Sit-down hydrofoil

The sit-down hydrofoil, first developed in the late 1980s, is a variation on water skiing, a popular water sport. When towed at speed, by a powerful boat or some other device, the board of the hydrofoil 'flies' above the water surface and generally avoids contact with it, so the ride is largely unaffected by the wake or chop of the water and is relatively smooth. The air board is a modified hydrofoil where the skier stands up.

Hydrofoils date back the early 1900s, however they were not part of a recreational sport. While the first hydrofoil boat was created in 1906 by Italian inventor Enrico Forlanini, the first waterski hydrofoil was invented in the early 1960s by Walter Woodward, an aeronautical engineer, with two skis attached to a bi-wing hydrofoil. In 1972, Mike Murphy and Bob Holst added a bi-wing hydrofoil to a surfing kneeboard, then reduced the bi-wing to a single wing. Murphy and Bob Woolley then applied the concept to sit-down hydrofoil, with Woolley riding the first "Sit Ski" in 1984. After various material changes, including the addition of a seat belt to keep rider and ride together and prevent separation collisions, they sold their first "Air Chair" in 1990. Advancements in materials and hydrofoil design have created stronger rigs, gentler rides (shock absorbers in the seat), more maneuverability, and boards that combine sit-down and stand-up (air board) for surfing.

Unlike water skiing or wakeboarding, a hydrofoiler's body and the board rise above the water, supported by a pair of front and rear hydrofoil wings which are still under the water. This reduces the drag of the water, allowing for both a smoother ride on rough water and a lesser need for strength.

The rider sits on the seat of the hydrofoil and is strapped in with a seat belt; their feet are strapped into bindings near the front of the board. When the rider is ready the boat will start to tow the skier. The water flowing past the hydrofoil wings generates lift, which can be controlled by the rider to move the board up and down or side to side above the surface of the water. The rider must be centered over the post of the hydrofoil; small body movements will cause great reactions with the hydrofoil. In order to "float" upward the rider leans back while maintaining balance to avoid pitching forward unexpectedly. To go back down the rider leans forward or pushes down with the feet. Turning is accomplished by pointing the knees in the direction desired; the hydrofoil will follow. To jump, the rider leans even farther back. This will give the rider a somewhat stable base to perform aerial tricks for example.

...
Wikipedia