Next Generation Propulsion Emerging Technologies

Foiling for leisure sailors

Tuesday, 28 March 2023

Niche gears up to engage new audience

“A new balance is created,” foiling boat designer Jurian Rademaker explains. He is one of the numerous boatbuilders around the globe that is trying to popularize sailing on foils. Creating boats that are easy to handle even when it sails at high speed above the water, his company Aeronamics is preparing ‘foiling for the masses’.

The new balance that Jurian indicates is created when the forces that drive the boat, lift the boat, push it over and keep it straight, all change position. With the hull in the water, buoyancy comes from the hull, the wind pushes in the sail to heel the boat over, the weight of the keel or the persons in the boat opposes this heeling force to prevent the boat from falling over. The drag is quite big, as the hull has to push all the water aside to move forward. All of these forces change when the boat speeds up, the hydrofoils start generating lift and the boat is lifted out of the water. The speed of the wind along the sails dramatically increases with the boat’s acceleration. With the higher airflow along the sail profile, the sideway forces that are experienced are reduced. From the perspective of the sailor aboard, the boat is a lot lighter on the sheet. With the hull out of the water, it is not hull buoyancy that withstands gravity, but the lifting force from the hydrofoils. With only these sleek appendages in the water, drag is reduced to maybe few percents of the hull drag before foiling. With less drag and little force on the sheet, is seems that forces on the boat have reduced. The boat is moving at great speed. Any change in the new balance may cause big changes in the speed, lift, drag and direction of the boat.


Maintaining the precarious balance of a foiling dinghy like the Moth or the Waszp is an art that can be mastered by athletes that put a lot of practice into it. These mini sailing rockets have one central hydrofoil under the hull and another at the rudder. That is why Aeronamics designed their Flo1 dinghy with two foils at the hull, that reach quite far out from the sides of the boat. With two foils stretching out besides the hull, a stable lifting force is generated by three hydrofoils: two on each side of the boat and one under the rudder. The mast is in the middle and the sailor can adjust the total balance by pulling in more sheet and positioning his body weight more outside to counteract increased heeling force from the wind in the sail. The stable lift generated by a triangle of three foils allows for less practiced sailors to experience the thrill and speed of flying above water, only powered by wind in the sails.


Foiling is gaining momentum in the sport of sailing. With the sport’s most prestigious event, the America’s cup, the technology of hydrofoil design and control has taken a big flight from the moment that the yacht from New Zealand for the 34th edition of the cup was able to sail on hydrofoils in 2012. The race of 2013 in San Francisco Bay saw gigantic 72 foot long catamarans racing elevated above the water on foils both upwind and downwind. The 35th edition in 2017 in the waters around the isle of Bermuda was raced on 50 foot foiling catamarans. The 36th edition, 2021 in New Zealand was sailed on 75 foot monohulls, with retractable foils alongside the hull, with one in the water on leeward to provide lift and stability while the windward foil was lifted out of the water. The foil under the rudder kept the boat horizontal over the length, with the stern also out of the water. Exciting footage of the races stirred the interest of competitive sailors for foiling.

Half the Olympics on foils

The 2024 sailing Olympics,  two out of four classes of one design boats for both women and men will be foiling: the Windsurfer iQ Foil and the Kiteboarder foiling. From the two mixed-crew sailing boats, the Nacra 17 catamaran is equipped with foils. Although Nacra designers declare that the foils are not primarily designed to lift the boat out of the water completely, but add windward lift and add righting moment to the boat, well trained Olympic sailors are seen to be able to race the boat while it does fly above the water.

Imoca 60 foot monohulls that are currently racing the globe in The Ocean Race use foils to reduce wetted surface and to gain windward lift and righting moment. These and other racing classes of foiling catamarans and monohulls strongly point towards a foiling future of competitive sailing.

Flap Control

Foils come in different forms and styles with different solutions to control the lifting power. A foil is basically a wing profile under water, that will generate vertical lift when water flows along. When the boat speeds up, the lifting moment increases. Foils in an upside down T-shape need flaps to control the lift. The size of the wing under water is always the same, at ow speed, at the speed that is just enough to lift the boat, and still when the speed increases. At some point the lift will be so big that the foil itself is at the surface and the boat become uncontrollable. Flaps can help increase the lifting power at low speed to help the boat come off the water surface early. Then they can be adjusted to the speed to control the height of the boat. At really high speeds, the flaps prevent the foils from lifting the boat too high. Aboard America’s Cup yachts, the flaps are controlled by the crew that is informed about the lift through numerous sensors. On Moth and Waszp dinghies, the flaps are controlled mechanically by a floater in front of the boat, that goes up and down with the height of the boat above the water and that is directly connected to the flaps.

Automatic Height

Another way to control the amount of lift generated by the foils is to use curved or V-shaped foils. This method is most commonly used in the boats that aim to enable amateur saiors to fly.  “Surface piercing foils are a self-regulating method to control ride height,” Rademaker says. At lower speeds, when the boat has its hull in the water, the foils are almost totally submerged. When the boat starts picking up speed, the foils create a lot of lift with all their surface in the water. When the speed increases and the hull comes out of the water, the part of the foil closest to the hull and the end of the foil on the opposite side of the curve of v-shape, also come out of the water. The surface of the foil in the water decreases. Therefor, the total lifting potential of the foil is less. When the boat speeds up even further, lift from the wing shape profile increases. Simultaneously, the surface of the foil in the water gets smaller. This way, the curved or v-shape of the foils automatically regulates the ride height. “Most drag when sailing above the water on foils, is generated at the surface, where water meets air,” Rademaker teaches. “An added advantage of surface piercing foils is the fact that they generate windward lift at the foil end that comes above the surface. The wing shape at that point helps to sail to the wind, both when the boat is flying above the water as when the boat is buoyant.”

Leisure potential

Like the Flo1, also the French foiling dinghy Peacoq utilizes this principle. While these dinghies aim to make foiling easy, bigger yachts like the third generation Beneteau Figaro, the Cub Swan 36 and the concept MW40oF from naval architects Wilson/Marquinez, aim at skilled sailors.  While the sport of sailing, with the Olympic foiling classes int the wake of the America’s Cup and the Ocean Race, show the speed and excitement of sailing on foils, the number of leisure foiling sailboats that are currently being produced herald a new branch for the aspiring leisure boater and a young audience that like to fly. The days of mass production foilers are near.

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