Hydrofoils

Last updated: March 10, 2009.

Walking through water takes much more effort than walking through air and this explains why ships and boats travel much more slowly than automobiles and aircraft. Water is almost 1000 times more dense than air, so most of the energy produced by a boat is taken up overcoming drag (water resistance). Hydrofoils travel much more quickly than ordinary boats not by pushing through the water but by raising the hull (main body) of the boat upward so it can glide above the waves.

Photo: This US navy hydrofoil has one foil at the front and two at the back. Note how the entire hull lifts clear of the water as the boat picks up speed. Public domain photo of the USS Taurus (PHM-3) patrol missile ship, shot in 1990, courtesy of Defense Imagery.

What are hydrofoils?

Hydrofoils are among the fastest boats on the water, with top speeds of around 100-110 km/h (60-70 mph). The most powerful hydrofoils have three different engines, two diesel enginesfor pushing the boat through water at low speeds and a powerful gas turbine engine to lift it onto its hydrofoil and power it along at top speed. Hydrofoils have been widely used as high-speed ferries and as fast military patrol boats.

A hydrofoil is like a cross between a boat and an airplane. It has three wings on stilts called "foils" just beneath the water level. As the boat begins to pick up speed, water travels faster over the curved top surface of the wings than under the straight bottom surface. This reduces pressure above the wings and produces an upward force called lift strong enough to raise the entire boat above the waves. Sharks have a pectoral fin on the sides of their bodies that produces lift in the same way.

Photo: You can clearly see the wing-shaped hydrofoil under the surface of the sea in this shot of the missile hydrofoil USS Hercules (PHM-2). Photo by Mark S. Kettenhofen courtesy of Defense Imagery.

Jetfoils

The fastest hydrofoils are pushed forward not by propellers but by massive jets of water forced backward at high speed. A gas-turbine engine pumps out up to 180 tons (164 metric tons) of water per minute, roughly the same as 75 fire engines working together. A typical boat of this sort, the Boeing Jetfoil, speeds along on three inverted T-shape foils. Each foil is fitted with sensors wired to an on-board computer. This constantly adjusts flaps on the foils to maintain lift and keep the ride smooth.

Photo: You can see how the hull of this hydrofoil boat rides completely clear of the water on three foils, two at the back and one at the front. This boat's the USS Taurus (PHM-3). Photo by Mark S. Kettenhofen courtesy of Defense Imagery.

Further reading

Books

• Ships and submarines by Chris Woodford. Facts on File, 2004. My book about the history of ships, from ancient craft to the latest wave-piercing catamarans.
• The Hydrofoil Mystery by Eric Walters. Puffin, 2003. A blend of history and fiction for young readers.

Websites

• Boeing: Hydrofoil history: Some great photos of fast-moving hydrofoils developed by Boeing in the 1970s.
• Alexander Graham Bell and the Hydrofoils: How the telephone pioneer developed hydrofoils at the end of his life.