How a Surf Foil works
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.
Composition of a Surf Foil
The 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 foil
The 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 energy
If 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.
It is thus a question for the practitioner of Surf Foil to leave lengthened on the board with the foil below and to row to take a wave (in SUP Foil one leaves directly upright and one paddles with the paddle to take the wave). Once on the wave, the accumulation of the speed generated by the slope of this one allows the takeoff.
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).
Foil 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.
Indeed, the specificity of the front wing profile will influence the overall lift of the foil. This can be the upper curve of the upper surface or the lateral curve, i.e. the one that goes from one side to the other in the direction of the length.
Finally, there are other external elements that influence the general lift of the Foil, such as the weight of the rider, the weight and size of the board and of course the power of the waves.
Along 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.
Secondly, the specific shape of the kite for the same surface area affects the speed of the foil. A wing with less chord and therefore more span will be faster. In addition, the profile of the wing (parameters mentioned above) is very influential on the speed and the ability to accelerate.
Finally, as mentioned before: the weight of the rider, the size and weight of the board, but also the power and speed of the swell are external factors that strongly influence the speed of the foil.
A 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).
A 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.
Our vision is to democratize board sports by offering products that are easy to learn and progress with, and that are affordable on the market while having excellent overall performance. We have particularly worked on our S-Foil with an easy, error-tolerant and stable kite profile to accompany every rider as easily as possible.
To achieve this, we have increased the surface area, chord, span and thickness of the front wing compared to our previous models to increase lift and stability. We have also reworked the stabiliser by adding surface area, side curve and a better thickness curve on the surface to maximise glide.
Faced with the influx of equipment that can be found today, the choice of versatility was a natural one. Versatility in terms of the type of practice (Wake, Wing, Surf, Kite, etc.) but also in terms of conditions (wide wind ranges and all types of waves).
By asking our athletes and combining our R&D efforts, we have been able to test the S-Foil for more than 6 different practices in very different conditions with users of different sizes. With 4 interchangeable front wing sizes (M 1250, L 1500, XL 1750 and XXL 2000) we have developed a hyper-versatile foil that adapts: to the practice and the conditions but also to the size and level of each person.
If the choice of accessibility and versatility does not always go hand in hand with performance, we have worked on the profile of our wings on the details that make the difference in order to obtain exceptional performance qualities for each front wing size.
To gain speed and handling we have chosen fast, efficient and very stable profiles and we have paid attention to all the details to reduce drag and friction by refining the trailing edges and moving the wingtip backwards… We have twisted the curvature profile of our wings to gain both stability and speed. Finally, we have validated our different sizes (surfaces) of front wings to gain in speed and performance when the conditions of practice and the level of each user allow to switch to a smaller size.
L’INFLUENCE DE LA PLANCHE SUR LE COMPORTEMENT DU FOIL
The length of the board will affect your surfing in two ways. A longer board with more volume makes it easier to catch waves and take off. However, a longer board will decrease the lift of the foil once in the air compared to a shorter board due to the extra weight at the front.
The width of the board can, as well as the length, make it easier to catch waves when paddling for take off. However, once in the air, the rails of a board that is too wide may touch the surface of the water in turns and slow down the rider.
Weight and Volume
The weight of the board has a negative influence on the lift of the board. The heavier the board, the more the lift of the foil will be counteracted (opposing force). On the other hand, a gain in volume allows the board to take off better on the waves.
The shape of the board may not matter much in the air, but there are several details that improve the flying experience: the rocker at the front and the bevel at the back and sides to keep the board out of the water when flying. Other parameters such as the rounded nose and the concave on the deck and under the hull allow for more comfortable paddling and more efficient wave catching.
NOS CHOIX DE PLANCHES DE SURF/SUP FOIL
Fast Flyer Surfboard
The Fast Flyer is designed for easy and efficient foiling. We’ve opted for a comfortable size of 4’6″ (140cm) long by 21″ (55cm) wide. With a rounded nose, and a compact overall volume, it’s the perfect size compromise for learning, progressing and performing in all conditions.
For ease of rowing, we’ve hollowed out the deck slightly for comfort with a progressive rocker on the nose. We’ve worked the hull with a double concave to maximise the glide at low speeds so you can go out on waves that haven’t broken yet. The entire deck of the board is padded for comfort and durability. Once in the air, the bevelled rails and cut tail keep the board off the surface when turning and pumping. The short length of the board makes it very responsive and stable in turns, and reduces inertia when pumping. For those who want to
SUP Foil Sky Rider and Sroka Air range
We have developed the Sky Rider range to combine performance and versatility. Our 7’8×30 allows you to practice up to 6 sports: SUP Foil, Wing Foil, Paddle Surf, Windfoil, Windsurf and Wake Foil. The generous dimensions of the board and its volume of 120 L make it easy for everyone to learn to foil. The Sky Rider 6’3 and 5’8 will convince the more experienced SUP Foil sailors, and will allow the practice of Wing Foil and Wind Foil. Some people also like to travel light. For this we offer inflatable foil boards. The Air 6′ and 6’4 are perfect for both SUP and Wing Foil.
Sky Rider 5’8 & 6’3
Sroka Air 6′ & 6’4