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 Post subject: Race Fin Boxes Straight or Toe In Some?
PostPosted: Sun Jun 17, 2012 2:54 am 
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After looking at the new AA and Mikes Lab and a handful of others, it's deceiving but is there any toe-in at all on these boards? I see some I think are straight and then I see some that are toed in to the eye a little.? Can anyone shed some 411 on this? What's best?

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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Sun Jun 17, 2012 1:10 pm 
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What do you mean by toe in? That the outside two fins are angle slightly towards the centre of the board and not perfectly parallel to forward direction? If that is the case I think the toe in is an optical illusion. Since the boards outline get wider, the fins can look like they are pointing inwards. That was the case on my board, but when I measure or look from a different angle then they are nicely parallel.


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Sun Jun 17, 2012 2:13 pm 
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ChristoffM wrote:
What do you mean by toe in? That the outside two fins are angle slightly towards the centre of the board and not perfectly parallel to forward direction? If that is the case I think the toe in is an optical illusion. Since the boards outline get wider, the fins can look like they are pointing inwards. That was the case on my board, but when I measure or look from a different angle then they are nicely parallel.


What board do you have?


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Sun Jun 17, 2012 6:11 pm 
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In the 2012 North LTD I got through the mail the other day, the front boxes are parallel.

But because of a misplacement of the aft fin (as reported on this thread), I ended up having one front fin toed-in and the other toed-out....


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Sun Jun 17, 2012 7:16 pm 
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ChristoffM, I was thinking it might just be an illusion. Thanks for the reply. I figured I was just seeing it wrong.


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Sun Jun 17, 2012 8:26 pm 
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If they were, it wouldn't make any sense on a board designed to be ridden flat.

The direction of travel of the board is slightly down wind of where it's pointing So the 3 fins (at zero toe) go through the water at postive angle of attack. and all create the kind of lift I refer to as upwind drive. (see my article about cant)

If the fins were toe'd in, the increase in angle of attack on one side would be offset by the decrease in angle of attack of the other side and although a net gain in upwind drive would still be produced it would be less than at zero toe and would have alot more drag....Not an efficient way to do this.

The same Idea applies to fin shape, although I haven't seen a lot of race board fins a free body diagram tell me fins should be symmetric and should not be cambered like the outer fins of a surfboard.

You could have some combination of toed in and cambered fins that would ultimately still create the same upwind drive as a symmetric parralel fin but of all the foils I've run lift drag curves for I can't see how this is done without an increase in drag from a zero toe and symmetric foil design.

A surfboard is a differnt animal as it's not ridden flat. Often times 1 or 2 fins are out of the water and so the negative lift caused by uneven toe is gone. Also the angle of attack is quite different because you now have significant speed from the wave to contend with. The basic design of a surfy is making it turn like hell. having a few degrees of rear toe makes the board more responsive to changes in roll. as soon as one of the fins comes out of the water or paritially out the board will begin turning due to rotational moment caused by the uneven rear lift This is done at the expense of drag when ridden flat, but the boards are only "flat" when running away from waves not when actually riding them.

Actual toe angle is very difficult to judge especially if the rail shape around the fin is not straight.


Last edited by Johnny Rotten on Wed Jun 20, 2012 6:41 pm, edited 1 time in total.

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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Mon Jun 18, 2012 6:10 am 
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Oh, my board is a quick home made one. Similar to GB70. Took 2.5 weekend to complete, which is why when I noticed the illusion I started measuring.


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 12:48 am 
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never one to believe in absolutes,
i'd like to persue this a bit more before jumping on the bandwagon.
there are some folks ( myself included) that believe the flow over the bottom of a surfboard looks something like this, when traveling straight and flat. (credit for the pic should go to the folks at swaylocks who have given this a lot more thought than me)


as long as the board's bottom must resist forces due to the riders weight or forces generated by turning, the water will seek diverging paths away from the board.
This is readily apparrant when you notice the wake/spray of your board is not flowing directly aft.
yes/no?
point a garden hose at a flat wall and tell me which way the water flows.
-bill


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 4:59 am 
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The drawing above does not take into account the drift angle
The flow under a board surfed (without a kite or sail) flat on a straight line is different than a board sailed flat on a straight line, because of the drift angle
http://www.blackdiamondfins.at/en/fin-theory--all-about-fins/18-26-die-abdrift


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 Post subject: Re: Race Fin Boxes Straight or Toe In Some?
PostPosted: Tue Jun 19, 2012 6:20 am 
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zfennell wrote:
never one to believe in absolutes,
i'd like to persue this a bit more before jumping on the bandwagon.
there are some folks ( myself included) that believe the flow over the bottom of a surfboard looks something like this, when traveling straight and flat. (credit for the pic should go to the folks at swaylocks who have given this a lot more thought than me)


as long as the board's bottom must resist forces due to the riders weight or forces generated by turning, the water will seek diverging paths away from the board.
This is readily apparrant when you notice the wake/spray of your board is not flowing directly aft.
yes/no?
point a garden hose at a flat wall and tell me which way the water flows.
-bill


zfennell you talking surfboards being surfed, kitesurfboard being pulled (edging + fin), or kite racing boards (riding flat)....All 3 will have quite different flow patterns.


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