Windsurf Fin Size: How to Pick the Right Fin Type and Length

Fins are a whole big topic, and I get quite a few questions about which fin and which fin size to choose. In this guide I'll show you how to know which fin fits which board, and my own fin sizes. Stay until the end, because there's one extra tip you should definitely consider.

Here's why there's no simple answer. Say there are just 10 board brands, and there are definitely more, each with five board ranges and five sizes per range. That's already 250 boards. Now add 10 fin brands with three kinds of fins in sizes from 30 to 40 cm. That's 300 fins, and 250 times 300 makes 75,000 combinations. Unfortunately I don't have a magic list that tells you which exact fin fits which exact board. But I can show you how to tell a right from a wrong combination.

To do that, we look at several aspects: your discipline and goals, the board size, the sail size, the conditions, your height and body weight, and the specifics of the fin. Admittedly, that sounds a bit like rocket science. But don't worry: if you get one or two of them wrong, you'll still be fine and you'll have a good time on the water.

1. Discipline and goals: which kind of fin

Just like with choosing any part of your gear, ask yourself what kind of windsurfer you want to be. Do you want to do manoeuvres (freestyle and wave), cruise around (freeride), or become as fast as possible (race)? The answer decides which board and sail you aim for, and those decide which kind of fin you aim for.

Each kind of board has a range of fin types you can use:

• In a freewave board you can use a pure wave fin, a freewave fin or a small freeride fin.
• In a freerace board you can use a freeride fin, a freerace fin or a race fin.

Knowing which area you want to improve in helps you decide which end of your board's spectrum you go for. Another example: you're on a freeride board and your goal is to get faster. Then you can use a freerace or a race fin in your freeride board.

The next question is whether your level allows for that step. If you're just starting to use the footstraps and the harness and you're still getting used to the freeride board, a freeride fin is probably your best choice. A race fin might be a bit too fast and too loose for you. Once you feel you've reached the limits of your board, you can upgrade, for example to a freerace or a race fin. The same applies on a freewave board when you want to become a better wave rider. (Just getting into the straps and the harness? That whole step is in my planing tutorial.)

2. Board size: the number one factor

Now that you know which kind of fin you need, let's get more specific. The size of your board is the number one factor for the fin size. To understand why, we first have to understand how a fin works.

How a fin works

The fin forms a vertical surface in the water. It's what gives you the drive forward.

Let's start without a fin. When the wind hits your sail, it tries to take you in the direction of the wind. Because you're standing behind the sail on your board, the tail starts rotating around the mast and pushes the nose into the wind.

Now put the fin in. The board again tries to rotate around the mast, but it can't, because the fin in the water holds the force. That force has to go somewhere, though. Thanks to the profile of the fin, with a relatively round leading edge, a camber in the front and a sharp trailing edge, the fin guides the water flow in the right direction. That creates forward drive, and from a certain speed it starts lifting the upwind rail.

So you're pushing the fin sideways. Even more so when you're racing: that's when you really push the rail of the board from the side. But also when you're freeriding and standing more upright, once you're planing you still apply sideways pressure through the grip on the board. So regardless of how much, there's always an element of pushing the board sideways rather than just from the top, and that becomes especially important once you go faster.

Why wider boards need bigger fins

The pressure you can put on the fin increases with the leverage your board lets you put on it. On a wider and thicker board you automatically apply more pressure to the fin, which is why wider boards need bigger fins. And the other way around: narrower boards need smaller fins.

• Board too wide for the length of the fin? You're more likely to overpush it and spin out. (More on that in Spinouts: Why They Happen and How to Stop Them.)
• Fin too big for the width of the board? You don't have enough leverage on the fin. You get overlifted and you lose control.

My rule of thumb, and why the tail matters more

To find the right size, I once came up with a rough rule of thumb: divide the width of your board by two and add a few centimetres, 1 to 2 cm. Take an 80 cm wide board and you get a fin of around 42 cm, which in most cases is quite accurate.

That being said, even more important is the tail width of your board, because that's the point where you apply the leverage. So a rule like this doesn't always work.

And because this is relatively complex, most board brands provide a chart, mostly on their website, that tells you which fin size to use in which board. That's definitely your best starting point for the size of your fin.

3. Sail size

From that starting point, there are a few more things to consider. One of them is the sail size. A bigger sail increases the sideways force I mentioned before, so you put more pressure on the fin. That means you may need to add 1 or 2 cm to the fin size your board brand suggests. On a smaller sail, you might want a fin 1 to 2 cm smaller.

You might have realised by now that this is something really important, so pay close attention: all parts of the gear relate to each other. Imagine a big fin with a big sail, just like I explained, but on a board that's too small. It just won't work. A bigger sail needs a bigger board and a bigger fin, and the other way around. For the sail size, too, the board brands typically give a good suggestion of what's suitable.

This all sounds a bit annoying, but you can use it to your advantage. Since sails don't come in 0.2 m² steps (think 5.2, 5.7, 6.2, 6.7 m² and so on), you can use the fin to fine-tune your setup to the conditions.

4. Conditions

Difficult conditions like strong wind or chop should be a sign to use a fin that lets you feel more comfortable and handle the pressure in the sail. A fin that's too big for the conditions gives you too much lift.

Similar to what I said before, your skill dictates how far you can go towards the higher end of your board's spectrum, and that even goes for the kind of fin. Fully powered up on your freeride kit? Take a freeride fin. Its thicker profile and bigger outline keep you slower and more in control.

When it's very windy and you're well powered up, there's more force coming from the sail:

• Expecting to be rather powered up on your board and sail? Choose a fin a centimetre smaller than you normally would.
• Expecting to be rather underpowered? Use a fin a centimetre bigger.

5. Height and body weight

Lastly, your height and body weight also play a role in the equation. As a rule of thumb, taller people can use fins more efficiently: at the same weight, they get more power from a smaller fin than shorter people. But since taller people also tend to be heavier, this evens out quite nicely.

• Taller than average but still in good shape? You might not need a bigger fin. You might simply need a bigger gear setup in general.
• More kilograms on a shorter body than average? You may need a fin slightly bigger than what your board brand suggests.

In short: tall and light, a shorter fin. Average height and weight, an average fin. Short and heavy, a longer fin.

Put it together: stay within 3 cm

So you have several factors that decide whether you go slightly bigger or slightly smaller: board size, sail size, conditions, weight and height. If you go for the bigger side on all of them, you might end up with a fin 4 to 5 cm bigger than what's suggested for your board. In my experience, that throws off the balance of your setup.

Always try to stay within a range of maximum plus or minus 3 cm of what is the suggested fin size of your board.

My extra tip: get to know your fin

At the start I promised you one extra tip, and my own fin sizes. Try to see it this way: for me, it's a lot about getting to know the fin. Every fin has a specific profile, layup and outline, and with that its own character. Personally, I want to know my fins so well that I know exactly how they'll react to every gust and every chop on the water.

So I try to stay within a small range of fin sizes for each board, and over the years I've also tried to stay on the same fin brand from board to board. That lets me trust the fin blindly and build sort of a personal relationship with it.

I use carbon fins. They have a lot of reflex, which gives you a very direct feeling, very direct feedback from the water. It also allows them to be built with a very slim profile, which makes them faster. And the extra reflex typically allows a fin 1 to 2 cm shorter than a traditional glass fin.

These are my sizes:

• On my small slalom board, 62.5 cm wide: between 31 and 33 cm, but mostly 32 cm.
• On my 72 cm wide board: 36 to 38 cm, but mostly 37 cm.
• On my 85 cm slalom board: between 44 and 46 cm, but mostly 45 cm.

A long guide with a lot of information. Now it's time for you to apply it and get some water time. Below you'll find everything that decides your fin size at a glance, plus the sizing guide. Save the card on your phone and take it to the beach.