italy. Static fins have also been used for one satellite, GOCE.
Temperature regulation Engineering fins are also used as heat transfer fins to regulate temperature in heat sinks or fin radiators.Siegel R and Howell JR (2002) Thermal Radiation Heat Transfer Chapter 9: Radiation combined with conduction and convection at boundaries, pp.335–370. Taylor & Francis. .Fin: Function in aircraft engines Encyclopædia Britannica. Retrieved 22 November 2012.
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."Female fish flaunt fins to attract a mate ScienceDaily. 8 October 2010.
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.Brandner PA and Walker GJ (2004) Hydrodynamic Performance of a Surfboard Fin 15th Australasian Fluid Mechanics Conference, Sydney.
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.Ichthyology Florida Museum of Natural History. Retrieved 22 November 2012. 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.
Evolution
There is an old theory, proposed by anatomist Carl Gegenbaur, which has been often disregarded in science textbooks, "that fins and (later) limbs evolved from the gills of an extinct vertebrate". Gaps in the fossil record had not allowed a definitive conclusion. In 2009, researchers from the University of Chicago found evidence that the "genetic architecture of gills, fins and limbs is the same", and that "the skeleton of any appendage off the body of an animal is probably patterned by the developmental genetic program that we have traced back to formation of gills in sharks".Evolution Of Fins And Limbs Linked With That Of Gills ScienceDaily. 25 March 2009.Wings, legs, and fins: How do new organs arise in evolution? Neil Shubin, University of Chicago. Recent studies support the idea that gill arches and paired fins are serially homologous and thus that fins may have evolved from gill tissues.
Fish are the ancestors of all mammals, reptiles, birds and amphibians."Primordial Fish Had Rudimentary Fingers" ScienceDaily, 23 September 2008. In particular, terrestrial tetrapods (four-legged animals) evolved from fish and made their first forays onto land 400 million years ago. They used paired pectoral and pelvic fins for locomotion. The pectoral fins developed into forelegs (arms in the case of humans) and the pelvic fins developed into hind legs.Hall, Brian K (2007) Fins into Limbs: Evolution, Development, and Transformation University of Chicago Press. . Much of the genetic machinery that builds a walking limb in a tetrapod is already present in the swimming fin of a fish.Shubin, Neil (2009) Your inner fish: A journey into the 3.5 billion year history of the human body Vintage Books. . UCTV interview Clack, Jennifer A (2012) "From fins to feet" Chapter 6, pages 187–260, in: Gaining Ground, Second Edition: The Origin and Evolution of Tetrapods, Indiana University Press. .
In 2011, researchers at Monash University in Australia used primitive but still living lungfish "to trace the evolution of pelvic fin muscles to find out how the load-bearing hind limbs of the tetrapods evolved."Lungfish Provides Insight to Life On Land: 'Humans Are Just Modified Fish' ScienceDaily, 7 October 2011. Further research at the University of Chicago found bottom-walking lungfishes had already evolved characteristics of the walking gaits of terrestrial tetrapods.A small step for lungfish, a big step for the evolution of walking" ScienceDaily, 13 December 2011.
In a classic example of convergent evolution, the pectoral limbs of pterosaurs, birds and bats further evolved along independent paths into flying wings. Even with flying wings there are many similarities with walking legs, and core aspects of the genetic blueprint of the pectoral fin have been retained.Vertebrate flight: The three solutions University of California. Updated 29 September 2005.