Hydrofoil

Aerodynamics

Hydrofoils are built on the principle that less friction means faster speeds. Reducing friction requires having the least amount of surface area in contact with the water, so the hull of the ship is actually lifted out of the water. Lifting a hydrofoil ship out of the water is done by the V-foils under the ship. They work the same way that aircraft does, but applied to water.

There are four fundamental forces acting on a lifting surface: thrust, gravity, drag and lift. Thrust is provided by the engines of the ship, driving it forward. Gravity is countered by water, but water also produces drag. A foil's upper surface is curved and water would have to travel a longer distance over it than the bottom surface, forcing it to move faster. Thus, the upper surface has less pressure and the pressure below the foil pushes it upwards. This is lift. Foils do not have to be wide to produce lift because of water's density.

Ship Systems

Two kinds of foils in use today are the surface-piercing foils and submerged-foil systems. Surface-piercing systems lift the boat until only the tip of the V-foil are in contacting the water. The amount of foil area above and below the surface must achieve balance for stability. Submerged-foil systems are foils underneath the surface of the water. Sonar and other devices watch the ship's condition in relation to the water and adjusts the angle of the foils to achieve stability. The foils may also have flaps on their trailing ends that can be extended, just like those on an aircraft. Propulsion is provided by either a screw or a turbine jet engine that pumps water through the vessel.