Sailing hydrofoil

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A sailing hydrofoil 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.

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

1 Typical configurations
2 Production designs
3 Experimental designs
4 See also
5 References
6 External links

[edit] Typical configurations

Some multihulls use three foils; two main forward foils provide lift so that the boat "flies" while 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.

[edit] Production designs

Hobie TriFoiler.

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. A Hobie TriFoiler still holds the Class A record of 43.55 knots on a 500 meter course, set in in Tarrifa Spain in 1993.^[1]

In 1998, Windrider LLC introduced the Rave hydrofoil, a popular one-person trimaran capable of lifting off in as little as 8–9 knots of wind. The Rave is capable of sailing between 1.5 to 2 times wind speed.^[2] The boat's mainsail has no boom. The Rave proved to be such an unbalancing factor in small-boat regattas, that now regattas are held exclusively for Rave boat owners.

The Australian company Fastacraft produces an International Moth monohull fitted with a hydrofoil kit. The boat features a carbon-fiber hull, inverted "T" foils on the centerboard and rudder, and it can reach speeds up to 25 knots.^[3] The company also manufactures human-powered hydrofoil boats.