Common hydrofoil configurations (left) surface-piercing foils; (right) fully submerged foils.
A hydrofoil is a structure which, when moved rapidly through water, generates lift in exactly the same way and for the same reason as does the airfoil. It is usually mounted beneath a vessel (also called a hydrofoil). Much of a conventional boat's power is spent in overcoming the drag (resistance) of the water; as a hydrofoil vessel builds up speed, it lifts out of the water until only a small portion of it (struts, hydrofoils, and propeller) is in contact with the water. Thus drag is reduced to a minimum. Hydrofoils can exceed 125 km/h as compared with conventional craft, whose maximum speeds rarely approach 80 km/h.