Fin

A fin is a thin part added to a larger object. Fins help things move, steer, or stay steady when traveling through water, air, or other liquids. They can also help spread heat or just look nice.

Fins first appeared on fish to help them swim. Fish fins give fish the push they need to move forward and help them turn and stay balanced. Fish and other sea animals, like cetaceans, use fins such as pectoral and tail fins to move and steer. As they swim, they use other fins like dorsal and anal fins to stay steady and change direction smoothly.

The fins on the tails of cetaceans, ichthyosaurs, metriorhynchids, mosasaurs, and plesiosaurs are called flukes.

Thrust generation

Foil shaped fins can create thrust when they move. The lift from the fin pushes water or air, moving the fin in the opposite direction. Many water animals use their fins to move through the water, often using their tail fin. Some animals use pectoral fins instead. Fins can also make thrust when turned in air or water. Machines like turbines, propellers, fans, and pumps use spinning fins to move things. Propellers turn to push airplanes or ships forward, while turbines use the movement of air or water to create power.

Moving fins can help create thrust. Fish move their vertical tail fins from side to side to swim. Some sea animals, like cetaceans, move their horizontal tail fins up and down. Stingrays use large pectoral fins to push through water.

Finlets can change how water moves around a tail fin. Very fast movement can cause problems called cavitation, where bubbles form and then burst, which can hurt propellers, turbines, or even the fins of fast sea animals like dolphins and tuna. Cavitation is more likely near the surface of the water where the pressure is lower.

Motion control

Once something is moving, other fins can help control how it moves. Boats use fin-like rudders to change direction and special fins to keep them steady. Airplanes also use small fins on their wings and tails to control their movement.

Fish, boats, and airplanes all need to control how they turn and stay balanced. For example, the dorsal fin of a shark helps it stay steady while swimming. Rudders correct side-to-side movement, keels stop rolling, and special fins on ships reduce rolling. Ailerons on airplanes control rolling, and elevators control up-and-down movement. Rudders also help control direction.

Stabilizing fins are used on things like arrows, darts, bombs, missiles, rockets, and torpedoes to keep them steady. These fins are usually flat and shaped like small wings, but sometimes they are grid-like. Static tail fins are also used to stabilize satellites in space.

Temperature regulation

Engineering fins help control temperature in things like heat sinks or fin radiators. They can keep machines cool or help spread heat.

Fins are used on motorbikes to cool the engine, and oil heaters use fins to spread heat. Even animals like sailfish use their dorsal fin to cool down or guide a group of schooling fish.

Ornamentation and other uses

Fins can also be used for showing off during courtship. For example, a female cichlid fish, Pelvicachromis taeniatus, shows a large purple pelvic fin to attract males. Some cars in the 1950s had tail fins that were mostly for looks.

Fins help people and animals move better too. Swim fins help swimmers and underwater divers kick faster. Surfboard fins help surfers steer their boards. Some fish, like butterflyfish, damselfish, and angelfish, use their fins to slow down and turn quickly, especially around coral reefs. Other fish, like flying fish, use big fins to glide above the water.

Evolution

Aristotle noticed that birds and fish have similar body parts arranged in different ways. Birds have wings on top and feet below, while fish have fins both in front and below their bodies.

An old idea suggests that fins and limbs might have started from the gills of ancient sea creatures. Recent studies show that the genes that build gills are also used to build fins and limbs. Fish are the ancestors of all animals with four legs, like mammals, reptiles, birds, and amphibians. These four-legged animals evolved from fish that used their fins to move on land about 400 million years ago. The fins of fish changed over time into legs and arms in their descendants.

Researchers studied living lungfish to understand how fish fins turned into legs. They found that lungfish that walk on the bottom already had some features of land animals. In another example of evolution happening in similar ways in different animals, the wings of pterosaurs, birds, and bats developed separately but still share some basic features with walking legs.

About 200 million years ago, the first mammals appeared. Some of these mammals later returned to the ocean about 52 million years ago, such as whales, dolphins, and porpoises, which evolved from land animals that are related to hippos. Another group of land animals, similar to bears, also returned to the sea about 23 million years ago and evolved into seals. Their walking limbs changed back into swimming fins. In whales, the back limbs turned into a tail with fins, while in seals, the back limbs became a tail ending in two fins. Fish tails move side to side, but whale tails move up and down because their bodies bend like other mammals.

Ancient reptiles called ichthyosaurs looked like dolphins and lived in the ocean. They first appeared about 245 million years ago and disappeared about 90 million years ago. They evolved a dorsal fin and a tail shaped just right for swimming, even though their land-dwelling ancestors did not have these features.

Robotics

Scientists study how fish move through water to make better underwater robots. Some fish can move very efficiently, and robots copy their movements to swim quietly and quickly. For example, a robot tuna was built to learn how real tuna move. In 2005, a group showed three robot fish that could swim around and avoid things, just like real fish.

Companies have made robots that copy penguins, manta rays, jellyfish, and barracuda. These robots help scientists learn more about how animals move. Robots let researchers test ideas more easily than with live animals, and they can measure forces and movements very accurately.