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wave fin ques....???
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Wind-NC-Hatteras



Joined: 28 Jun 2008
Posts: 772
Location: Cape Hatteras, NC

PostPosted: Fri Jan 04, 2013 9:36 am    Post subject: Reply with quote

Oh, wait, bred2shred already said that... Sorry for the repost!
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isobars



Joined: 12 Dec 1999
Posts: 13834

PostPosted: Fri Jan 04, 2013 10:23 pm    Post subject: Reply with quote

As did a couple of others. Wink
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jingebritsen



Joined: 21 Aug 2002
Posts: 2373

PostPosted: Sat Jan 05, 2013 5:39 am    Post subject: Reply with quote

nature's been designing fins and wings far longer than humans have been trying to manipulate nature. look at the critters and what they do to help you decide what you want to do in windsurfing and choosing fins and sails. fascinating stuff going on all around us.
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rich1



Joined: 10 Apr 2000
Posts: 156

PostPosted: Sat Jan 05, 2013 3:39 pm    Post subject: Reply with quote

Quote:
nature's been designing fins and wings far longer than humans have been trying to manipulate nature. look at the critters and what they do to help you decide what you want to do in windsurfing and choosing fins and sails. fascinating stuff going on all around us.


I must disagree with this. This has been the attitude driving a lot of design in the surf/windsurf industry and doesn't really make sense. Animals have very different design parameters than we do. Nature has to make it's fins and wings out of living tissue, wings have to fold, and both wings and fins are also sensory and variable in geometry. That's not to say that we can't learn by studying what nature has done, just that we shouldn't blindly copy a dolphin fin and expect it to be the best design for a wave fin.

That being said, I've done quite a bit of reading, and devoted a lot of thought to figuring this out.

So boiling it down, a looser fin has a lower aspect ratio, higher taper ratio and more sweep than a pointer.

The first two (aspect and taper) seem to be pretty straight forward, ie centre of lift is concentrated closer to base of the fin, so we have more leverage over it as mentioned in previous posts. The sweep however is a bit more perplexing. With the addition of sweep, the effective span of the fin changes with the planing angle of the board. What's kind of interesting here, is that the fin becomes effectively longer as we initiate a turn because the angle of attack of the board becomes greater. So, going from rail to rail, the load on the fin transitions from one side to the other. At the initiation of the turn, the lift being generated is actually pushing the tail in the direction that aids the rolling of the board, then as the load moves to the other side of the fin, it begins to resist the continued rolling of the board. Based on this, it would seem a vertical (or very slightly swept like 5 or 6 degrees) fin would be a tiny bit stiffer up to the transition point, but looser as you continue to set the rail. Which, I would think for agressive rail-to-rail riding would be looser overall, so where does that leave us?

The other thing that sweep does is raise the cavitation speed of the fin a bit. This may be important, but I'm not sure. It may allow us to use smaller fins without suffering spin out. So rather than being a design plus for looseness, sweep may be a necessary design compromise to get away with a smaller fin.

How's that for muddying the waters?
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U2U2U2



Joined: 06 Jul 2001
Posts: 3056
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Sat Jan 05, 2013 5:49 pm    Post subject: Reply with quote

the water is muddy only if you want to look too far

this interpretation of the concept of probability can be seen as an extension of logic that enables reasoning with propositions whose truth or falsity is uncertain.

seems to me, to be a abstract concept, that we assign theoretically, for the purpose of representing a state of knowledge, or that we calculate from previously assigned probabilities.

referred to as theory

cavitation:
Why do my fins hum?

If your fins make a noise like humming, it is due to cavitation. Cavitation occurs when low pressure bubbles in the water begin to collapse and create tiny shockwaves which are the source of the noise.

To fix humming fins: sand the sides of the trailing edge with a medium grade sand paper. Several passes with sand paper is usually enough.

the other theory is that they hum because they don't know the words.

there are fins that do indeed look like animal this a whale fin.



with many looking like the dorsal fin of the shark.

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isobars



Joined: 12 Dec 1999
Posts: 13834

PostPosted: Sat Jan 05, 2013 7:03 pm    Post subject: Reply with quote

U2U2U2 wrote:
If your fins make a noise like humming, it is due to cavitation.

Or it's loose in the box.
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westender



Joined: 02 Aug 2007
Posts: 612
Location: Portland / Gorge

PostPosted: Sat Jan 05, 2013 11:47 pm    Post subject: Reply with quote

I have some Swell fins for the non waves around here but depend on the board for maneuverability more than the fin. For some reason I'm anti technical when it comes to my gear. If it works, I keep it till it breaks and wish I had another.

My go to fin is a old Finworks Pro300 Tuttle design race type fin that turns great.
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rich1



Joined: 10 Apr 2000
Posts: 156

PostPosted: Wed Jan 09, 2013 1:41 pm    Post subject: Reply with quote

So after a bit more research I discovered something I had been looking for for a while.

Bottom line, sweep tends to move the centre of pressure more towards the tip.

I think this may explain things. Say you're sailing with a fin with about 20 deg of sweep, and the board is trimmed at about 5deg. The effective sweep now becomes 15 deg just sailing along. As you initiate the turn, there is more lift towards the tip due to the 15 deg of sweep and it aids in the initiation of the turn to the point of the fin beginning to load on the opposite side. As the nose comes up to more like 20 deg, the effective sweep is 0 deg. The centre of pressure migrates toward the base of the fin making it feel a bit easier to continue to roll the board into the turn.

These numbers are "made up" for illustration purposes, I'd like to know the real ones, but I think they're pretty close.

Just a theory, but it all does seem to match observation. Plus I never liked the "higher cavitation speed" explanation anyways.
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