Sailing hydrofoil
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 planform of a catamaran or trimaran.
Typical configurations
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.
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, but with some additional drag arising from the dihedral support of the outboard ladder foils.
Types of hydrofoils
There are four main types of hydrofoils used in sailboats, in which each type employ similar and different advantages to each other. Overall, 'T' Foils fit into their own category of single direction lift, while 'C', 'L', and 'S' fit into a category of multidirectional lift.'T' Foils
'T' Foils, commonly seen in the International Moth class and the AC75 class, contains a vertical component connecting to a horizontal wing, providing lift for the foil. 'T' Foils only provide lift in the perpendicular direction from the horizontal wing component and hence are typically used only for upwards lift instead providing lift in the vertical and horizontal direction. 'T' Foils are often equipped with an adjustable trailing edge that attaches to some flight control sensor to ensure steady flight. This system was easily implemented in the Moth class.'C' Foils
'C' Foils also known as curved daggerboards are shaped as a constant curve and are allowed to travel up and down within its casing, changing the area of the board exposed to the water. When the board starts at its maximum height above the water, it is near vertical and employs a lifting force normal to the exposed surface or otherwise horizontal lift. As the board is pushed lower, the 'C' shape forces a larger portion of the board to become horizontal, introducing a larger vertical component in the lift. This variation in configuration allows for many boats to provide a higher upwind angle due to the horizontal lift in combination to the vertical lift. The curved boards also solve the issue of increased drag at high speed, while no longer requiring a strong horizontal lift by adjusting the exposure of the board to the water.'S' Foils
The 'S' Foil contains the slight shape of a 's' and operates in a similar manner to the 'C' Foils, with the reverse order of operations, such that when lowering the 'S' Foil the horizontal lifting force exerted increases due to a more vertical orientation in the board.'L' Foils
'L' Foils consist of a curve edge followed by a sharp 90-degree bend such that the bottom of the foil sits upright when not submerged in the water, and sits at a 45-degree angle from horizontal when submerged entirely. Similarly to the 'C' and 'S' foils, the amount that the foil is lowered changes the amount of vertical and horizontal lift being created.Foiling classes
Monohulls
International Moth
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. Initially Ian Ward in Sydney, Australia developed the first centerline foiling Moth which demonstrated that sailing on centerboard and rudder foils alone was feasible. 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.
Waszp
The Waszp class founded in 2016, is almost identical to the current configuration of the International Moth class. The class contains the same "T" foils design on the centerboard and rudder, with the same narrow hull shape. The largest difference between the two classes is that the Wazsp is heavier due the foils being constructed out of aluminum instead of carbon fiber. Due to heavier design, the top speed of the waszp is only 26.7 knots, however its cheaper materials have made it a popular and accessible class, despite its relative youth. Most recently, the Waszp class has partnered with SailGP to develop a pathway for young sailors to work their way into the professional field of sailing.69F
The 69F also known as the Persico 69F is a 3-4 person fully carbon foiling monohull, resembling that of a skiff, designed by Wilson-Marquinez and built by Persico Marine. The newly founded one-design class had its first racing series in 2020, starting in Europe and then making its way over to the United States in 2022. The boat consists of a singular t-shaped rudder foil, two 'L' foils on each side, and a singular daggerboard in the center of the boat for stability. The 69F is 6.9 meters long and contains a fully battened main sail and jib alongside a gennaker, totaling a sail area of 69.0 meters.Laser dinghy
The first time a Laser foiled was in December 2009 by Ian Ward in Sydney, Australia.Top speeds of around 23-25kts are possible with this kit. The foils can be removed and the boat remains class legal. This foiling system is unique in that it is retractable, has flapless foils and the height sensing 'wand' is integrated within the daggerboard.
Optimist
The "world's least advanced sailboat" was converted to hydrofoiling by the Chalmers University of Technology, in Sweden, in 2017.The Optimist, only 2.3 metres in length and with a sail area of 3.3 square metres, is normally limited to speeds below 4 knots. The hydrofoils allows the optimist dinghy to achieve 12 knots in only 12 knots of wind.