Traditional sailboats are displacement hulls with keels or centerboards. Foiling boats are ultra-light and have a wing on the centerboard and rudder, which allows the whole boat to rise out of the water.

A sailing hydrofoil, hydrofoil sailboat, or hydrosail is a sailboat with wing-like foils mounted under the hull. As the craft increases its speed the hydrofoils lift the hull up and out of the water, greatly reducing wetted area, resulting in decreased drag and increased speed. A sailing hydrofoil can achieve speeds exceeding double and in some cases triple the wind speed.

Both monohull and multihull sailboats can be retrofitted with hydrofoils, although greater stability can be achieved by using the wider platform of a catamaran or trimaran.

The most widespread use of hydrofoils in sailboats to date has been in the International Moth class. Andy Paterson of Bloodaxe boats on the Isle of Wight is widely considered to have developed the first functional foiling Moth, though his boat had three foils in a tripod arrangement. Brett Burvill sailed a narrow skiff Moth with inclined surface-piercing hydrofoils to a race win at the Moth World Championships in 2001 in Australi, which was the first time a hydrofoil Moth had won a race at a World Championship. This hydrofoil configuration was later declared illegal by the class, as it was felt to constitute a multihull, which is prohibited by class rules.

Some multihulls use three foils; two main forward foils provide lift so that the boat “flies” while a horizontal foil on the rudder is trimmed to drive and control altitude. On catamarans, a single main foil can be attached between the hulls just in front of the center of gravity and at 2 degrees of incidence, spanning the tunnel with supporting struts. Hydrofoil catamarans are also called foilcats.

Multihull sailboats can also employ hydrofoils only to assist performance. Just as daggerboards and rudders are foils that enhance the control of a boat, assisting hydrofoils provide lift to the hull to reduce the wetted area without actually lifting the boat completely out of the water.

Based on the 11ft Moth dinghy, foiling began to spread throughout the sport of sailing. And it was not long until hydrofoiling boats of all different shapes and sizes were taking the water.

Over time, some traditional classes converted to foiling – the A-Class and C-Class catamarans being examples. But more new boats were also designed specifically with hydrofoiling in mind.

In 2013 Emirates Team New Zealand built their 72ft America’s Cup catamaran to be a foiler, forcing their competition for the Cup, Oracle Team USA to convert their AC72 into a foiler to stay competitive – ultimately Oracle Team USA won the Cup in one of the biggest sporting comebacks of all time.

To date the America’s Cup has not looked back with the competition taking place in smaller hydrofoiling AC50 catamarans in 2017 and the newly conceived monohull foilers, the AC75s, in 2021.

In 2021 the Olympics Games introduced the first ever foiling catamaran in the Nacra 17.