foil design

[rigo]

Hai there,

As I started into a long testing project on foil flexibility I'd like to have as many information as I can get on this issue.

I'm talking here of things like chances in flexibility in relation with a change in lift drag performance of foils in choppy conditions.

So if anyone has some leads I hope you can let me know.

good winds

rigo

[Eric Sponberg]

Rigo,
What are you talking about in relation to "foil flexibility"? That is, in relation to what--sails, keels, rudders, daggerboards, surface-piercing foils? In general, foils to not want to be flexible, but rather they should be as stiff as possible for the least amount of profile and section shape, consistent with strength and performance. This is why America's Cup and Grand Prix sails are made of cuban fiber these days--extremely stiff and lightweight so that they don't change shape and are easy to raise and lower. If a foil is to be flexible, then is has to be so in a controlled manner. It is a complex problem to pre-determine a shape based on the lift and drag produced. I am not saying there is not an application for such a foil, but I would recommend some outside controllability for whatever you develop.

Eric

[Karsten]

Hi Ringo,
From my experience with dinghies it seams that "soft" centreboards can have some advantages in heavy/gusty conditions. They tend to dampen the gusts a bit. You can have the same effect with a smaller centreboard which should be faster than a big and flexible one (keels and centreboards are usually too big once the boat has accelerated). I once sailed a boat with the keel swept forward. If you do this correctly you could twist the keel with the weight of the boulb to increase the angle of attack near the keel tip. This could help to keep the boat upright.
If you have a long and narrow keel or rudder you can get flutter problems at high speeds. Basically the keel acts like a torsion spring. If you twist it the lift changes and creates a twisting moment that wants to further increase the twist. If this twisting moment is larger than the torsion stiffness of the keel it just keeps twisting. Not good.

Cheers,
Karsten