The two main forces acting upon your surfboard’s fins as you maneuver the face of a wave are lift and drag. Understanding the effects of these forces reveals exactly how your fins preform and the trade offs between different design elements.
You can get real technical with hydrodynamics, foil science, and drag testing when you start to think about fins – and I think that begs the question – is there a “perfect” surfboard fin?
While I think you can certainly design a fin that outperforms other fins in a fluid chamber in terms of drag and the occurrence of vortices – I don’t think chasing a perfect fin is a worthy cause.
Depending on how you surf, what kind of board you’re riding, and what kind of waves you have to work with – different fins are going to allow you to do different things on a wave.
I don’t think any surfer surfs the same wave the same way every time. For me, a lot of the enjoyment I get from surfing is through learning and trying new things – fin experimentation included.
Perfect fins aside, it does still help to understand how your fins work.
Surfboard Fins Create Lift
Lift is a force acting perpendicular to the direction of your board and fins.
Surf fins create horizontal lift that allow you to build speed into and out of turns.
If you’ve ever slid out when trying to plant a turn, you’ve likely experience a lack of lift in your board’s fins. The right amount of lift in a fin allows you to dig into and speed out of turns.
The characteristics of a fin will affect how lift is produced at different speeds. Different foils, thicknesses, fin templates, and surface areas will allow a fin to produce more lift at higher or lower speeds – the tradeoff being drag and drive.
Surfboard Fins Create Drag
Drag is a force acting in the opposite direction of the motion of your board and fins.
Fins also create drag, which is again affected by the shape and characteristics of the fin. Surfing a completely finless board would have very little drag – the tradeoff being a loss of lift and difficulty turning.
Drag is caused and influenced by the surface of the fin, the fin’s foil shape and positioning, the fin’s form (template and angles), and the speed at which you’re surfing.
Drag also occurs as the fin moves through water in the form of pressure difference and vortexes. Water with a higher pressure moves towards water with a lower internal pressure. This can create turbulence around the fin that will slow you down.
If you’ve ever experienced a buzz or hum in your fin, you’ve experienced a less than ideal vortex around your fin. This can be alleviated by thinning the fin’s trailing edge.
Just like lift, drag will increase or decrease at different speeds depending on the design of the fin.
Thinner fins will typically create far less drag than a thicker fin – the tradeoff being a loss of lift and less speed and control while turning.
Manipulating Lift and Drag
Drag and lift are created and influenced by both your surfboard and your fins. Depending on the design of the board, you may be able to create enough lift with little to no fin at all – while other surfboard designs may feel unrideable without a proper fin.
Some general ways to affect lift and drag with your fins is as follows:
- Keep the leading edge rounded.
- To increase lift – thicken the foil.
- To decrease drag – thin the foil.
- Use a fin that twists towards the tip rather than bends.
Next time you’re surfing, try to think about lift and drag as your doing turns or trying to drive down the line. Feeling slide-outs or slippage when you’re planting into a turn? Find a fin with more lift. Feeling sticky or slugging on take-offs or down through sections? Find a fin with less drag.