Surfboard Fin Design Characteristics & Hydrodynamics
Surfboard fins have a huge effect on how your surfboard performs. Even the slightest changes in fin design can make a world of difference.
If you’ve ever broken a fin and continued surfing or swapped the fins on your favorite board, you’ve probably already got a sense for it.
Surfboard Fin Hydrodynamics
Why are your surfboards fins so important?
Think about it, the fins are the one part of your surfboard that are almost always in contact with the water. They have a huge influence on drag and drive, the flow of water over and off of the surfaces of your board, and how you’ll be able to move into, through, and out of turns.
With different fin setups on the same board in the same conditions, it can feel like you’re surfing something completely different.
Certain fin designs are better for speed, some for quick maneuvers, some for solid turns, and still others for drive, stability, and nose riding.
Once you wrap your head around some of the basics of surfboard fin design, you’ll be able to make better informed decisions about the best fin for the type of surfing you want to do in the type of wave and conditions you’ve got to work with.
If your board isn’t performing the way you want it to – if it feel sticky through turns, squirly in the pocket, or too slow when you need more speed and drive, a simple fin swap may be the answer you’re looking for.
The Characteristics of a Surf Fin
The major characteristics of your surf fins that affect performance are as follows:
- Fin Foil
- Fin Flex
- Fin Base
- Fin Depth
- Fin Rake/Sweep
- Fin Size
- Cant and Toe
- Positioning & Configuration
Surf Fin Foil
The fin foil is the changes in width of the surf fin from its leading edge to its trailing edge. Fin foils can be described as symmetrical or asymmetrical.
Symmetrical surf fins, also called a 50/50 foil, are often found on the center fin for thrusters, single fins, and sometimes the rear fins on a quad set up.
Asymmetrical surf fins, which can be flat on one side or have a different foil ratio on each side (e.g. 80/20, 70/30, inside foil, etc.), are often found on the side/front fins on thrusters and quads.
A fin’s foil controls the board’s lift and drag forces as water moves over the bottom of the board. It affects the pressure and velocity of the water flowing around your fins and your board’s surfaces.
The largest or thickest part of a fin foil is often best set towards the front center of the fin. Generally, a thicker fin foil equals more lift, but a thicker foil will also create more drag at higher speeds.
The leading edge of a surf fin’s foil should typically be round rather than sharp. This allows the water to flow over the surface of the fin smoothly and reduces turbulence.
Sharp edges or knicks in the surface of the fin can lead to break off points in which the water will break off from its uniform flow over the fin too soon. Just like imperfections on the bottom of your board, that’s no good.
Surf Fin Flex, Twist, and Recoil
A surfboard fin’s flex or twist has a lot of influence over how your board will feel through turns and the speed you’ll be able to generate out of them.
While all the characteristics of a fin can affect fin flex, it is heavily impacted by the material used in the fin’s construction. Two major components of fin flex are stiffness and recoil time.
Stiffness of a fin refers to how much force it takes to deflect the fin from its resting point. Recoil refers to how quickly or slowly it takes for a fin to return to its original position.
As some general rules of thumb, a more flexible fin is good for long, drawn out carves and turns, while a stiffer fin is generally good for quicker maneuvers and speed.
Stiffer fins with less flex reach maximum flex during turns and release quicker.
Flexy fins take longer to reach their maximum flex and take longer to release.
Ideal fin flex allows for a fin that matches the intentions of your maneuvers and your body’s movements. A fin that’s too slow to recoil or hard to flex may feel sluggish and sticky, while a fin that’s too fast may feel like you’re surfing a step behind your board out of turns.
Surf Fin Base Length
The fin base is the portion of your surf fin that is connected to the bottom of the board. In other words, it’s the bottom most portion of the fin that’s in the water.
A longer base gives you more drive, hold, and more drawn out turns. A shorter fin base results in a board that’s easier to pivot quickly, but sacrifices some drive and stability down the line.
Surf Fin Depth
The fin depth refers to how deep the fin is in the water. It’s measured from the fin base to the fin’s tip.
A deeper, or longer, surfboard fin is going to have more hold and stability in the water and through turns. A shorter fin makes it easier to throw out the tail and surf loose.
Surf Fin Rake/Sweep
Fin rake or fin sweep refers to the angle between the base and tip of the fin. It has an influence on how your board will turn. This is what gives your surfboard fin it’s profile or shape.
The larger the angle (or the further back the tip ends), the more drawn out your turns and carves will be.
A fin with a smaller sweep angle (or a more upright looking tip profile) will have more release and allow for quicker pivots.
Surf Fin Size/Surface Area
Fin size refers to the overall surface area of your fin and is a result of the fin characteristics above. This will have a big effect on how the fin performs depending on the surfer’s weight and the dimensions of the board.
In general, discounting surfer weight for a moment, a smaller fin will have less hold, and will be easy to pivot and slide, while a larger fin will have much more stability, hold, and drive.
A fin’s surface area should be determined by the individual surfer (weight, ability, and style of surfing) as well as the surfboard’s tail width.
Fin Cant & Toe
Fin cant refers to the angle at which the fin leans relative to the bottom, perpendicular surface of the board.
More cant is associated with more turning control, but increases drag. Less cant, or a fin that sits more upright, will produce more speed and drive with less turning control.
Fin toe refers to the angle the fin (the side or rail fins) sits relative to the center line of the board.
Greater toe-in allows for more responsive turns, while less of a toe angle leads to more drive.
Fin Positioning & Configuration
Just like the characteristics of the fin itself, the position of the fin or fin cluster will also have an effect on how the board performs. Fins set closer together or further apart will affect turning and drive, as will fins set closer or further from the back end of the board.
You’ll also find that fin configuration will change how your boards feel. A single fin feels much different from a quad. Each type of fin configuration has its strengths and weaknesses depending on the waves you surf and the style you’re hoping to achieve.