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Old 21st March 2007, 12:19 PM   #2
Ian Fox
Join Date: Aug 2006
Posts: 533
Default RE: Q for Ian: Fin Stiffness for Slalom and Wave?

Hi SIN909,

Yes, for sure with respect to slalom. In fact, even though the other thread kinda referenced FW fins (historically), the real essence of those comments related to slalom fins in today's typical sizing 30-50cm, plus further into the speed range 20-30cm.

With wave fins it is further complicated by two more factors :
1> the behaviour of the fin under "surfing" load, where in many cases the rider may prefer a fin that is less ideal in pure windsurfing mode, but gives a better feel or response when pushing the edge in hi torque surfing moves. Wave fins used in non surfing mode (so B&J etc), then the stiffer fin/higher wind and softer fin/lighter will still usually apply. 2> With the extra rake, the deflection mechanism of the fin under load (and subsequent breakdown of the foil flow) is subtly different to the extra vertical race fin, so if a soft/er wave fin is designed really well, it can still retain a reasonable performance under deflection if designed right so the flow remains pretty good under deflected (as opposed to static, unloaded) foil shape.

A further variable is the generally short lengths 20-24cm of wave fins, and the use of (generally) thicker foils in the wave sections means that with a greater cross section and smaller leverage length, there is the option of producing a reasonably laterally stiff fin made with "softer" materials and process that would not work so well in a hi load 30-40+cm race fin. Or at all in a 70cm FW fin.

Something as simple (????) as the material used (G10 machined vs specific carbon laminated) can significantly influence "twist" (pun) the discussion - with a really well designed and made carbon fin, the designer has the option to (much more accurately c/w G10) control/influence the behaviour of the fin under load; for example, it is possible to retain a lot of lateral stiffness whilst allowing relatively soft torsional behavior. So scrape beneath the surface finish of this discussion and quickly "stiff" means more than one thing.

You see where it's all heading If the foil shape is optimised to take advantage of accurately controlled twist, you can see the potential. And the potential for massively more complex and expensive, hi accuracy manufacture. Conversely, just ripping that same design and grinding it identically into G10 will generally not give the same result.

Of course, taking the discussion to even finer levels of accuracy becomes quite technical, as you really need to factor in many specifics/variables. There are instances where real world practice can deny or contradict many broad "generalisations", so again highlighting the concept of not taking these "rules of thumb" to apply specifically to every fin in the world.. But as a generalisation, they are applicable.

Again, these are the short version answers of very long discussions, but you get the overview..

Cheers ~ Ian
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