underwater glider

Hydrofoil surfing is just one of those things that doesn’t look right when you see it for the first time. These guys are surfing, riding down the face of a wave, and yet the board itself is more than a foot above the surface. Huh?

In this video, the foil is attached to the bottom of the board via a single strut. After a jet-ski pulls the surfers up to speed, allowing the hydrofoil to push board and rider up out of the water, they let go of the tow rope and let the power of the wave take control. Terry Hendricks, a physicist and long-time surfer who has designed an innovative wave-rider of his own, says the foil effectively acts like an underwater glider. When the surfers are coasting down the face of the wave, the water itself is rushing upward, getting sucked up by the energy of the swell. This rushing water acts like an updraft in the air, generating lift—only in this case it’s keeping the foil flying instead of a glider.

The real trick, though, is  balance. Hendricks compares the form of hydrofoil surfing practiced in this video to riding a unicycle. The rider is balanced over that single strut, and there are probably 30 inches between the bottom of the board and the foil below the water. His own model uses two foils, front and back, and a bodyboard approach. The rider lies down, kicking into the wave. This makes it easier to balance but produces a much bumpier ride, since the leading foil stays at water level. With the surfers shown here, on the other hand, the single foil is a good distance below the surface most of the time, so they’re completely avoiding wind chop, and smoothly cruising down the face of waves that look like mogul hills. Ready to try it out?—Gregory Mone