Introduction

Surfboard fin placement is a critical factor in board performance. Unlike thrusters, twin fin boards offer flexibility in the position of the trailing edge. The size and style of twin fins can shift their base forward or rearward relative to the fin box. Understanding this variation helps you dial in drive and control that works for your surfing style. In this post, we explore how fin selection and placement interact and how to choose the right setup.

Understanding Fin Layout

Fin Example

Shapers mark fin placement from the trailing edge of the fin rather than the front of the box. On thrusters, the trailing edge sits at the back of the box, so longer fins shift forward. Twin fins and keels can hang beyond the back of the box when their base is longer than the box slot. This means twin fin base lengths from around 4 3/4” up to 6 3/4” will end up with different positioning of the trailing edge. Always confirm the effective trailing edge location before selecting your fins.

If you are having trouble picturing this, take a look at the image above. From top-bottom and left-right the fins are:

  1. NVS Large JL thruster side fin
  2. Futures AM2 thruster side fin
  3. Album Fascination split keel main side fin
  4. Futures Al Merrick Keel

Starting with the JL thruster side fin (1), you can see that the fin base is the same length as the fin plug. This means that wherever the shaper intends the trailing edge of the fin to be, they place the rearmost part of slot of the fin box. Similarly, the AM2 side fin (2) trailing edge lines up with the back of the fin plug. However, you can see that the fin base is longer than the plug and extends in front of it. Moving on to the Fascination split keel, it too is longer than the fin plug. However, the trailing edge of this fin extends past the back of the fin plug, essentially moving the trailing edge placement aft by 1/4” or so. The AMK has the largest base, which is to be expected for a keel fin. This fin extends forward by about an 1/8” or so, and extends aft roughly 1 3/4”.

Impact on Board Performance

Moving twin fins forward loosens the tail and increases pivot speed. With less support behind the fins, the board can feel lively but risks sliding out. Shifting fins rearward adds drive and stability until a point of diminishing returns. Small changes in fin base length can have noticeable impacts on control and speed.

  • Forward placement increases looseness and pivot
  • Rearward placement enhances drive and rail hold
  • Excessive rearward shift can make the board feel stiff and like it’s “on rails”

Choosing the Right Fins

Before purchasing fins, consult your shaper or review their recommendations. Shapers often test specific fin models during the board design process and can advise which sets deliver the intended performance. Once you find a fin that suits your wave conditions, use that set to test future boards and make changes based on your experience.

Example: Rozbern HMB and Simon Shapes DTDN

On my Rozbern HMB (convertible thruster/twin+trailer), the back of the fin base sits 10 1/2” from the tail. Initial testing with Futures T1 fins felt too loose because their base and plug trailing edge align. To add drive I chose Lost Evil Twin + Trailer fins with a 5 5/8” base. Their base extends about 3/4” past the plug, moving the trailing edge rearward. This adjustment delivered the balance of support and looseness I needed for small to medium waves.

When I first started testing my Rozbern HMB with the twin+trailer setup, I had expected it to work better with the T1 set because I had previously had success with the Simon Shapes diamond nose-diamond tail model. The fin placement was a little different at 10” from the tail (though the trailer was up at 3 3/4”). 1/2” is a lot in terms of fin placement, but I had found the Simon Shapes to be a bit stiff in small waves (it’s designed for stronger surf, more like a hpsb). This goes to show how important it is to experiment with fins before writing a board off.

Conclusion

Twin fin performance varies significantly with fin placement and base length, moreso than typical thrusters. By understanding how different fin bases interact with your fin box, you can tailor drive and looseness to your surf conditions. Always start by asking for your shaper’s guidance and maintain consistency when testing new fin setups. With careful selection you can refine your board’s responsiveness to meet your performance expectations.