Fin Encyclopedia
INTRO: Fins perform a much more technical and complex job than one might think. A fin can radically contribute to the overall performance of the board during sessions.
Below are some concepts to understand just a few aspects. From here, you can sense that a good fin is an incredible mix of engineering and technology, something often overlooked in stock fins supplied with boards. Fin design makes a considerable difference in performance.
OUTLINE: is the contour of the fin itself. Each fin model has its own outline
BASE: The base is the lower part of the fin, that is, the length of the fin between the leading edge and the trailing edge (the part “attached” to the board).
A wider fin base will help with drive, offering more thrust and acceleration. This translates into longer turns.
A narrower base gives more speed and rotation and allows for slightly easier and sharper turns. The length of the base determines how the board will turn and track. The longer the base, the longer and more extended the turn and the greater the tracking. The shorter the base, the tighter the turns but less drive.
LENGTH/DEPTH: The distance between the base of the board and the top of the fin, that is, how far the fin extends and immerses into the water. A deeper fin offers more grip, stability, and control with greater resistance compared to a shorter fin. A shorter fin will feel less grippy and be more prone to sliding. The shorter the fin, the more release you get when moving the tail in a turn.
AREA: The area of a fin can be described as the amount of surface within the outline. Generally, a larger area means more hold and thrust, while a smaller area means faster and smoother riding.
The way the fin’s surface is arranged affects the performance of a particular board shape. More height and a wider base: greater surface, more hold. Less height and a narrower base: less surface, less hold, greater tendency to slide or drift. A larger surface will offer more hold but will be stiffer for a given length.
SWEEP/RAKE: When you look at the fin’s profile curve and how much it leans or sweeps backward, you are looking at the fin’s rake. The greater the rake angle, the longer the turns. This is great for more challenging days with a nice long wall to work on. Less rake, meaning a more vertical fin, means you get more pivot, great for less perfect and weaker days. The angle of the fin’s leading edge relative to the base. The more the leading edge is tilted toward the tail, the greater the rake. The more vertical the angle, the less the rake. Rake describes how the fin’s area is arranged. Greater rake = longer turning radius. Less rake = shorter turning radius. A more vertical fin with less rake will offer tighter turns and more pivot. A more swept-back fin will offer better control and wider turns. Fins with a longer tip swept back (or rake) will offer more grip and direction, better for wider arcs and powerful turns. A simple way to understand rake is how “swept back” versus “vertical” the fin’s stance is. The more a fin is “swept back,” the greater the rake. A more technical definition is the angle between a line running parallel to the fin’s base and a line passing through the fin’s front corner and tip. Greater rake offers more stability and a tendency for long, drawn-out turns, while less rake offers more responsive tracking with a tighter turning radius.
Fins with a shorter tip will be snappier and turn tighter, better for staying in the critical section of small waves. They may also be faster.
FLEX: Simply put, it is the “flexing” of a fin, indicating how much it will bend or twist laterally and perpendicular to the flow direction. Flexible fins feel completely different from stiff fins. Flexible fins have the ability to store and release energy during a turn. This contributes to projection out of the turn, which in turn determines speed and influences control. When the turning pressure is released, the fin returns to its original position (reflex), releasing energy and producing forward thrust. The flexible fin responds better to rider inputs during the turn. With a stiff fin, it is harder to make tight turns but it is more forgiving for beginner or intermediate surfers. The flexible fin is a bit slower to respond at the start of the turn but provides thrust coming out of the turn. The amount of flexibility of a fin is determined by the foil and the type of material used. Flex is one of the features directly related to fin performance. We see how flex, fin shape, and material go hand in hand. Important qualities include the force needed to flex the fin (i.e., the stiffness of the flex).
REFLEX: is the ability to return to the original shape after deformation from load, and the speed and reliability with which the fin returns to center. This factor is extremely important. A fast reflex translates into increased performance. Reflex also helps resistance to breakage because it relates to the flex-stretch value of the raw material, but mainly to restoring its profile to the optimal position as quickly as possible. Flexibility and return (elasticity) of a material are key mechanical properties: flexibility is the ability to bend easily without breaking, while reflex is the ability to deform under load (bending, stretching). The combination of these properties allows the material to adapt to deformations without damage. Factors influencing it include geometry, density and fill pattern, thickness. Definition: Ability to regain original shape after deformation. Our materials produce fins with consistent flex that return to center quickly and reliably, and with this as a starting point, we make each fin model with optimal stiffness.
FOIL: The fin’s foil is the shape of the aerodynamic surfaces extending from front to back, and the internal and external shape of the fin. Just like airplane wings, this foil generates lift under the board. Similarly, as an airplane wing profile influences the airflow over it, the fin’s profile influences the water flow. Looking at a cross-section of a fin, you notice the thickest part is in the center, while the leading and trailing edges are thinner. Different types of foil affect the water flow over the fin’s surface and have a significant impact on fin and board performance.
There are different types of foil. The most used profiles are 50/50 (center fins) which have symmetrical contours on both sides. Profiles can vary from 50/50 to 70/30 to flat inner profiles (side fins). Whatever the profile ratio, continuous profiles have the thickest section toward the center of the fin and thinner sections at the leading and trailing edges. The profile plays an important role in producing lift and thus speed.
LIFT: Regarding fins, lift is produced by differences in surface pressure between one side and the other of the fin and how they interact with the water. The flat side of a fin and the foiled surfaces behave differently when moving through water and will produce lift in a specific direction, generally outward, toward the wave’s surface. Multiple factors can influence the amount of lift generated, such as fin size, foil thickness, angle of attack, rake, etc. Lift is used to generate speed and thrust, but it’s a balancing act because as lift increases, potential drag also increases. Lift is also used to help insert the board’s rail into the wave’s surface. When a surfer pushes with the fin(s), acceleration occurs due to lift and the amount of pressure applied in a given direction, thus contributing to turning, stability, and increased momentum.
TOE-IN: is the angle at which the fins converge toward the centerline. This happens for the side fins of wave boards. This can help create pressure on the outer foil of the fin, allowing for greater board responsiveness. Tilting the front angle of the fin toward the stringer makes the board freer and allows for tighter, more responsive turns. But be careful because excessive tilt creates drag and slows the board down. Reducing the tip or positioning the fins more parallel to the stringer produces longer, drawn-out turns, minimizes drag, and increases speed. Relevant for all types of side fins, toe-in essentially indicates how much the side fins are angled toward the stringer. In other words, it is the measure of the angle between a line parallel to the stringer and a line parallel to the fin’s base. If these two lines are parallel, the fin has no toe-in, meaning it points straight forward. Often fins are “set” at a few degrees, which results in greater control and maneuverability, while fins without toe-in maximize speed by causing minimal water flow resistance.