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Tribal techtalk

Tribal techtalk

De techniek achter de juiste vin keuze

Het meest kritische onderdeel van je surfuitrusting is met zekerheid je vin. Met een relatief kleine investering kan je je board maken of breken. Om een juiste keuze te maken is enige kennis van vinnen essentieel. Tribal heeft Chris Lockwood een intervieuw afgenomen om een aantal geheimen bloot te leggen. 

G10 vs Carbon fins

When machining out of a pre-cured block of material, or injecting a homogonous material (plastic) into a mould, you are constrained by the material mechanical properties. With G10 you have fibres running at 90 degrees of even distribution. This means that along the fibres the product is stiffer and at 45 degrees it is more flexible. The stiffness and flex can only be controlled by the 3D shape (inluding rake, how the blade is oriented on the panel= fibre alignment, thickness and chord distribution etc).

With upright fins there is some advantage to be gained by allowing the fin to flex along the blade, whilst remaining tortionally rigid. The tortional rigidity ensures the angle of attack remains constant over the length of the blade, as it flexes. When the blade is flexed it resembles a J foil, which provides a degree of vertical lift (up or down), and when correctly raked this has a stabilising effect on the board trim angle, and can help to provide some vertical lift through light wind as long as the fin is loaded.

Unlike for sails, twist is generally not useful in a performance windsurfing fin.

Flex is useful for upright blades for the aforementioned reason. However in shorter blades with extensive rake, this flex becomes detrimental since it manifests as twist.

When a blade is tortionally rigid, and upright, the blade can flex without impact on the angle of attack of the profile over its length. When the blade has rake, twist is introduced toward the tip as the blade is bent. If you take a point on the leading edge, the trailing edge shifts more windward as the blade flexes- even if it is tortionally rigid on the blade axis. This has been called "geometric twist". This effectively lowers the fin angle of attack from where the blade is flexing towards the tip. Basically this means the lift reduces towards the tip, and the fin becomes more easily overloaded at the base, often resulting in sudden spinout when a gust hits. It also means less efficiency since a larger part of the fin begins to operate at a lower L/D than the base of the fin. Overall it results in a fin that will not handle the gusts as well as a stiff fin.

Raked fins therefore need to be very stiff along the blade, as well as tortionally rigid.

The Tribal G10 range is designed to be stiff for this reason.

We could change the material to carbon fibre, but it would make no difference as it is already stiff enough. We could introduce flex like we use in high performance slalom fins, but that would lower the performance for speed sailing.

It’s true there are carbon speed fins on the market that do flex a bit due to their thickness and layup, but this limits the amount of load that can be applied, increases drag, and heightens the risk of catastrophic spinout in big gusts.

Spin out / Ventilation

Spin out is the result of ventilation which occurs when an air channel develops between the low pressure (upwind) side of the fin to the surface. The water opens up behind the board leaving the fin dry on the upwind side, and unable to generate significant lift.

The trailing vortex has a low pressure core that extends behind the tail of the board. If it is strong enough it can pull air from the surface behind the tail, upstream through the vortex and onto the low pressure side of the fin causing sudden ventilation (AKA spin out). This mechanism was addressed in the '90s with the popularity of flappers on the tail of wide (at the time) boards where the fin trailing edge was very close to the board tail.

Cavitation

As a foil moves through water, it pushes the water; accelerates it, so that it can squeeze around the profile. When the water accelerates, the pressure of the water drops. This local surface pressure is referred to as dynamic pressure.

Cavitation is the formation of gas bubbles on the fin surface due to the dynamic pressure being low enough for it to form. It’s a bit like when you open a bottle of beer or soft drink- the high pressure of liquid in the sealed bottle drops suddenly as the bottle is opened, and dissolved gas comes out of the liquid as bubbles. You can see this just with unpressurised water if you are lucky enough to fly in planes. In windsurfing, the gas can be simply air that’s naturally dissolved or increased by wave action on the water, or in extreme cases when a near vacuum is created can be formed from low pressure “boiling” of water. In windsurfing, it’s the former that occurs first.The bubbles can sit on the surface, or can be shed off in a sheet or in "blobs" (collapsing as it enters the free stream where the pressure is back to normal).

Cavitation can trigger ventilation by its effect on lift. Generally, cavitation reduces the amount of lift for a given angle of attack (with the same speed). This is known as the lift gradient, or lift slope. Cavitation has the effect of unexpectedly higher drag and loss of lift slope with increasing speed. If load is increased at speed, it can cause the fin to start cavitating due to the lower dynamic pressure, then the angle of attack increases more than a non-cavitating foil for the same load increase.

This increase in angle of attack makes the cavitation worse so it is a sudden onset effect that appears as ’hitting a wall’ with speed. A cavitating foil will have higher turbulence in the wake and a more intense trailing vortex, so this makes it more prone to a "spontaneous" spin out once it is cavitating. If the bubbles are shed, it takes time for the bubble to collapse (water has to be accelerated to fill in the void), so the low pressure in that volume of gas assists in creating the low pressure channel that can trigger ventilation. All this moving around of water uses up a lot of energy- and that energy can only come from whatever is pushing the fin through the water- so its adding to the drag.

Some questions we asked Chris Lockwood:

You state that the profiles you use are designed by yourself. How important are those profiles? Looking at other brands, they invest mainly in the outline and rake of the fin and use standard profiles.

The profiles are critical to achieve the performance. With a standard profile the fin will be more prone to spin out, and you would have to choose a fin more carefully to suit the conditions due to the narrower performance range. Our profiles have a significantly wider performance "envelope" - they are performing optimally over a much greater range of speed and load, so you don’t need to worry so much about having the fin size perfect. After all, if the conditions change, it’s much better
to keep using the same fin than be constantly swapping. Aside from the benefit of the lower surface area (and lower drag), you can get away with the same fin if the conditions change. You can go a size up or down and not worry about losing performance.

How important is a Carbon Speed fin? Will I be able to reach over 50 knots with the TRIBAL G10 speed fins?

The first production fin to reach 50kts was the G10 KA Asy Speed fin from 2008. That is still an amazing fin. The TRIBAL Asy Speed fins were designed to reach 56kts without cavitation. To achieve this, we had to make the fin a bit thinner (in terms of thickness to chord ratio). There is no way around this when it comes to avoiding cavitation. The consequence of this thinner profile is that the low speed performance is reduced - meaning it’s harder to get up to speed, especially if there's a little bit of chop around. You NEED good lower speed performance to reach high speed unless you have kilometres of perfect conditions (if you’ve found these, PLEASE tell me). So we increased the chord a little bit to increase the surface area to maintain the acceleration capability. This is especially important in the smaller sizes, otherwise they would have impractically low lift to even get past the low-40kt range. Carbon? The needs have determined the shape, and it turns out that this shape in G10 is more than stiff enough! Carbon is certainly useful if you want a fin with lower surface area, choosing poorer acceleration and/or greater susceptibility to spin out. There is nothing stopping the Tribal G10 Asy speed from breaking the record out of the bag, except for a willing & able pilot and exceptional conditions. It’s an advance on the KA series which were the fastest fins in the world year after year since they were released, up until the Luderitz canal opened.

Your Weed fin looks very different to the rest out there. How does this work? Will it not take on weed?

The reason for the different shape is that boards are designed for upright fins. A standard Weed fin with a straight leading edge raked over from the base shifts the centre of pressure too far back to be balanced properly on the board. This is why almost always you need to run the mast track right back. And it gets worse with bigger sizes for this reason. In most places where an upright fin catches weed all the time, you don’t need to run rake all the way to the base to shed the weed. Unless the weed is particularly sticky, the design of the TRIBAL WeedSpeed effectively accelerates the surface weed down the leading edge and over the fully swept tip area. Sub-surface weed is swept away like a normal weed fin, due to the high rake at the middle of the blade. The benefit in having the base area upright is that it keeps the fin centre of pressure more forward so that it’s a lot less out of balance than a traditional weed fin.

For average speed sailing on lakes do I need a SYM Speed, WeedSpeed or Asy Speed? Can you state the main differences between these fins?

For average sailing the best speed fin is the WeedSpeed. It’s very robust and hard to spin out, offers a lot of security in chop and big gusts, and it is very, very fast. Stick it in and GO! The bigger sizes turn your slalom board into a speed board. We've had customers pushing past 43kts using the 33cm fin in 68cm boards in chop! The WeedSpeed shape offers huge stability at speed, and doesn’t force the nose down off the wind like conventional fins. The forward centre of pressure allows you to easily build up sail pressure and board speed before the slingshot. The SYM Speed was designed for flatter water, tighter courses where there is an advantage in more surface area - if you are a bigger guy using a speed board with a bigger sail for example. It has less rake than the WeedSpeed so the centre of pressure is a bit further forward. The Asy Speed is designed for flat water conditions, where you have a course on one tack (port or starboard). The fin is much thinner than the symmetric fins and has a bit less lift for its area. It also has lower drag for the area, so if the conditions are for ‘pure speed’ then this is the go-to fin.

What attributes make a good fin?

This applies to all fins! The most important attribute is that you shouldn't notice the fin ... or have to work around its deficiencies. A great fin just disappears. You don't notice that it’s great until you get back onto something you thought was great before, and then suddenly you notice all the deficiencies you got used to and developed reflexes to avoid. A good fin works in more than one set of conditions. Conditions always change.

A good fin lets you get away with doing something silly without unceremoniously drilling you into the water in front of an audience. A great fin lets you push harder than you thought possible - out of your comfort zone, to new levels!

Apart from the obvious objective of ‘speed’, what attributes make a good Speed fin?

Ability to provide lift at low speeds so that you can hit the speed course at the highest speed possible. Enter fast, exit faster ... The ability to bear away in chop without being scared of spin out or lifting. Here the profile is critical: in these conditions it is subject to high shock loads and a lot of entrained air in the water so it needs to be extremely robust. A good fin lets you go for it even if there's chop. A mediocre fin has you holding back for fear of spinning out. It might go fast but you won’t be as fast as you could be if you are worrying about crashing. A great fin allows you to sail into anything without fear, without pulling the board around in unexpected ways, and without thinking of the fin at all.

Apart from the obvious objective of ‘speed’, what attributes make a good Slalom fin?

A good slalom fin has free trim for the downwind legs and good top speed, with plenty of bottom end to push upwind in the lulls when you have to, and doesn't have any sudden instability when you tail walk, hit chop or get nailed by a gust. A great slalom fin is one you can put in and not worry about changing sizes if the conditions change - not thinking at all about the fin when you are sailing. You can just GO where you want, upwind, downwind and always passing your mates.

Apart from the obvious objective of ‘control’, what attributes make a good Freeride fin?

A good Freeride fin has to be super stable and easy to use. Literally stomp on it and go. No spin out, plenty of bottom end, and it needs to give the board a nice free trim across the wind and off the wind. It has to be super stable in the gybes, no matter how out of balance we might be on the rail. The TRIBAL Powermax does all this in a package that also goes really, really fast. There is no compromise on speed.

When do I need to use Asymmetric or Symmetrical fins? Which is best?

The Asymmetric fins from TRIBAL are designed for higher speeds on flat water. They have higher top speed capability and lower drag for the size. You need to choose the tack based on the course. They still work enough on the off tack to get back upwind, but they have to be sailed carefully on this tack since they offer much less lift. The Symmetric fins are best if you want one fin that can go fast on both tacks and where you need good performance to get onto the runs from both tacks. Its easier to get more runs using a symmetric fin since they go upwind better. In a competition this can be important- to increase the chances of getting a good gust. If you are sailing behind a bank, the Asy fin is likely to be the best choice. For open water, shallow areas, or places where both tacks can be sailed in a session, then the Symmetric fins are a better choice.

How important is fin flex looking over the range of fins you design?

Controlling fin flex is critical. Raked fins need to be as stiff as possible. Upright fins need to have a little bit of flex, with minimal twist in the tip unless the tip is raked. It is very difficult to achieve this in G10, so the designs are carefully engineered to get the most out of the material.

The base of the fins is quite short compared to the length of the box, which gives you range for positioning. How do you decide upon this?

The centre of pressure of the fins is calculated and the offset in the base is set to keep it lined up with our reference fins between sizes. This means the fins should just work in a wide range of boards.