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- - In order to add to the half-baked science often posted in this
group and waste some time I will attempt to explain basic hydrodynamics.
Lift on a foil ~ surface_area * (velocity)^2
- - meditate on this over and over and over.... If you really understand
this equation and then draw the vector force diagram of windsurfing
you'll understand most of the "science" posts in this group.
>I've often wondered about what fin size to use for a given sail size.
- - The only way to figure this out is trial and error. It totally depends
on your sailing style, body weight, board design. It's too bad you can't
rent fins to work this out. I have a collection of about 20 fins of which
I really only use 4-5.
>Everyone knows that as the wind strength increases (sail size decreases)
>and fins get smaller .... but why?
- - Because you are sailing faster...
>I can understand that a bigger fin is needed in marginal conditions to
>get you planing earlier, but after that?
>Lets assume that you are fully powered up and the water is perfectly
>smooth (ideal situation). Also assume that you sail at the same speed
>whether on a 7.0 or 4.0 or anything between.
- - This is just not true, even in an ideal situation. Since the velocity
is squared, even a small change in velocity creates a large change
>Then isn;t the sail
>producing the same amount of lift?
- - No, because you are not sailing the same speed. You are probably
sailing at roughly the same efficiency (i.e. % of true windspeed ).
Your arguement assumes that sailing with smaller sails has a dramatically
lower efficiency. Why would this be true? Everything that contributes
to drag increases as you increase the size of the foil.
- - There's a whole bunch of other effects involved ( end plate,
form drag, angle of attack (both horizontal and vertical)... )
but you don't need these to explain the shinking fin size.
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