Although the fighter is two and a half times lighter than the small passenger plane, its engines need to produce a force 1.8 times that of the passenger plane's thrust. By this example it is obvious that much fuel is saved by taking off through means of a runway as opposed to vertically. Hydrofoils are interesting sea-going vessels which work on the same principle as airplanes. They have foils which extend from steel girders down underneath the boat. When the engines propel the boat forwards, the water rushes over and under the hydrofoils as air does about an airfoil.
Hydrodynamic lift is created, lifting the boat partially out of the water.
Since the boat remains only partly in the water much of the friction with the water is reduced and greater speeds are more easily attained. Since the density of water is 840 times the density of air, the foils needed to lift the vessel are much smaller than those needed to lift an airplane. More water will run over and under the foil than air possibly could. A large hydrofoil can weigh up to 305 tonnes while offering about half the resistance of a planing boat. Hydrofoils can attain speeds of up to 20 to 30 metres per second.