Hydrofoils Incorporated Dictionary

THE US STANDARD INDUSTRIAL CLASSIFICATION NUMBER FOR HYDROFOILS IS 3732

hy-dro-foil (hi'dro-foil' ) n .

1. One of a set of blades attached to the hull of a boat and aligned in the water at a small angle to the horizontal so that when the boat is in motion the fluid striking each blade's underside creates a high-pressure region below the blade, low pressure above it, and a resultant lift that raises the craft out of the water for efficient high-speed operation.
2. A boat equipped with hydrofoils." The American Heritage Dictionary of the English Language.

"...hydrofoils are small wings, almost identical in section to those of an aircraft, and designed to generate lift. Since water has a density some 815 times that of air, the same lift as an airplane wing is obtained for only 1/815 of that area (at equal speeds)." JANES Hovercraft and Hydrofoils.

hydrofoil {hy'-droh-foyl} A hydrofoil is a device shaped like a fin or foil that is designed to provide lift as it moves through water. The principle of a hydrofoil is identical with that of an airplane wing (an airfoil) moving through air Water is 815 times denser than air, however, and therefore a hydrofoil can be much smaller than an airplane wing to give the same amount of lift. Ships equipped with hydrofoils belong to a group of vehicles called surface skimmers which also includes hovercraft (see AIR-CUSHION VEHICLE).Because it was thought that boats could go much faster with their hulls out of water--thus eliminating the "drag," or friction between hull and water--inventors had tried for many years to make boats that could rise up on "sea wings," or hydrofoils. Alexander Graham Bell perfected (1918) a craft that used ladder foils, three ladder-like cascades of foils mounted under the hull. The faster the boat's speed, the greater was the lift and the further the ladders rose out of the water, until at full speed only the lowest foils remained in the water. Ladder foils proved to be expensive and heavy, however, and more practical hydrofoils have been developed, with the most successful work beginning in the 1950s.

Surface-piercing and submerged-foil systems are the two major types of foil systems in use. In Surface-piercing foil systems the foils lift the boat until only the amount of the foil area required to lift the hull remains in the water. This lift is directly related to and controlled by the speed of the craft and the shape and size of the foils. Lift increases with speed. Stability is maintained by balancing the amount of foil area to the weight, center of gravity, etc. of the boat. If the boat banks (heels over), more foil is submerged on one side and less on the other, creating increased lifting forces on the side towards the roll, which tend to right the boat automatically. Because of this design, these boats are classified as "inherently stable." This is important in higher speed vehicles since there is no lag time for the balancing to take place. It is a function of physics, gravity, wave action, and vessel attitude and design, and not reliant on secondary input from sensors, computers, hydraulics, and the lag time involved for these controls to right the craft.

Submerged-foil systems are not automatically stable; the vessel's height and attitude is monitored by sensors, and controlled by hydraulic devices that change the angle of foils relative to the boat or extend flaps on the trailing edges of the foils (as on a plane's wing), to adjust for changes in wave height or vessel speed and direction. Because of this, these boats are classified as "inherently unstable" since loss of the computer means loss of balance and the ability to fly or function as a true hydrofoil. Propulsion for these vessels is provided either by a diesel powered screw propeller, or a turbine-driven water-jet propulsion. Boeing, under contract to the U.S.Navy, manufactured seven Patrol Hydrofoil Missile Ships (PHMS). This military boat could travel at 40 knots (74 km/h;46 mph) on retractable foils powered by jet propulsors. The ships travel 1.4 m (4.6 ft) above the water's surface, kept on an even keel by arrays of sensors and computers. Because of this, these boats are classified as "inherently unstable" since loss of the computer means loss of balance and flight. High costs have forced these ships from operation. Technology gained from this contract enabled the development of the "Boeing JetFoil" ferry boat. These boats, like their predecessors, are also inherently unstable. Hydrofoils have been used, primarily as ferries, on waterways in Europe and Asia. The Mitsubishi jetfoils that ferry passengers between Hong Kong and Macao make up one of the largest such fleets currently in service. Hydrofoil ferries also cross the English Channel. The Russian Republics utilizes hydrofoils for passenger transport on rivers and lakes.

Portions of the above text are from Bill Gunston Bibliography : Gunston, William T., Hydrofoils and Hovercraft (1969); McLeavy, Roy, ed., Jane's Surface Skimmers (annual); McLeavy, R., and Wood, J.W., Hovercraft and Hydrofoils.