Types of fins

Fins

The fins that are most commonly used in scuba diving are fins made of plastic or rubber. These are the most basic fins. A pair of simple stiff plastic, composite, or rubber blades that work as extensions of the feet while kicking. Some fins have a water vent through the blade, opening backwards on the underside and forwards on the upper side, as in the third image. As the hip joint is flexed, a jet of water blows backwards out of the vent in the fin. This type of fin is sometimes called a jetfin. Other paddle fins have convulated channels and grooves to improve power and efficiency though it has been shown that the desired effect does not occur. Fins are widely believed to be the most versatile and have improved swimming economy in men. Tests in women showed a more flexible fin to be more economical, most likely due to lower leg power.

Some fins have the end of the blade split. In hydrodynamic principle, split fins operate similarly to a propeller, by creating suction and lift forces to move the swimmer forwards. The theory is that water flowing toward the center of the fin's "paddle" portion also gains speed as it focuses, creating a "scooping" effect. Due to the design of these fins trey're also commonly used for snorkeling due to the fins being able to break the tension of the water easier than other designs when swimming on the surface and that results in using less energy.
When you're scuba diving in cooler water you usually wear fins with an open heel which you wear over shoes made of neoprene.

These are very similar to paddle fins, except they are far longer, and designed to work with slow stiff-legged kicks that conserve energy and oxygen. Though primarily intended for high efficiency at variable depths, they can still deliver impressive acceleration if necessary. The vast majority are made in the "full-foot" design with very rigid foot pockets, which serves to reduce weight and maximize power. Freediving fins are commonly made of plastic, but are also often made from materials such as fiberglass and carbon fibre.

Freedivers can also use a monofin. A monofin is actually one big fin with two foot pockets attached. The monofin swimmer puts his arms forward and keeps his hands together. He puts his head between the upper arms. The swimming movement starts in the shoulders, with maximum amplitude towards the hips, the legs almost don't bend to transfer the movement to the monofin. This technique is called the dolphin kick.