Windsurf Fin Doesn't Fit? How to Fit It Without Cracking It

This one is really important if you want to upgrade your board with a third-party fin. Boards are made in different factories, fins are made in different factories, and fins are made by hand. They're kind of semi-custom products, so there can be minor differences in how they're made, and that means not every fin fits plug and play into every board.

So here I show you three things:

1. How to fit the fin into the fin box perfectly.
2. How to make sure you don't use screws that are too long, and damage your fin that way.
3. How to fine-tune your board by raking the fin.

Which fin type and size to put in is a different question. I answer that one in Windsurf Fin Size: How to Pick the Right Fin Type and Length.

What you need to fit a windsurf fin

• A screwdriver for your fin screws. I typically use Torx screws.
• A small screwdriver. It can be a flathead or a different kind of head, it just needs to be really small. You measure the screw length with it.
• A couple of screws in different lengths, and some washers.
• Sandpaper, 120 and 180 grit, and a sanding block. The block can be made of wood or aluminium.
• The fin cover, because fins are quite sharp.

Why it's good if a new fin is a bit too big

It's actually good if the fin is a little bit too big. It's easier to make it fit into the board when it's a bit too big rather than when it's a bit too small: you can always sand material off, but you can't add it back.

Step 1: How to fit the fin into the fin box

Push it in by hand, as far as it goes

Put the fin into the box. It's pretty normal that you have to use a little bit of force and wiggle it in. Always use the cover while you do that, fins are quite sharp.

Fit the fin in as far as you can with your hand. If it doesn't go in completely, don't worry: the last few millimetres, the fin moves in when you screw it tight. When it looks like that, it's actually already a really good fit, because you can take off a little bit of material with sandpaper and make it fit perfectly with very little effort.

Read the scratch marks

Now take the fin out again and look where it's a bit too big. Wherever the fin rubbed in the box, the box scratched material off the fin head. That tells you where you need to sand.

Check both sides. On the fin I fitted, the essential part was probably at the back, the other side looked fine, with just a little bit here and there. So that fin was a little bit too big in the back part.

Sand only the scratched spots

Take the 120 paper on the sanding block and sand off a little bit, only where you see the material scratched off. Be careful with 120: it takes off quite a lot of material.

Then check the fit again. Mine was already slightly better, with just a few millimetres missing. Next I screw it in gently to see how far it goes, because it might move a bit further. Before any screw goes in, measure its length (Step 2).

Step 2: How to get the fin screw length right

This part is really important. If you don't get the screw length right and you damage the fin that way, with pretty much all fin brands the warranty is gone, and your fin is damaged.

Measure with the small screwdriver

Once the fin is almost inside the board, measure how long a screw you can use. That's what the small screwdriver is for:

1. Put the small screwdriver all the way down into the hole. Make sure it really goes all the way down.
2. Place your finger right at the start of the fin box.
3. That's the maximum screw length you can use.

Then use a screw that is a few millimetres shorter than that. When I fitted mine, the first screw I picked was pretty much the same length as what I had measured, so it was probably a little bit too long, and I used a slightly shorter one.

At this point the fin isn't fully screwed in yet, so this is not your final screw length. You do this step once again later.

A washer, then hand tight

Always use a washer between the screw and the board. Then tighten the screw hand tight: you shouldn't have to force it. I start with the front screw, then screw in the back one, then go back to the front again. As you do that, use your other hand to feel underneath the board whether the fin is moving in.

On the beach without the right screw? Add washers

Sometimes you're on the beach, it's hectic and you don't find the right screw size. Then put more washers in between: you essentially shorten the length of the screw. It's a little bit more weight and not ideal, but it does what it needs to do.

Repeat until it's flush (typically two or three rounds)

With the right screw and a washer in, just by screwing it in slightly, not too hard, my fin already dropped into the box quite significantly. But if you look really closely, there's still a little bit of distance I want to get rid of to fit it in perfectly.

It's very normal that you don't get it right after the first bit of sanding. Typically this is a process you repeat two or three times to really fit it in perfectly. Don't over-sand it, because then the fin sinks in too much. So take the fin back out, look at the scratch marks again and take off just some more material, not too much.

Optional: finish it off nicely

With rough sandpaper you take off quite some material. That makes the job a bit faster, but it doesn't look quite as nice. It doesn't really matter, you don't see it once the fin is in the box. But if you want to finish it off nicely, take very fine sandpaper, like 600, and sand it wet. Use some water to flush off the material you just sanded off.

Fin flush? Measure one last time

Once the fin sits nice and flush, it's really important that you check the potential length of your screw one last time. Here's why.

The screw goes into the inserts and the threads in the fin head, but the hole ends at some point. Once the hole ends and you keep screwing, you push the screw with a lot of force into the box, or even into the blade of the fin. Depending on what kind of head your fin has, several things can happen: you can push the fin blade out, or immediately crack the head of the fin. Then you can essentially throw your fin away.

Step 3: Fin rake, the last bit of fine-tuning

One last thing: there's a way to tune and trim your board a little bit with the fin, and that's raking the fin. It changes the angle of the fin, and with that you can change the trim of your board: raise the nose or flatten it.

With some boards, or some board and fin combinations, you might feel that the nose of the board is a little bit too low. Say you've already done all the adjustments on your sail, your boom and the mast base. (More on those in Windsurf Sail Trim: 3 Rigging Mistakes Almost Everyone Makes and the board and sail tuning chapters of Freeride Windsurfing Guide: Speed, Tuning, Tacks and Jibes.) Then raking the fin is the last bit of fine-tuning you can do.

• More rake raises the nose a little bit: sand the back of the fin head a little bit more, so that you can sink in the screw a little bit on the back. That changes the angle of the fin to the back, meaning you have more rake, and that raises the nose a little bit.
• Less rake lowers the nose: screw in the front of the fin a little bit more, so you have a little bit of a gap at the front. That lowers the nose, and you have less rake in the fin.

Of course, the fin won't sit completely flush anymore then. But to be honest, it doesn't make that big of a difference if it's sunk in a little bit more at the front or at the back. As long as the fin doesn't stick out anywhere. That's really the worst that can happen: it creates a lot of resistance.

I did it in competitions, and I achieved some really good results with the fin sunk in and raked a little bit more. So don't be worried if your fin sits slightly too far in at the front or the back. For the OCD guys among you: I know it's not that nice to see, and to know that somehow it's not perfect. But it will definitely work.

Your fin fitting checklist

Save this on your phone for the next time you fit a fin. With that, your fin is in the board, and it's time to get on the water.

Below are the steps once more, one picture each.