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FIN INFO

 
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isobars



Joined: 12 Dec 1999
Posts: 13834

PostPosted: Tue Aug 08, 2000 7:59 pm    Post subject: RE: FIN INFO Reply with quote

Find a friend who has a stash of old Windsurfing magazines. They discussed fins ad nauseum, and hydrodynamics havent changed recently. In a nutshell, curved fins carve curves and fight spinout easily at the expense of max speed, whereas straight fins go straight and upwind really well at some cost in turning. The nuances fill many magazine articles.

Mike \m/
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spyder



Joined: 24 Sep 1996
Posts: 2790
Location: oahu

PostPosted: Thu Aug 10, 2000 12:23 am    Post subject: RE: FIN INFO Reply with quote

I know you probably want design details of fins, that would tell you exactly why fins work..I thought I would post this msg on Spinout, there is a lot of info that is practical here:

Spin Out:
Causes and cures
Adapted by Marc Lefebvre from
the SpinoutFAQ by Tom von Alten

There you are, sailing along, powered up in some conditions a little beyond where youve been before, you hit a little chop and then

-- WHAT THE @%#*?! Did I break off my fin?! --
Youre still sort of on course, but the board is pointing about 45 degrees closer to the wind, and blasting along sideways. Youre still on a plane, but youre not sure you should be.

Whoa. Now that youve stopped and jumped in the water and turned your board over, you can see that the fin is still there, and just fine. (Or not, in which case you need to look for the busted fin FAQ.) What happened?

Youve just experienced SPINOUT.

That nice, smooth flow of water on either side of the fin that was providing the (sideways) lift to keep you on course, and heading a little upwind is not so nice and smooth when this happens. Your fin has stalled, and at the new angle of attack, 45 degrees or so, theres hardly any lift, but plenty of drag. Whats more, its a long way back to that smooth flow!

There are three causes of spinout, and even more ways to cure it. The actor in the scene described above has come to be known as ventilation and is the way most of us first encounter spinout; after going airborne off a wave, we lose it on re-entry. If air can get at the root of the fin, it can be drawn into the low-pressure flow on the windward side and lead to a stall. The turbulence resulting from chop and the landing helps this happen, as does putting the fin back in the water at the wrong angle - it needs to be pretty close to on course.

The other term that gets tossed around is cavitation, which is low pressure boiling. Its a known and well-studied problem for propellors, and for hydrofoils at speeds above 40-45 knots. You probably werent going that fast, were you? Whatever the threshold, any nicks, dings and other surface imperfections will bring it closer, and increase speed-robbing drag as well.

The third cause is a fin that doesnt match the conditions. Too small a fin for that big sail will do you in, as will too big a fin when the wind is blowing small sails. (The latter problem is from loss of control as the vertical component of lift from your fin wants to make your board fly.)

How do you keep it from happening? Keeping those dings tuned out of your fin is certainly important, and better fairing (smoothing of the transition) between the fin and the board can help. Technique has a lot to do with it, too. Will Estes (westes@usc.com) suggests the fix I use:


As soon as you spin out, try to remove all weight from the back foot. I find that in most cases this alone will allow the board to correct on its own. Usually the spin-out is initiated by too much weight on the back foot, and once the spin-out begins, weight on the back foot keeps it going.
As you get better, you will find yourself becoming very familiar with the sensation of the fin just as it is about to begin to spin out. There is a moment before the spin out when you can feel the fin lose force against the water. If you act quickly to remove weight from the fin just at the moment when the force of the fin is lost, the spinout will correct before it even happens.


Learn to grab the board with the back foot and literally yank it toward you (to windward). The idea is to force the fin back to the direction youre going and re-establish smooth flow around it.
The side force that you apply to your board and fin is what drives the fin. If you drive it too hard, or when its not completely in the water (or if you dont have enough fin to start with) it stalls. In the worst case, you have to get out of the straps and move your weight closer to the mast to get it off the fin.

Paul Billings (pab@maui.com) describes a more agressive technique:


You must absorb the bumps with your legs. When going over the top of the chop, let the sideways pressure off a bit (dont push so hard with your back leg).

If it happens, pull HARD with your back leg and push with the front. Actually its more of a jerk than a pull.

And another approach is to just do a little chop hop to get the fin out of the water and situated properly while in the air. This assumes you can land without spinout, however. Smile

There are plenty of experiments in fin design going on, and you can certainly join in that fun. Hydrofoil designers have been working around cavitation for more than 20 years, and with the sailing speed record topping 50 knots, its certainly a problem for windsurfings leading edge.

If your designer hasnt made any gross mistakes (or found the Holy Grail), fin SIZE is the key parameter you need to pay attention to. If you can control your sail in the conditions youre in, but you spinout easily, or cant point as high as you want to, you need MORE fin. If the board is getting squirrely, trying to fly on its own, leaving you overpowered and out of control, you need LESS fin (and maybe less board, too).



--------------------------------------------------------------------------------

Marc A. Lefebvre (lefebvre@ultranet.com)
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joeyyyy



Joined: 18 Jul 2000
Posts: 162

PostPosted: Fri Aug 11, 2000 12:44 am    Post subject: RE: FIN INFO Reply with quote

So Now I know that those nasty nicks in my fin are making me
DE-CAVITATE.
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JeremyHermann



Joined: 08 Aug 2000
Posts: 2

PostPosted: Wed Aug 16, 2000 11:49 pm    Post subject: RE: FIN INFO Reply with quote

I keep hearing that with bigger sails you need bigger fins and with smaller sails, you need smaller fins. Is that really the case? Or is it that when you are more powered you need a bigger fin and when you are less powered you need a smaller fin?

It seems to me that it is the force on the sail and not the size of the sail that is the critical factor. If you were overpowered with a 4.0, wouldnt you need a bigger fin than if you were optimally powered with a 6.0? In other words, it is the size of the sail relative to the wind speed that is relevant. Or am I missing something?

-Jeremy
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joeyyyy



Joined: 18 Jul 2000
Posts: 162

PostPosted: Thu Aug 17, 2000 2:23 am    Post subject: RE: FIN INFO Reply with quote

I beleave (correct me if I am wrong) that the fin provides lift to your board. it provides resistance to sideways movement which in turn creates forward excelleration. the size is more surface errea in light winds to utilize as much power as possible without the fin riding up and out of the water.
When using a smaller sail then naturely you have more wind and hence more speed. with more speed less fin is needed to provide the lift.
the vareble of board Speed is what your missing to understand fin size.
So in a nut shell: small sail is more wind and MORE SPEED, so smaller fin.
And big sail is less wind and LESS SPEED, so bigger fin.
hence the term: big sail big fin,,,small sail small fin...
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