Page contents
- What is a surf fin for, and how does it work?
- A specification that changes every instant
- Symmetrical or cambered profiles: lift and drag curves
- Drag, lift and stall: what variable geometry changes
- The skiing analogy: skidded turn or carved turn
- Acceleration: lift projected forward
- Rail grip and heeling: a centre of thrust close to the board
- Tip vortices: the soft lip at the fin tip
- Safety and biomimicry
- The FYN designers' story
- Frequently asked questions
What is a surf fin for, and how does it work?
A surf fin is a small vertical wing under the tail of the board. In a straight line it stabilises the course and should go unnoticed; in a turn it provides a pivot through its lift, so the board turns without sliding; correctly oriented and cambered, it tilts that lift forward and accelerates the board (drive, pumping). Its specification therefore changes every instant: the water direction relative to the fin (the angle of attack) goes from 0° in a straight line to 30° and more in a tight turn, where a fixed profile stalls (spin out). A symmetrical profile drags little but bites late; a cambered profile bites from 10° but drags ten times more in a straight line. The variable-geometry fin (ADAC system of FYN fins) solves the dilemma by adapting its angle of attack and camber, from 0 to 35 %, to the line. This page covers the physics of the fin; the guide below covers choice, safety, history and shape.
A specification that changes every instant
Take-off, straight line while gaining speed, bottom turn, cut-back, tube, pumping: in each of these phases the fin's functions vary and sometimes conflict. The surfer, their weight, the board and the wave change the demand again. A fin must bite hard at the moment of the turn and disappear hydrodynamically the rest of the time. To measure this we use the notion of dynamic weight: the forward resistance of the fin, which goes from 70 g for a well-aligned fin to 900 g when its camber and angle do not follow the water. Static weight varies by 0.1 kg between two fins: thirty times less.
You can first test your intuition with the surf fin hydrodynamics quiz, or follow the full explanation on video (in French).
The evolution of the fin since Tom Blake, and the eleven "genes" it left, are covered on the history of the surf fin page.
Symmetrical or cambered profiles: lift and drag curves
The performance of hydrodynamic profiles is listed in profile databases, and we know how drag and lift vary with angle of attack, camber and speed. The curves below, computed with Heliciel for a surf fin, give drag (green), lift (blue) and lift-to-drag ratio (black) from 0 to 90°.
Symmetrical profile (NACA 0008): efficient at low angles, but it stalls sharply at high angles; in a turn, the angle imposed by the curved line causes the loss of grip, the spin out.
Cambered profile (NACA 9505): efficient at high angles, but it drags even at zero angle, which penalises acceleration phases.
Drag, lift and stall: what variable geometry changes
Drag. At zero angle of attack the symmetrical profile drags 0.7 N, the cambered profile 8 N. On a twin whose shaper set 4° of toe, resistance reaches 9 + 9 = 18 N, nearly 2 kg to push with the legs. The ADAC system aligns the fins at zero angle and removes the camber when the line is parallel to the board axis, while keeping them parallel: 18 − 1.4 = 16.6 N of drag removed.
Lift. The cambered profile lifts from low angles. At the slightest slide, the ADAC profile becomes cambered: more lift, hence more camber, hence more lift; this feedback loop restores the line almost immediately. The symmetrical fin must reach 40° to deliver 120 N; the variable-camber fin gives them at 10° and rises to 160 N at 15°, an involuntary tail slide four times smaller. Variable camber thus combines the symmetrical profile, optimal in a straight line, with increasingly cambered profiles from 5° to 25° of angle of attack, without sharp stall.
Stall. Beyond about 30°, every fixed profile stalls (fig. 2.2 and 2.4). Adaptive angle of attack keeps an efficient relative angle by pivoting the profile into the flow, and delays stall down to the tightest turns.
The detailed figures, with the influence of thickness, are on the high-performance fins page.
The skiing analogy: skidded turn or carved turn
Turns with static fins and with ADAC fins compare to skidded and carved ski turns. The skidded turn is unsuited to high speeds (centrifugal force pushes the surfer off the line), allows neither precise control nor a turn in a tight space, loses speed and puts the body too far back to re-accelerate. The carved turn allows precise driving at high speed with very little speed loss; the support stays centred throughout the turn, control is intuitive and re-acceleration immediate.
Acceleration: lift projected forward
The variation of angle and camber is programmed mechanically by a controlled elastic deformation in a structure forming an articulated kinematic loop (patented technology). The kinematic chain reacts to lateral loads as soon as the line includes a drift component, and tilts the lift direction forward: a propulsive component appears. Two consequences: a natural ability to generate speed through alternating lateral movements (pumping, comparable to a dolphin's tail beat) and acceleration in turns, opening sequences impossible without drive.
Rail grip and heeling: a centre of thrust close to the board
A centre of thrust close to the board limits heeling and the effort needed to bite the rail into the wave. Long fins increase the heeling effect, which tends to tip the board over when the load is high. The ADAC system obtains drive and 70 % more thrust thanks to its dynamic leading edge and camber, while keeping a centre of thrust very close to the board. Fin position and number are covered on the choosing your fins page.
Tip vortices: the soft lip at the fin tip
Short wings with high lift shed tip vortices that degrade lift (tip losses). Some birds, such as eagles, carry feathers at the wing tip forming an inclined lip that exploits and limits these vortices. Soft, profiled lips are built into the tips of the ADAC structure and deform naturally under lift, following this principle.
Safety and biomimicry
The ADAC system combines higher performance with higher safety: its articulated structure with tapered but soft parts removes cuts, and a fuse part protects the board's fin box. The slow-motion tests below show the twisting of the structure and the fuse protection under impact. The surf fin safety page develops this point; the biomimetic fins page shows how the analysis of fins, ultra-efficient and harmless in the games of dolphins, guided the design.
The FYN designers' story
"We were far from imagining a new type of surf fin: there are already so many models that you get lost. Surfers and developers of hydrodynamic calculation software, we were simply curious to understand how the fins of our boards work. A few decades of surfing and conversations with shapers had given us the basics: the pivot, the fin position, area, speed, flexibility. We had tested alaias and finless boards, planed and even reversed the upper and lower sides of fins. But some questions remained open: the dynamic evolution of the angle of attack along the line, stall, the antagonism of opposite fins and the drag it creates. We applied the dynamic principle in a series of prototypes, then measured ADAC objectively in wake surf on a constant wave: impossible to go back to static fins."
The FYN fin configurator illustrates the influence of the surfer's position relative to the fins on speed and manoeuvrability. Shapers will find the fitting dimensions in the shaper's guide. To license the ADAC system (US patent 14859193/2015 and FR 1402226/2014; Gasquet, Annecy, and Cionca Law Group, Irvine) in other fields of hydrodynamics or aerodynamics, write to contact@mecaflux.com, subject "ADAC system info".
Frequently asked questions
What is the fin of a surfboard for?
It stabilises the course in a straight line and provides, through its lift, the pivot that allows turning without sliding; correctly oriented and cambered, it projects that lift forward and accelerates the board.
Why does a fin stall in a turn (spin out)?
Because the curved line increases the angle of the water on the fin; beyond about 30° the flow separates from a fixed profile and lift collapses. A variable-angle fin pivots into the flow and delays that stall.
Symmetrical or cambered profile for a surf fin?
Symmetrical in a straight line (0.7 N of drag against 8 N), cambered in a turn (120 N of lift at 10° against 40° for the symmetrical one). A variable-camber fin takes one or the other depending on the phase.
How do I calculate the lift and drag of a fin?
With the online calculator or the software that produced the curves of this page. The online lift and drag calculator gives the forces on a profile from its area, speed and angle of attack, and the Heliciel software plots the complete curves of a fin, foil or keel. For a specific project, Mecaflux also offers the study of your project or training on the software.