# Hydrofoil

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A hydrofoil is any liquid-based structure that provides lift because of its attack or camber. This can be artificial, such as on a boat, or occur naturally, as with fish, aquatic mammals or other creatures like the sand dollar.

The term is most commonly used for the wing-like structure mounted on struts below the hull of a boat, which lifts the boat partially out of the water during forward motion, in order to reduce drag. As a synecdoche, the term "hydrofoil" is often used to refer to any boat that utilizes hydrofoil technology.

Hydrofoils are similar in appearance and purpose to airfoils.[1] As the watercraft increases in speed, the hydrofoils develop enough lift to raise the hull up and out of the water. This results in a great reduction in drag, and a further corresponding increase in speed.

## Contents

### Hydrodynamics

Since air and water are governed by similar fluid equations, albeit with different levels of viscosity, density, and compressibility, the hydrofoil and airfoil create lift in identical ways (see foil (fluid mechanics)). The foil is shaped to move smoothly through the water with faster flow over the top face of the foil, creating a pressure drop and consequently an upward force on the foil. This upward force lifts the body of the vessel, decreasing drag and increasing speed. The lifting force eventually balances with the weight of the craft, reaching a point where the hydrofoil no longer lifts out of the water, but remains in equilibrium. Since the force of the waves acts over a smaller area of the hydrofoil, there is a marked decrease in turbulence drag.

### Foil configurations

Early hydrofoils used U-shaped foils. Hydrofoils of this type are known as surface-piercing since portions of the U-shape hydrofoils will rise above the water surface when foilborne. Some modern hydrofoils use inverted T-shape foils which are fully submerged. Fully submerged hydrofoils are less subject to the effects of wave action, and are therefore more stable at sea and are more comfortable for the crew and passengers. This type of configuration, however, is not self-stabilizing. The angle of attack on the hydrofoils needs to be adjusted continuously in accordance to the changing conditions, a control process that is performed by sensors, computer and active surfaces.