Fin

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Fins are used by aquatic animals, such as this orca, to generate thrust and control the subsequent motion
Caudal fin of a great white shark
Aquatic animals typically use fins for locomotion
(1) pectoral fins (paired), (2) pelvic fins (paired), (3) dorsal fin, (4) adipose fin, (5) anal fin, (6) caudal (tail) fin
Comparison between A) the swimming fin of a lobe-finned fish and B) the walking leg of a tetrapod. Bones considered to correspond with each other have the same color.
In a parallel but independent evolution, the ancient reptile Ichthyosaurus communis developed fins (or flippers) very similar to fish (or dolphins)
In the 1990s the CIA built a robotic catfish called Charlie to test the feasibility of unmanned underwater vehicles

Thin component or appendage attached to a larger body or structure.

- Fin
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Ray fins on a teleost fish, Hector's lanternfish 
(1) pectoral fins (paired), (2) pelvic fins (paired), (3) dorsal fin,
(4) adipose fin, (5) anal fin, (6) caudal (tail) fin

Fish fin

Ray fins on a teleost fish, Hector's lanternfish 
(1) pectoral fins (paired), (2) pelvic fins (paired), (3) dorsal fin,
(4) adipose fin, (5) anal fin, (6) caudal (tail) fin
Skeleton of a ray-finned fish
Lobe-finned fishes, like this coelacanth, have fins that are borne on a fleshy, lobelike, scaly stalk extending from the body. Due to the high number of fins it possesses, the coelacanth has high maneuverability and can orient their bodies in almost any direction in the water.
The haddock, a type of cod, is ray-finned. It has three dorsal and two anal fins
Cartilaginous fishes, like this shark, have fins that are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers
Caudal fin of a grey reef shark
Shark fin
Comparison between A) the swimming fin of a lobe-finned fish and B) the walking leg of a tetrapod. Bones considered to correspond with each other have the same color.
In a parallel but independent evolution, the ancient reptile Ichthyosaurus communis developed fins (or flippers) very similar to fish (or dolphins)
Similar adaptations for fully aquatic lifestyle are found both in dolphins and ichthyosaurs
In the 1990s, the CIA built a robotic catfish called Charlie, designed to collect underwater intelligence undetected

Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish.

Dorsal fin of a shark

Dorsal fin

Dorsal fin of a shark
Most fish, like this Prussian carp, have one dorsal fin
Sharks typically have two dorsal fins
The yellowfin tuna also has two dorsal fins
Haddocks have three dorsal fins
Differences of dorsal fins of orcas between male and female
The dorsal fin of a white shark contains dermal fibers that work "like riggings that stabilize a ship's mast", and stiffen dynamically as the shark swims faster to control roll and yaw.<ref>Lingham‐Soliar T (2005) "Dorsal fin in the white shark, Carcharodon carcharias: A dynamic stabilizer for fast swimming" Journal of Morphology, 263 (1): 1–11. {{doi|10.1002/jmor.10207}} pdf{{dead link|date=September 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
Large retractable dorsal fin of the Indo-Pacific sailfish
Various species of Ichthyosaurs displaying different types of dorsal fins
Dorsal fin of a perch showing the basals and radials of the pterygiophore that support the dorsal fin.
Closeup of the dorsal fin of a common dragonet

A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom.

Streamlines around a NACA 0012 airfoil at moderate angle of attack

Foil (fluid mechanics)

Solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag (force generated parallel to the fluid flow).

Solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift is substantially larger than the drag (force generated parallel to the fluid flow).

Streamlines around a NACA 0012 airfoil at moderate angle of attack

Other types of foils, both natural and man-made, seen both in air and water, have features that delay or control the onset of lift-induced drag, flow separation, and stall (see Bird flight, Fin, Airfoil, Placoid scale, Tubercle, Vortex generator, Canard (close-coupled), Blown flap, Leading edge slot, Leading edge slats), as well as Wingtip vortices (see Winglet).

Feather fletching – these are shield cut with barred red hen feathers and a solid white cock.

Fletching

Feather fletching – these are shield cut with barred red hen feathers and a solid white cock.
Plastic fletching (also known as vanes) – this example is parabolic cut with pink hen vanes (the ones put oblique to the bow when nocked on the string) and a green cock (the one – or ones, with even-numbered vanes – put perpendicular to it).

Fletching is the fin-shaped aerodynamic stabilization device attached on arrows, bolts, darts, or javelins, and are typically made from light semi-flexible materials such as feathers or bark.

Traditional target arrow (top) and replica medieval arrow (bottom).

Arrow

Traditional target arrow (top) and replica medieval arrow (bottom).
Modern arrow with plastic fletchings and nock.
Warring States bronze arrowheads
Schematic of an arrow with many parts.
A sideprofile of an Easton Carbon One arrow with a spine of 900, taken with a scanning electron microscope (SEM). The arrow is a bond of two carbon tubes, an inner and an outer tube (black wires). In between both carbon layers, another fiber is used (white fiber). This second fiber is an Mg-Al-Si-fiber. The "white" fiber is twisted around the inner carbon tube. The fibers of the carbon tubes are not twisted, to ensure a maximum of possible mechanical tension of the arrow. The Mg-Al-Si-fiber enhances the flexibility of the arrow. The diameter of a single carbon fiber is approx. 7 µm.
Obsidian broadhead
Ancient Greek bronze arrowhead, 4th century BC, from Olynthus, Chalcidice
Various Japanese arrowheads
Native American arrowheads
20th century field points
Modern replicas of various medieval European arrowheads
Straight parabolic fletchings on an arrow.

An arrow is a fin-stabilized projectile launched by a bow.

The Vympel NPO R-77 is a beyond-visual-range missile that uses grid fins. The gray KAB-500 OD guided bomb to its left has conventional "planar" tail fins.

Grid fin

The Vympel NPO R-77 is a beyond-visual-range missile that uses grid fins. The gray KAB-500 OD guided bomb to its left has conventional "planar" tail fins.
Closeup of MOAB grid fins
Initial design aluminum grid fins on the Falcon 9 launch vehicle, undeployed. February 2015.
Grid fins (here folded against the payload fairing) are part of the launch escape system of Soyuz spacecraft.
Grid fins stowed against the base of an SS-20 ballistic missile
Grid fins on a SpaceX Falcon 9 rocket. They guide the rocket's first stage during landing.
First test of grid fins by SpaceX during a Falcon 9 controlled-descent test on 11 February 2015.

Grid fins (or lattice fins) are a type of flight control surface used on rockets and bombs, sometimes in place of more conventional control surfaces, such as planar fins.

Pelvic fins from a Java barb (Barbonymus gonionotus)

Pelvic fin

Pelvic fins from a Java barb (Barbonymus gonionotus)
The pelvic fin appears at roughly 21 days post fertilization in zebrafish
Pelvic fin skeleton for Danio rerio, zebrafish.
Gobiids have modified their pelvic fins into adhesive suckers.
Lumpsuckers use their modified pelvic fins to adhere to the substrate.

Pelvic fins or ventral fins are paired fins located on the ventral surface of fish.

Full foot fins

Swimfin

Full foot fins
By 1974, modern-looking swimfins in regular use in landlocked, “second-world” Hungary.
1959 Soviet postage stamp with image of finned recreational diver in tribute to DOSAAF sport organisation.
Swim fin sole showing compliance with German standard DIN 7876:1980
An assortment of fins in a diving shop. Fins on the right are full foot and those in the middle are open heel.
Monofin and pair of freediving bifins
An open-heel vented paddle Jetfin
Underwater divers using paddle fins
Fin design intended to reduce fatigue
Swimfins designed for swim training.
Swimfins designed for bodyboarding or bodysurfing.
Swim fin strap attachment with simple rubber strap and wire buckle
Swim fin strap attachment with swivelling plastic buckle and clip
Aftermarket stainless steel spring fin strap attached with long D-shackles for security
Open heel fin with stainless steel spring strap with rubber padding
Fin with bungee strap
Figures 1-3: Fin grips before and after fitting.
Figures 4-7: How fin grips are fitted on full-foot swimming fins.
Two pairs of early fin grips: Beuchat Fixe-Palmes and Mares Fissapinne
Underwater hockey fins with yellow and red pairs of fin grips.
A fin grip positioned to secure a full-foot swimming fin on the foot.

Swimfins, swim fins, diving fins, or flippers are finlike accessories worn on the feet, legs or hands and made from rubber, plastic, carbon fiber or combinations of these materials, to aid movement through the water in water sports activities such as swimming, bodyboarding, bodysurfing, float-tube fishing, kneeboarding, riverboarding, scuba diving, snorkeling, spearfishing, underwater hockey, underwater rugby and various other types of underwater diving.

Gravity Field and Steady-State Ocean Circulation Explorer

The first of ESA's Living Planet Programme satellites intended to map in unprecedented detail the Earth's gravity field.

The first of ESA's Living Planet Programme satellites intended to map in unprecedented detail the Earth's gravity field.

Model of GOCE
GOCE flares to magnitude +2 as the 67.5 degree solar panel briefly mirrors sunlight (3 January 2010, 17:24:23.15 UTC).

The satellite's unique arrow shape and fins helped keep GOCE stable as it flew through the thermosphere at a comparatively low altitude of 255 km. Additionally, an ion propulsion system continuously compensated for the variable deceleration due to air drag without the vibration of a conventional chemically powered rocket engine, thus limiting the errors in gravity gradient measurements caused by non-gravitational forces and restoring the path of the craft as closely as possible to a purely inertial trajectory.

Surfboard fins can help surfers control their boards

Surfboard fin

Hydrofoil mounted at the tail of a surfboard or similar board to improve directional stability and control through foot-steering.

Hydrofoil mounted at the tail of a surfboard or similar board to improve directional stability and control through foot-steering.

Surfboard fins can help surfers control their boards
US Box fin
FCS/FCS II fin
Futures fin

Fins can provide lateral lift opposed to the water and stabilize the board's trajectory, allowing the surfer to control direction by varying their side-to-side weight distribution.