We had a look at the mechanical requirements of the fin, and realized that we were looking at a very different set of requirements to those of the applications previously mentioned. Rather than working mostly at one speed and load, we push our foils over a wide speed range and load range, without the practical ability to add flaps to enhance the low speed performance. Upwind and accelerating, we apply the highest loads, but these reduce as speeds increase when we accelerate off the wind (due to apparent wind going more side on to start with and then forward, but lighter when board speed passes the wind speed).
We developed software to accurately predict the maximum lift of a foil over speed.
> The standard foils used on most fins show a similar maximum load over speed, steadily increasing with speed. When the maximum lift point is reached, the lifting ability reduces, with the angle of attack increasing more suddenly as we go past this point, and drag increases significantly so we consider this the edge of performance envelope. “Stall” is one of the conditions that defines this maximum. You feel this when you try to push the fin at low speed and it doesn’t offer much resistance until you are going a bit faster, then suddenly it gets more “solid” under foot.
This linearly increasing maximum load with speed is actually the opposite of what we need, since it forces us to use a lot more surface area for low speed/acceleration phase performance than we need for high speed operation.