Fins: setups, sizes, shapes and fin boxes

7 min read

Fins give the board hold, so it goes where you point it instead of sliding sideways, and drive, so it keeps speed through a turn. Changing fins is the cheapest way to change how a board feels. It will not turn a bad board into a good one, but the right size and setup make a clear difference.

Setups

• Single fin: one big fin in a long box, the classic longboard setup. Smooth and stable, suited to long, drawn-out lines and nose-riding rather than sharp turns. Sliding it forward in the box makes the board looser; back gives more control.
• Twin: two side fins, no centre fin. Fast and loose, the usual setup on fishes and small-wave boards. Some twins add a small centre fin (a stabiliser or trailer) for more hold.
• Thruster: two side fins and a centre fin, now the most common setup. Drive, hold and control in most waves; a good default.
• Quad: two side fins and two rear fins, no centre fin. Fast down the line and holds well in hollow, fast waves; also common on fishes and step-ups.
• 2+1: a longboard box in the middle with two small side fins. Keeps the smooth feel of a single fin with more drive and hold in turns; common on mid-lengths and longboards.
• Five boxes: a thruster and a quad in one board. Try both and keep what you like; how well each works depends on the board's tail.

Size by body weight

Fin makers size fins by the surfer's weight, because a heavier surfer pushes harder on the fins. The bands overlap on purpose: if you are in between, go bigger for bigger, more powerful waves or more hold, smaller for small waves or a looser feel.

• FCS (as published by FCS and reproduced by Surfer Today and UK shops): XS under 55 kg, S 55 to 70 kg, M 65 to 80 kg, L 75 to 90 kg, XL over 85 kg.
• Futures gives sizes in pounds on each fin's page, for example Small 105 to 155 lb (about 48 to 70 kg) and Medium 145 to 175 lb (about 66 to 79 kg). Retailers' Futures charts give Medium up to about 88 kg and Large from about 80 kg; the bands differ a little from page to page.
• FCS Specialty fins use wider bands: S-M under 75 kg, M-L 70 to 90 kg, L-XL above 85 kg.
• Other brands use their own charts. Check the size on the fin's own page, not a general table.

What the numbers mean

• Base: the length where the fin meets the board. A longer base gives more drive and speed; a shorter one turns more sharply.
• Depth (height): how far the fin reaches into the water. Deeper fins hold more; shallower fins let the tail slide and release.
• Area: the total surface of the fin, from base and depth together. More area means more hold; less is looser.
• Sweep (rake): how far the fin leans back. More rake gives longer, drawn-out turns; upright fins turn tighter and pivot.
• Foil: the shape of the fin in cross-section. Centre fins are usually symmetrical (50/50, both sides curved). Side fins are curved on the outside and flat or slightly curved on the inside ("inside foil").
• Flex: how much the tip bends under load. Flexier fins feel lively and playful in small waves; stiffer fins are more predictable in fast, hollow waves.
• Cant: how far the side fins tilt out from vertical. Less cant gives more drive and speed; more cant feels looser and more responsive in turns. It is set by the fin box, not the fin.
• Toe: side fins point slightly towards the nose ("toed in"), set by the box. Guides describe it as making the board quicker to respond in turns, though they describe the effect in different ways.

Materials

• Plastic (moulded): cheap and tough, stiffer and less lively. Often supplied with beginner boards; useful as spares.
• Fibreglass: the classic material, with a natural flex and a good balance of drive and feel.
• Honeycomb core: a light core wrapped in fibreglass or carbon; lighter, with a flex between fibreglass and softer fins.
• Carbon: stiff and light, for drive and control. More brittle than fibreglass, and can feel too stiff for light surfers.
• Soft fins: flexible plastic or rubber with no sharp edge, fitted to most soft-top boards. They give up some speed and drive.

In the app: Soft fins are a safety choice. Most surfing injuries come from a surfer's own board, and the sharp trailing edge of a hard fin causes many of the cuts. On a beginner board, or a soft-top in a crowded surf school lineup, keep the soft fins until you can control the board.

Fin boxes: what fits what

• FCS (original): two tabs per fin, held by small grub screws in two plugs. Tightened with a fin key.
• FCS II: no screws; the fin clicks into the box and comes out by hand. FCS II fins do not fit original FCS plugs. Original two-tab FCS fins do fit FCS II boxes, with grub screws and a small insert (FCS sell a compatibility kit).
• Futures: one long tab along the whole base, held by a grub screw at the front of the box. Futures fins and FCS fins are not interchangeable.
• Single or longboard box (often called US box): a long slot that takes a single fin held by a small plate and screw. Most single fins fit most longboard boxes, whatever the brand.
• Look at the box before you buy fins. If in doubt, take the board to the shop.

Changing fins

1. With screws (FCS, Futures): back the grub screw out with the key just enough to clear the slot; you do not need to take it out.
2. Seat the fin. On Futures, hook the back of the fin into the latch at the rear of the box first, then press the front down so the base sits flat.
3. Side fins go flat side in, logo facing out, angled the way the box sets them.
4. Tighten until snug, then stop. Overtightening strips the screw or damages the box. Wiggle the fin: it should not move.
5. FCS II: push the fin in until it clicks; follow FCS's instructions to lever it out.

• Rinse the boxes after sandy sessions. Grit in the thread is the main reason screws seize.
• Use the right key: FCS and Futures keys differ slightly, and a worn key rounds off the screw head.
• Keep a spare key and a couple of grub screws in your car or travel bag. They are tiny and easy to lose on the beach.

Sources: evo: how to choose surf fins (setups, sizes, rake, foil, cant, systems) · FCS: find your fin (size by weight) · FCS Australia: specialty fin sizing · Surfer Today: what are the best fins for your surfboard · Futures Fins: R4 Blackstix Small (spec table and weight range) · Wikipedia: FCS (fins), original FCS, Fusion and FCS II · Kona Surf Co: how to install Futures fins · 3D Fins: surfboard fins and safety

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