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 two times the wind speed.
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 attitude. 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.
Monohull boats typically employ a "ladder" arrangement of hydrofoils splayed out with a dihedral angle of 50 degrees, with a stabilizing rudder foil. One of the earliest examples is the Monitor boat from 1957. This design offers the advantages of maximum lifting foil area at slow speeds and less at higher speeds, with rolling resistance arising from the dihedral support of the outboard ladder foils.
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 Australia, 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. Subsequently, Garth and John Ilett in Perth, Australia developed a two-hydrofoil system for the Moth with active flap control for the main foil via a surface sensor. John's company Fastacraft was the first to produce a commercially available hydrofoil International Moth. Fasta Craft's Prowler design, superseded in 2008 by the F-Zero, features a carbon-fiber hull, inverted "T" foils on the centerboard and rudder, and can reach speeds of over 27 knots. Fasta Craft has since been joined in producing hydrofoil Moths by several other companies, including Bladerider, Assassin, Exocet, and Aardvark Technologies.
Although initially debated fiercely within the class, the adoption of hydrofoils has proven a success for the International Moth class, with rapid fleet growth in the years since 2001. All World Championships since 2004 have been won by hydrofoil-equipped Moths, which can become foilborne in as little as six knots of breeze when steered by an experienced sailor of lighter weight. The class rule remains open to development of all boat components including hydrofoil systems, and development within the class continues to be spurred by both commercial and individual/amateur efforts.
The 2013 America's Cup featured daggerboard catamarans. Under the terms of the protocol, these daggerboards may not feature trim tabs, may not exceed the beam of the boat when raised and may not be adjusted when lowered, but a loophole exploited by three teams was to create T-shaped rudders and L-shaped daggerboards of which the leeward appendage serves as a hydrofoil on all points of sailing conditions in winds over 10 knots. On September 6th, 2012 in Auckland, during Team New Zealand's fifth day of trials, their boat achieved circa 40 knots (74 km/h) with a level trim and no heeling with 17 knots of breeze.
Recent C-Class Catamarans have been foiling, and further development is expected.
A-class rules never specifically outlawed foils, but the 2014 A Class Catamaran World Titles in Takapuna New Zealand, brought foils to the class. 
The Nacra17 has entered the olympic classes, and is capable of elevated foiling under some conditions.
In the 1990s the Hobie Cat company manufactured the TriFoiler (no longer in production), a twin-sail trimaran with a mainsail on each outrigger capable of 30+ knot speeds in typical sailing conditions, making the TriFoiler the fastest production sailboat in the world. The prototype of the Hobie TriFoiler, called Longshot, was developed by brothers Dan and Greg Ketterman in conjunction with Russell Long. Though more streamlined than the Trifoiler and having smaller hydrofoils, Longshot still holds the Class A speedsailing record of 43.55 knots on a 500 meter course, set in Tarrifa Spain in 1993. Until recently, it was the only existing speedsailing record held by a hydrofoil, but the recent records of Hydroptère have added to the list with record breaking runs across the English Channel.
In 1998, WindRider LLC introduced the WindRider Rave, a popular two-person trimaran hydrofoil capable of lifting off in as little as 12-13 knots of wind. The Rave is capable of sailing between 1.5 to 2 times wind speed. The boat's mainsail has no boom. The Rave broke new ground in the development of flapped foils and control systems.
Non-production experimental designs have been built:
- The French experimental Hydroptère has set numerous speed records. The boat attained a record speed of 47.6 knots with a goal of breaking the "50 knot barrier" in 2008. On 4 September 2009, l’Hydroptère broke the world record, sustaining a speed of 51.36 knots for 500m.
- The Vestas Sailrocket 2, a boat specifically built for breaking the world record. It broke the world record on 18 November 2012, and further pushed it on 24 November 2012 by sailing 65.45 knots.
- A Hobie 18 foilcat prototype called Kangalope with foil kits attached.
- Mirabaud LX is a Swiss 1.2 meter hydrofoil boat with traditional hull replaced with centerfloat, supporting spar for foil, sail and crew perch similar to moth sailing boat.
- Rich Miller's hydrofoil sailboard.
- "Monitor specs". International Hydrofoil Socieity. Retrieved 2008-06-03.
- Team NZ's new catamaran in full flight, TVNZ, 2012-09-06
- http://www.yachtsandyachting.com/news/172395/Foiling-Little-Cup-Cats-set Missing or empty
- http://www.a-cat.org/?q=node/359 Missing or empty
- "Hobie TriFoiler History". Retrieved 2008-05-15.
- "WindRider Rave". Retrieved 2008-05-15.
- Burns, Thom (1998). "The Rave Hydrofoil". Sailing Breezes online magazine. Retrieved 2008-05-15.
- "Assault on the World Sailing Speed Record". 2008-02-11. Retrieved 2012-09-09.
- "l’Hydroptère, new version". 2007-12-14. Retrieved 2012-09-09.
- "L'Hydroptère, monstre de vitesse". L’Équipe. 2009-09-06. Retrieved 2012-09-09.
- Carlson, Dave. "Hydrofoil! (Hobie 18 foilcat Kangalope)". Retrieved 2008-05-15.
- "Mirabaud LX".
- "The Miller Hydrofoil Sailboard". International Hydrofoil Society. 2007-09-11. Retrieved 2008-05-15.
- Foils.org gallery of sailing hydrofoils
- Miller Hydrofoil Sailboard - a hydrofoil windsurfer experiment
- Sailing Cat with Foils
- Trimaran Foil Assisted
- Aspects of a Foiled Multihull
- Design Forum and information exchange