How to do Foil Surfing

Surf-Foiling emerged in the 2000s. Among the pioneers were big names like Laird Hamilton, Dave Kalama, and Paolo Rista. Originally, it was just a way to fly above the water while tow-surfing in large swells. In 2016, Kai Lenny caused the number of practitioners to explode. The discipline immediately conquered Hawaiian, Australian, Californian, and Brazilian surfers. The level soared. Gradually, surf foiling arrived in Europe, and in France. The advantages of foiling are to increase the number of possible sessions, practicable spots, and offer new sensations. In this article, we will explain how a Surf Foil works, its different parts, and we will detail the importance of the front wing's characteristics.

This article explains how a Foil works for the following sports: Surf Foil, SUP Foil, Downwind SUP Foil, and Tow-in Foil (tow-in waves).

The operating principle of a surf foil comes from fluid dynamics. The front wing is pulled upwards when it moves forward, because the water molecules passing over the extrados of the front wing accelerate to catch up with the water molecules that passed over the intrados (which is shorter because it is flat). The acceleration of molecules on the upper part of the wing creates a depression, and the deceleration of those on the lower part 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).

We can easily compare the operation of a foil to that of an airplane, which has roughly the same shape if we omit the mast and the board. The speed of movement creates lift and allows it to take off. The difference in angle given to an airplane's wings allows it to stabilize in the air. A decrease in travel speed reduces lift and thus allows an airplane to land (at a constant angle of incidence). Finally, to change the foil's angles of incidence, the rider can, by alternating pressure on the front or rear foot, make the foil go up or down or stabilize it. It's all about dosing the pressure on the supports.

To take off, the speed must be high enough to create a lift force on the front wing. Combined with a high angle of incidence (thanks to pressure on the rear foot), the board takes off. To stabilize, pressure on the front leg reduces the angle of incidence, thus the lift, and accelerates.

In parallel with the Foil's lift, its speed depends on the surface area and angle of incidence of the front wing. A front wing with a small surface area and/or little thickness will offer more speed, and a front wing with a smaller angle of incidence will also be faster.

Next, the specificities of the wing's shape for the same surface area affect the Foil's speed. A wing with less chord and thus more span will be faster. Moreover, the wing's profile (parameters mentioned above) is very influential on speed and the ability to accelerate.

Finally, as mentioned earlier: the rider's weight, the board's size and weight, and also the power and speed of the swell are external factors that strongly condition the Foil's speed.

A relatively slow Foil with a lot of lift (i.e., large surface area, wing thickness, and a bit of angle of incidence) will be more stable than a less lifting and faster foil.

The main elements that allow the Foil to gain stability are the length (chord) of the front wing, its thickness, and the surface area and span of the rear wing, also called the stabilizer.

The rear wing (stabilizer) plays its full role in stabilizing the Foil. Its lift is inverse to that of the front wing and thus slightly counteracts the front wing's lift by creating opposing forces to stabilize the entire foil.

External factors influencing the stability of the Foil are necessarily the rider's weight and technical skills, but also the type of body of water on which Surf Foil is practiced. A choppy body of water with current, eddies (backwash), and breaking waves will be more unstable because these elements disturb the depressions and overpressures that generate the upward attraction of the Foil (like an aerial disturbance when in an airplane).

While a Foil with little lift (i.e., small surface area, angle of incidence, and thickness) is faster and indirectly more maneuverable, this is not the only parameter that affects its maneuverability.

The major element concerning maneuverability remains the shape of the front wing, particularly the lateral curve, which, when accentuated, allows for easier turning with the Foil. A high lateral curve will result in a more maneuverable but slower foil because for the same lift surface area (projected shadow on the ground), the total surface area will be larger and will create more drag. Conversely, a flatter lateral curve will turn less but will be faster.

Span also influences maneuverability. The larger the span, the longer the foil will glide, but the less tight the turns will be. It's a bit like a longboard versus a shortboard.

The overall shape and dimensions of the Foil vary these four characteristics:

• The larger, thicker, and higher the angle of incidence of a wing = the more lift it generates and the slower it goes
• The smaller a wing, with little thickness, little angle, and little chord relative to its span = the faster it is
• The more span a wing has, the longer it will glide and thus the more efficient it will be for pumping.

• The larger and thicker a wing with a longer chord = the more stable it is
• The smaller a wing, less thick with more curve = the more maneuverable it is
• The more span a wing has, the less maneuverable it will be.

When we developed our S-Foil, we spent several months of R&D and testing to achieve the best compromise of accessibility, versatility, and performance.

While the choice of accessibility and versatility does not always align with performance, we have worked on the profile and shape of our wings to find a compromise that allows for easy pumping, maximum glide, and maneuverability.

By adapting the length of the fuselage and the size of the stabilizer, we can make a foil much more maneuverable and thus achieve a compromise between glide, ease of pumping, and foil maneuverability. This is what we have done in the UHA range; combined with shorter fuselages, we achieve an almost perfect compromise.

Length

The length of the board will influence your Surf Foil practice in two ways. A longer board with more volume makes it easier to catch waves for takeoff. However, a long board reduces the foil's lift once in the air compared to a short board due to the extra weight at the front.

While the choice of accessibility and versatility does not always align with performance, we have worked on the profile and shape of our wings to find a compromise that allows for easy pumping, maximum glide, and maneuverability.

By adapting the length of the fuselage and the size of the stabilizer, we can make a foil much more maneuverable and thus achieve a compromise between glide, ease of pumping, and foil maneuverability. This is what we have done in the UHA range; combined with shorter fuselages, we achieve an almost perfect compromise.

Width

The width of the board, like its length, can facilitate paddling into waves for takeoff. However, once in flight, the rails of a board that is too wide risk touching the water surface during turns and slowing the rider down.

Weight and Volume

The weight of the board negatively impacts the board's lift. The heavier the board, the more the foil's lift will be counteracted (opposing force). However, an increase in volume allows for better take-off on waves.

Shape

One might rightly assume that the board's shape matters little once in the air, but several details improve the flying experience: the rocker at the front and the bevel at the back and sides to avoid touching the water while flying. Furthermore, other parameters such as the rounded nose shape and the concave on the deck and under the hull help improve paddling comfort and efficiency in catching waves.