g10 versus carbon

When machining out of a pre-cured block of material, or injecting a homogeneous 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.

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.

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.

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.

Flex in carbon speed fins

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.

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