Finology 101 – Fin Properties
Ok, so we talked about the body parts of a fin. In this section, we introduce the different properties of a fin. This information is specific to VMG Blades’ designs and foils. All properties are interrelated but we’ve tried to isolate different properties as much as possible to keep it simple. This isn’t exhaustive, that is a text book’s job, but you can consider this the cheat sheet to making you look good.
Click on a title below for more info.
Rake is the angle of the fin. We measure it in degrees, using an imaginary perpendicular line coming from the base of the board, against the leading edge. If the LE is perpendicular to the board, it would be measured as 0°. Generally speaking 4° – 5° is the most common rake.
Raked back, or more rake, is when the angle increases, such as to 5° – 6°. Weed fins are an extreme example of being raked back. Increase rake when you want to keep control in stronger winds. Rake will increase Geometric Twist thus depowering the fin. Raking a fin back will also change the vertical lift the fin produces. Less vertical lift in the tail allows the nose of the board to ride higer. Boards with flatter rocker lines require fins with more rake to allow the nose of the board to ride higher.
Is how much the fin bends from side to side, it’s also known as flex stiffness. A fin’s bend is determined by how soft or hard it is, usually denoted by XXS, XS etc. Bend is a good thing to have, as it can act like suspension for your fin, giving you a smoother ride or absorbing power for more lift.
Soft fins are very popular as they are more powerful. As the fin bends it generates vertical lift, taking the board out of the water. The downside is that if the fin is too soft, it over-bends and the energy is absorbed into the fin instead. It’s like the fin has gone all rubbery.
VMG Blades favourite fins are those with soft tips. They are more responsive and auto trim the board. Soft tips are also more versatile and can be used in a broader range of conditions.
Stiff or hard fins are useful too. A heavier person can overpower a soft fin so a stiffer fin is usually needed, giving them the ability to put more power into the fin.
So what happens with ultra hard fins? As mentioned, the fin can act as a shock absorber, if the fin is too stiff, the shocks are not absorbed. Instead the shocks are transferred to the board, making it harder to ride and tiring. Also, you may not get enough rail pressure (or vertical lift), so you’ve got more wetted surface in contact with the board and more drag.
Which conditions you are in also plays a significant role. If you are in a lot of chop or swell, the fin will experience varying loads as you sail through the chop. You want a fin that can react quickly to the changes in this energy and allow you to maintain your trim. In these conditions, VMG Blades can even consider changing the amount of soft area within the fin.
“I want the most torsionally stiff fin possible.” Do you? Are you sure? Let’s have a look at TS and then you can decide. We’ll talk about outlines in this section to help you get a better understanding why the same shoe doesn’t fit every foot.
Torsion’s definition is the twisting, or the screwing, of an object around an axis. Stiffness is the resistance to that force. The way TS is measured is by how far the screwing occurs and how much force was needed.
TS and twist are not the same, but they are related. Torsional Stiffness is the twisting around an axis and a fin with high TS needs more force to turn around the axis. Twist occurs to the fin under pressure and is the release of force, just like how a sail uses it’s leech. Torsional stiffness can affect twist, but not vice versa.
Neutral Outline is a design which has little or no twist existing within the design. It is mainly the layup that controls twist. In a neutral outline, you don’t need to create high TS in order to control the twist. Funnily enough, it is the opposite, you do not want high torsional stiffness as the fin won’t be able to twist sufficiently.
While you are sailing, the fin comes under load and the trailing edge twists to de-power the fin and settle the board. It is very similar to the leech of your sail and how it ‘twists’ to let some of the wind pass.
Twist can help in reducing drag if you don’t have a tapered outline, such as in large cutdowns. By creating twist in the fin this modifies the lift distribution, and drag is reduced because the strength of the trailing vortices are reduced
Having more twist will allow the fin to depower in strong winds. It’s very handy downwind, helping to settle your board and give you a smoother ride.
So when would you want little or no twist built into the fin?
- When the plan shape and rake combined are already creating Geometric twist
- When you have an elliptical or tapered plan shape, which creates even lift distribution and downwash
What’s Geometric Twist?
There are two different types of twist. Normal twist, which is mainly what we’ve been referring to above, is created by the layup of the fin. Geometric twist is the twist that is built into the design and shape of fin. When the fin bends, it will naturally curve creating additional twist.
Short fins are those which are below the standard FW length of 70cm, eg 66cm. You would want a shorter fin in the following situations:
You will be travelling through some very very strong conditions, and similar to a smaller sail size, a smaller fin size is preferable.
If you are very light.
Junior competitors will sometimes want a smaller fin to avoid being overpowered.
You could also consider a cutdown fin. A cutdown is when a fin is built in its standard foil with extra length, eg 76 cm and cut to meet requirements of 70cm. Cutdowns have the benefit of increasing the surface area of the fin, which gives you more lift. These fins definitely perform better in light winds.
Cutdowns aren’t just a simple snip of the fin (so please don’t try this at home). The tip of the foil is thicker, and the outline becomes generally less tapered. All of this increases induced drag. The layup for a cutdown fin needs to be adjusted for maintaining the correct twist and bend in the tip.
Thin foils are fast foils. With a thin foil, you can reach top speed faster because there is less frontal surface area creating drag. The flip side is that in certain conditions, you may not have enough power to lift the board. If you can’t get planing, well, you’re never going to win that race.
Thicker foils have the power. The thickness gives more surface area for traction, giving you both vertical and horizontal lift. Unfortunately, thicker fins are harder to get started as you need to contend with more drag and the significant change in water direction around the foil.
In between thick and thin foils there are so many properties which determinne how a foil performs, it is best left to the experts to understand.
VMG Blades uses foils carefully chosen and developed by experts such as Boogie to provide the best performance for you.
High torsional stiffness is needed in a curved outline as the swept back shape creates more geometric twist. There will always be twist in this outline. You actually need to increase the TS of the fin so that you reduce twist.
Each outline has its benefits but generally, low TS in a neutral outline is a good thing and low TS in a curved outline is a bad thing.
A Common Misconception
Do you think you know your stuff? Try our quick quiz…
Which is the incorrect answer?
A. A fin with low torsional stiffness will always have high twist.
B. A fin with low TS can be built to twist only a little.
C. A fin with high TS can twist a lot.
And your answer is …. A. It’s a common misconception that a fin with low TS will have high twist. It all depends of the outline and layup of your fin to how TS affects twist.
Finish & Flow
Finish is how the surface of the fin feels and has a large part in how the water flows over the foil. This is why looking after your fin is very important. Nicks, scratches or dents all affect the flow and can create drag or separation bubbles. If you have a tendency to spin out unexpectedly, then this is the section for you to read.
Laminar flow is where the water stays attached to the fin and follows the foil shape closely. Theoretically this creates the least amount of drag (always a good thing as we want to minimise drag). However, laminar flow can stall dramatically and you find it much easier to spin out. This is because large separation bubbles happen in the boundary layer of the water flow around the foil. The separation bubbles separates the fin from the water, you suddenly have no lift and you spin out.
Turbulent flow is like little swirls of turbulence along the fin. More energy is created in the boundary layer and the separation bubbles are tiny. As the bubbles are tiny, stalling is less likely to occur and a rider is able to push against the fin as it is more stall resistant. This gives you more control and allows you to be more daring in your tactics. There is slightly more drag but at least you don’t have to worry about spinning out in the water.
A rougher finish forces a fin into making turbulent flow and a smoother fin is more likely to become laminar flow. Sailors often experience different sensations when sailing new fins. This is because of their style and experience. It can also take a little while to get used to a new fin and adapt your style. Different finishes could be recommended to different sailors, this is so that you get the best possible ride for your style.
In this section we’ll cover off what various conditions, namely wind and water, will do to a fin.
You often experience a lot of different water conditions while windsurfing. Going from flat water, to chop, swell and even waves.
Flat water has the fin in even contact with the water, creating a constant load on the fin. If you sailed in just flat water conditions, you could get away with a very soft fin to give you more lift. It’s not just that the water is flat though, it’s also that typically when the water is flat, the wind is also light. So you need more power to get lifted up and going.
In swell or choppy conditions, the load on the fin spikes as you travel over the swell; or get slightly airborne over chop. You’ll want a fin that is softer but not too soft, so that it can react and balance out to the changing load along the fin. This will help you to cut cleanly through the chop and ride above all the mess that will slow you down.
Fresh and salt water also affect what is needed in a fin. Salt is more buoyant and lets your board sit higher on the water than it would in fresh water. As salt water already gives you some ‘lift’, you don’t need to as much compensation for lift and can concentrate on speed. In very salty water, you would be absolutely flying.
Wind strength can play a large part in what fin you will feel comfortable riding while out in that weather. You don’t want a fin that is too powerful, as it would be like taking a 12m sail out in 35 knot conditions. You probably wouldn’t perform very well and wouldn’t feel very secure being out of control. So we’ll break down what’s the key thing to look out for in the different wind conditions.
Light winds need a powerful fin so that you can get your necessary lift and minimise board drag. A soft fin is usually the best here.
Medium winds needs a fast reacting fin, with a wide range. This is so that you can sail high or low in the groove in order to find the most efficient path through chop and swell.
Windy conditions need to have more twist built into the fin. This is so that you can depower and keep in control. A soft tip will automatically trim the board to adapt to the changing loads during chop and gusts.