What is a Hydrofoil?

A hydrofoil boat is a boat with a hydrofoil submerged under the hull. When sailing at high speed, the weight of the boat is fully supported by the lift generated by the water to the hydrofoil, so that the hull completely leaves the water surface, reducing the water resistance, so it can reach a higher speed. Hydrofoil works in a similar way to aircraft wings. Some speedboats are only equipped with hydrofoil in the capital, so that the front part of the boat leaves the water and the rear part still glides on the water. Commonly used for passenger and military use. ^[1]

The inventor who first thought of hydrofoil and researched in this area was a French priest named Ramis in the mid-19th century, who was inspired by kites. Another Frenchman of Russian descent named Drumbel knew that he had just invented

A hydrofoil is a wing that works or cuts the surface of the water. It is the basic component of a hydrofoil boat. It moves in the water and generates a certain amount of lift just like an airplane wing moves in the air. Because the density of water is about 800 times the density of air, hydrofoils are much smaller than the wings under the same lift. Hydrofoil boats that rely on hydrofoil lift to support the weight of the boat have less drag, less waves, and are less affected by wave interference than planing boats, so they have good fastness and seaworthiness.

A pair of hydrofoils generally consists of a wing plate that generates lift, a pillar connecting the wing plate and the hull, and some other accessories. The lift characteristics of a hydrofoil mainly depend on the geometric features and geometrical elements of the airfoil. These elements mainly refer to the chord length, wingspan length,

According to the number of hydrofoil, hydrofoil boats are divided into single hydrofoil boats with only front hydrofoils (also known as hydrofoil boats) and double hydrofoil boats (front and rear hydrofoils). The single hydrofoil bow is generally supported by shallow submerged hydrofoils. The hull of the hull is a gliding surface. The two hydrofoil boats use the front and rear hydrofoils to support the hull. The two hydrofoil boats can achieve wing navigation status. According to whether the hydrofoil crosses the water surface during the wing, it is divided into divisions. According to whether the angle of attack of the hydrofoil can be adjusted, it can be divided into self-control and non-self-control (fixed).

Part of the hydrofoil hydrofoil is exposed above the water surface during navigation, and only part of the area is submerged in the water. It relies on changing the hydrofoil flooding condition to adjust the hydrofoil lift without the need for other auxiliary equipment. Therefore, the structure is simple and reliable to use. The advantages of surface-cut hydrofoil are good self-stability, large area of reserve hydrofoil, and low speed of entering the wing. Its disadvantage is that it is more sensitive to wave disturbances, and it is not easy to maintain stability in waves.

The main advantages of hydrofoil are:

Low resistance when sailing at high speed. When a hydrofoil is in wing mode, the hull is lifted out of the water by the hydrofoil. The only underwater components are hydrofoil, rudder and shafting. Therefore, the water resistance is greatly reduced, and the speed is greatly increased. The current maximum speed of the existing hydrofoil boat can reach 70 kn.

Good wave resistance. Since the hull is lifted off the water surface in the wing state, the disturbance of the wave to the hydrofoil boat is reduced, so the sway motion of the hydrofoil boat in the wind and waves is reduced, and its wave resistance is greatly improved.

The waves rising at high speed are small. Because only the hydrofoil, rudder and other components are in the water during wing flight, the rising waves are very small. Therefore, hydrofoil is very suitable for inland navigation, which can greatly reduce the damage caused by waves to the bank.

Low physical field radiation. When a hydrofoil boat is in the wing mode, because the hull is lifted off the water surface, the physical field radiation of the hull will be weakened, especially the acoustic radiation will be greatly reduced. The low physical field radiation characteristics of hydrofoil boats are conducive to improving the stealth of ships and reducing the risk of underwater attacks.

Good maneuverability. Hydrofoil boats use the combined action of flaps and rudder to make the hull have a safe inward slewing during wing sailing, so it has good slewing.

The main disadvantages of hydrofoil boats are: there are hydrofoil under the hull, which is not suitable for shallow waterways; the width of hydrofoil is often larger than the width of the ship, and it is inconvenient to dock at the dock; high-speed internal combustion engines or gas turbines are often used to reduce the weight of the host, and the service life is short. It has high fuel requirements and poor economy; it is difficult to scale up due to the limitations of hydrofoil lift and structural strength. ^[4]