The active parameters of the wing
The surface
A large surface area generates a large anti-drift force. Anti-drift force (lift) is proportional to the surface area, but it also increases with the square of the speed. This means that the fin’s surface area must be designed for a given speed. Boards for fast waves, like Guns, use fins with reduced sail area, while slow, gentle waves, such as those ridden on longboards, require large sail areas to provide effective lift. Comparing this to the sail area that is adjusted on a sailboat according to wind speed illustrates the adaptation to fluid speed. To obtain a fin that promotes sliding, simply reduce its surface area until it no longer catches at low speeds, such as those achieved in maneuvers involving board braking. This can even lead to a finless setup (a board without fins).
The plan view (aspect ratio)
The distribution of the aileron’s surface area modifies the center of lift. “Base,” “Rake,” and “Height” influence the overall characteristics of the aileron.
o The length of the base provides directional stability but also trajectory rigidity; the longer it is, the more “directional” the surf will be (the board’s tendency to impose the trajectory and increase the turning radii, which is the opposite of maneuverability).
o The height allows the fin to be submerged deep in the water. Remember that Simmons proposed the twin fin setup to position the fins close to the rail in order to avoid loss of grip in turns or vertical waves that tend to lift the central part of the board out of the water (Ventilation).
Thanks to new composite materials that allow for greater elongation than wood, Greenought has recentered the fin on the board’s axis and increased its height to ensure it no longer vents. Recentering the fin reduces cant (see fin placement), but a center of lift that is too low increases cant and slows the transition from one rail to the other due to leverage. The right compromise between cant, slowing the roll, and ventilation lies in choosing the correct height. In longboard waves (gentle slope and curve), there’s no need to seek depth; it’s better to maintain the maneuverability provided by good rail penetration. For radical maneuvers that vent the central area, side fins can be used in various placement options (see fin placement).
o The Height/Width ratio: Aspect ratio is a determining factor in the distribution of lift; an elongated wing generates less vortex loss than a wide wing.
The rake shifts the center of lift towards the rear. This increases directional control. Considering that ventilation is less pronounced at the rear of the board, which is more frequently submerged, the rake contributes to reducing ventilation by keeping the lift point further aft. By moving the lift point away from the fin’s fixed base, the rake increases the mechanical lever arm, generating more flex for a given fin thickness. Therefore, the rake increases flex. Decreasing the rake moves the fin’s center of lift forward, effectively moving the fin forward. Various fin rakes can be used to advance or retreat the pivot point generated by their center of lift, compensating for improper fin box adjustments.
The Flex
By lengthening the shapes and adding rake to its fins, Greenought brought fin flex to the board. Maneuvers are made smoother and destabilizing impacts are reduced by this shock absorber, which stabilizes the trajectory in large waves. Flex is also used by powerful surfers to store energy by exaggerating the maneuvering effort in order to recover the spring’s rebound at the exit of the turn.
The profile
If you cut a slice of fin, you get a fish. There are 3 types of profiles:
o Symmetrical profiles:
This type of profile is used for ailerons that need to work identically on both sides; the central fin is constructed using this type of profile.
o Flat profiles:
This type of profile is simpler to manufacture using injection molding; only one half of the mold is needed to produce the part. It is used to produce low-cost side fins.
o Curved profiles:
A cambered profile generates better performance (the performance of a profile is its lift/drag ratio, also called the hydrodynamic efficiency of the profile), provided that its upper surface (the profile’s hump) is on the side where you want to grip. This necessitates the use of profiles with opposing upper surfaces on either side of the board.
The future of the surf fin
For a hydrodynamics specialist, or a naval architect, it is easy to see the obvious improvements that surf fins can receive:
- Variable profile geometry, to always expose the upper surface on the correct side, throughout the entire trajectory.
- Angles de toe variables, synchronisés conservant tous les ailerons parallèles pour ne pas générer de freinage et produire les performances optimales sur tous les ailerons en permanence, même dans les phases ou tous les ailerons sont immergés (90% de la trajectoire d’une vague ordinaire),
- Adjusting toe angles to tighter or wider trajectories to prevent stalling
- Applying soft trailing edges to eliminate trails at the exit of profiles, never mind if it’s safe, the daredevil surfers will shave elsewhere.
- Application of soft leading edges, just for safety and the joy of seeing our passion remain a pure moment of happiness without unnecessary risks, even if the manufacturing costs are higher.
Isn’t it a bit utopian to produce such fins that exploit these logical hydrodynamic optimizations? How can one profit from it, producing in France a fin technology with an articulated skeleton, allowing for variable profile geometry? Isn’t it too complex, since it would also require mastering the fusion of materials with varying degrees of flexibility, demanding investments in industrial tooling that necessarily incorporate profit objectives impossible to achieve in order to satisfy shareholders? Unless you’re a professional passionate about hydrodynamics and surfing, wanting to invest in research and development, without depending on or waiting for commercial returns. Just for the pleasure of exploring new trajectories with a group of friends, some more skilled than others, who simply love gliding along with the currents…
Jf Iglesias,
Software developer at HELICIEL MECAFLUX,
Consultant and trainer in fluid mechanics applications,
R&D manager at Fynsurf
