Hydrofoil is a boat whose hull can lift above the surface of the water when traveling at high speeds. The craft has wings that are designed to move underwater. These wings, which are called foils, have a curved upper surface and work like airplane wings to lift the hull above the water.
The foils are attached to the hull of the boat by long, thin supports called struts. At low speeds, the hydrofoil moves through the water on its hull. When the craft reaches higher speeds, water rushes over the foils at a faster rate. This water action decreases the pressure on the top of the foils and causes them to rise in the water. The hull of the hydrofoil then “flies” above the water on its foils. The lift of the foils supports the hull on its struts.
Hydrofoils use less engine power than other kinds of vessels when traveling at high speeds because their hulls do not have to be pushed through the water. They generally travel at speeds from 30 to 55 knots (nautical miles per hour). Experimental craft have reached speeds of more than 80 knots.
An important feature of hydrofoils is their ability to ride smoothly in rough water at high speeds. The foils act to reduce the effects of waves on the craft, both when it is foilborne (traveling on its foils) and when it is hullborne (traveling on its hull).
Commercial hydrofoils carry hundreds of thousands of passengers and many tons of supplies and equipment each year. Passenger hydrofoils operate between the islands of Greece, across the English Channel, and in many other parts of the world. Russia operates the largest fleet of commercial hydrofoils.
Military hydrofoils perform such duties as carrying out patrol missions and tracking enemy vessels at high speeds. Some carry guided missiles. Russia has more military hydrofoils than any other country. China, Italy, the United Kingdom, and the United States also operate military hydrofoils.
Kinds of hydrofoils
Hydrofoils range from about 15 to 200 feet (4.5 to 61 meters) in length. Most are powered by gas turbine or diesel engines, or both. Propellers or waterjets propel the craft.
When a hydrofoil is foilborne, its weight is generally distributed between two foils, one at the bow and one at the stern. In the canard system, the foil at the stern is larger than the one at the bow. In the tandem system, both of the foils are of equal size. The airplane system has a larger foil at the bow than it has at the stern.
In rough water, hydrofoils change their “flight” path to keep their hulls above the waves. When the waves are small in comparison to the hydrofoil, the craft easily clears the tops of the waves while maintaining a fixed path. This method is called platforming. When the waves are large in comparison to the hydrofoil, the craft adjusts its path to follow the shape of the waves. This method is called contouring. Intermediate response combines features from both platforming and contouring for traveling over small and large waves. The hydrofoil maintains a fixed path when moving over the small waves, but it follows the shape of the waves when traveling over large ones.
Hydrofoils are classified according to the way their foil systems operate when they are foilborne. There are two basic types: (1) surface-piercing hydrofoils and (2) fully submerged hydrofoils.
Surface-piercing hydrofoils
are most commonly used on rivers, lakes, and other areas where the water is relatively calm. Two popular types of surface-piercing craft are the V-foil hydrofoil and the trapeze-foil hydrofoil. The V-foil hydrofoil has foils that form a “V” shape. The trapeze-foil hydrofoil has foils that form part of a trapezoid.
Surface-piercing hydrofoils use the surface area of the foils for stability and control. When a surface-piercing hydrofoil is in “flight,” the upper parts of the foils stick out of the water. The speed of the craft determines the distance of the hull above the water. As the speed of the craft increases, the foil area required for lift decreases, and the upper part of the foil therefore rises out of the water. When the speed decreases, more area is required to lift the craft, and the foils settle deeper into the water.
Fully submerged hydrofoils
are used most often in oceans or other areas where exceptionally rough water might be encountered. The foils of a fully submerged hydrofoil stay completely underwater when the craft is in “flight.” The craft is kept stable by changing the angle of the foils, called their angle of attack, to keep water flowing over them at an even rate. The angle of attack of fully submerged hydrofoils is changed through the use of automatic stabilizing systems. These systems are either electrical or mechanical.
Electrical stabilizing systems use sensors (detecting devices) to pick up changes in the size of the waves. The sensors feed this information into a computer, which sends signals to the craft’s stabilizing equipment to adjust the angle of attack. Some craft make such adjustments through the use of movable flaps on the foils. Others have air pumps that create an air cavity around the foils.
Mechanical stabilizing systems use floats attached to the foils by mechanical links to change the angle of attack. The floats ride along the surface of the water. As they travel up and down the waves, they move the mechanical links. The action of the mechanical links then rotates the foils.
Fully submerged hydrofoils called shallow draft hydrofoils have the self-stabilizing ability of surface-piercing hydrofoils. The foils of these craft lose lift as they approach the water surface and do not rise higher than one chord (the distance from the front edge to the back edge of the foil) from the surface. They maintain this height as long as their speed is kept constant. These craft are used on calm, shallow waterways.
History
Early hydrofoil experiments
began in Europe during the late 1800’s. In 1906, an Italian engineer named Enrico Forlanini successfully tested the first full-sized, self-propelled hydrofoil. In 1907, the Scottish-born inventor Alexander Graham Bell started work on a series of hydrofoils he called Hydro-Dromes. In 1918, one of these craft, the HD-4, set a world water speed record of 61.6 knots. This record remained the top speed for a hydrofoil until 1963, when an experimental hydrofoil developed in the United States, the Fresh I, set the new record of 84 knots.
Modern hydrofoil development.
In 1927, Baron Hanns von Schertel, a German engineer, started work on designing a hydrofoil for commercial use. After nine years of trials, Schertel developed a surface-piercing V-foil that eventually became the first passenger hydrofoil to be put into regular service. During World War II (1939-1945), Schertel and his design team created 15 different types of hydrofoils for the German military.
In 1945, Rostislav Alekseev, a Soviet scientist, began experiments that led to the development of the shallow draft hydrofoil. In 1957, Alekseev’s first shallow draft vessel, the Raketa, began passenger service on the Volga River in western Russia.
In 1947, the United States Navy began extensive research into hydrofoils and soon became the world’s leader in the development of oceangoing hydrofoils. In 1958, it launched Sea Legs, the first fully submerged hydrofoil craft to meet with great success. During the 1960’s, the U.S. Navy launched a number of fully submerged hydrofoil vessels, including the Tucumcari and the Flagstaff, both gunboats. During the late 1970’s and early 1980’s, the Navy commissioned six fully submerged hydrofoils called Patrol Hydrofoil Missileships (PHMs). They were designed for tracking enemy vessels and launching missiles against them.
