Surf-foil appeared in the 2000s. Among the precursors, there are big names like Laird Hamilton, Dave Kalama or Paolo Rista. Originally, it was just a way to fly over the water in a surf-tow, in large swell waves. In 2016, it was Kai Lenny who made the number of practitioners explode. The discipline immediately conquered Hawaiian, Australian, Californian and Brazilian surfers. The level of surfing soared. Gradually, foil surfing arrived in Europe, and in France. The advantages of foil surfing are to multiply the number of possible sessions, of practicable spots, and to offer new sensations. In this article we will explain the functioning of a surf foil with its different parts and we will detail the importance of the characteristics of the front wing.This article explains how a foil works for the following sports: Surf Foil, SUP Foil, Downwind SUP Foil and Tow-in Foil.
Composition of a Surf FoilThe Foil consists of the following components: mast, fuselage, front wing and rear wing (stabilizer). The mast is attached perpendicular to the board and fuselage. Thus, the wings are parallel to the board. In this way, it is possible to control the foil by simply pressing the feet. The front wing has a curved top surface and a flatter bottom surface. The front wing has a rounded leading edge that quickly increases in thickness and then tapers to a thinner trailing edge. The width of the front wing (from the centre of the leading edge to the centre of the trailing edge) is called “chord” and the length (the right/left end) is called “span”. Finally, the horizontal stabiliser has a similar shape to the front wing, with equivalent properties. However, it has a smaller surface area and a different profile.
Principles of operation of a foilThe principle of operation of a surf foil is based on fluid dynamics. The front wing is pulled upwards as it moves forward, as the water molecules on the top surface of the front wing accelerate to catch up with the water molecules on the bottom surface (which is shorter because it is flat). The acceleration of the molecules on the top of the wing creates a vacuum and the slowing down of those on the bottom creates overpressure. The consequence is an upward suction effect. The higher the speed of the foil, the more the foil is “pulled” upwards (with equal profile and angle). It is easy to compare the functioning of a foil to that of an aircraft, which has more or less the same shape if you forget the mast and the board. The speed of displacement creates lift and allows for take-off. The difference in angle given to the wings of an aircraft allows it to stabilise in the air. Decreasing the displacement speed decreases the lift and therefore allows an aircraft to land (at a constant angle of incidence). Finally, to change the angle of incidence of the foil, the rider can press alternately on the front or back foot to raise or lower the foil or the stability. It’s all a question of adjusting the pressure on the supports. To take off, the speed must be high enough to create a lift force on the front wing. To stabilize, pressure on the front leg decreases the angle of incidence, thus the lift and accelerates.
The swell as a source of energyIf the plane uses the suction force of air molecules, the foil uses the suction of water molecules which have a much higher density than air. This density will increase the lift (the upward suction effect) of the foil even more with speed. In this article we talk about Surf Foil, which means using water movements (wave, swell, hollows, boat wakes) to fly. As opposed to wind, which in kite or windfoil allows the use of a constant force external to the water, the water movements of waves are variable forces with precisely localised lift zones on the water surface.
The Surf Foil and Downwind Foil in pictures
HOW FOIL CHARACTERISTICS VARY
When it comes to fluid dynamics, every little detail or change to the foil is important and affects its characteristics. We have looked at 4 major characteristics of a Surf Foil to explain which parameters influence them (there are many more but we won’t go into them in this article).
LiftFoil lift, or the upward suction effect, varies primarily with the size of the front wing, its thickness, its airfoil shape and its angle of incidence. The larger the surface area of a wing with thickness, the more lift it generates. The higher the angle of incidence, the more lift it generates but the more it slows down the foil. On the other hand, the lower these parameters are, the more lift is reduced. With the same surface, angle and thickness, two different profiles will generate different lift and speed. The surface area as the only reference parameter is not sufficient to determine whether a foil has a lot of lift or not.
SpeedAlong with the lift of the foil, the speed of the foil depends on the surface area and the angle of incidence of the front wing. A front wing with a small surface area and/or thickness will offer more speed, and a front wing with less angle of incidence will also be faster.
StabilityA relatively slow foil with a lot of lift (i.e. large surface area, wing thickness and a small angle of incidence) will be more stable than a slower, faster foil. The main elements that make a foil more stable are the length (chord) of the front wing, the thickness of the front wing and the surface area and span of the rear wing, also known as the stabiliser.
The rear wing (stabiliser) plays a full role in stabilising the Foil. Its lift is opposite to that of the front wing and thus slightly counteracts the lift of the front wing by creating opposing forces to stabilise the whole foil.The external factors affecting the stability of the foil are necessarily the weight of the rider and their technical ability, but also the type of water on which the foil is being surfed. A rough water surface with current, backwash and breakers will be more unstable because these elements disturb the depressions and overpressures that generate the upward pull of the Foil (like an air disturbance when flying).
HandlingA foil with little lift (i.e. little surface area, angle of incidence and thickness) is faster and indirectly more manoeuvrable, but this is not the only parameter that affects its handling. The most important factor in handling is the shape of the front wing, namely the side curve, which is accentuated to make it easier to turn the foil. A high side curve will result in a more manoeuvrable but slower foil because for the same amount of lift (ground shadow) the total surface area will be larger and will brake. Conversely, a flatter side curve will turn less but will be faster.
The general shape and dimensions of the Foil vary these four characteristics
- The larger the wing, the thicker the angle of incidence = the more lift it has and the slower it goes
- The smaller the wing, with little thickness, little angle and little chord compared to its wingspan = the faster it goes
- The bigger and thicker the wing with more chord length = the more stable it is
- The smaller and thinner the wing with a curve = the more manoeuvrable it is
OUR CHOICES IN DEVELOPING THE S-FOIL
When we developed our S-Foil, we spent months of R&D and testing to find the best compromise between accessibility, versatility and performance.