Hydrofoil

A hydrofoil is a wing-like lifting surface that, at speed, generates enough hydrodynamic lift to raise the hull clear of the water, cutting wetted area and wave-making resistance so the craft rides on the foils alone. Lift follows L = 0.5 rho V^2 S C_L, rising with the square of speed, so foilborne flight begins above a takeoff speed where foil lift equals displacement. Foil systems are surface-piercing (self-stabilizing through changing immersed span) or fully submerged (needing active flaps for height and attitude control). The speed ceiling is set by cavitation on the suction side once the local pressure reaches vapor pressure, which caps practical surface-piercing foils near 40 to 50 knots before supercavitating or base-vented sections are needed.