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Sanding a "cut down" fin - Necessary?

 
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d0uglass



Joined: 28 May 2004
Posts: 1088
Location: Bonita Springs, Florida

PostPosted: Mon Jun 24, 2013 5:12 pm    Post subject: Sanding a "cut down" fin - Necessary? Reply with quote

In honor of the late, great Robert Swift I'm posting this question:

Background- I just got a formula board with an older Curtis 85 cm fin that has been cut down to 70 cm. It's cut square across the tip; no sanding or bevel or anything.

Question- Is the fin fine the way it is, or could I improve its performance significantly by some kind of filing / sanding / reshaping around the tip?

Thanks.

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U2U2U2



Joined: 06 Jul 2001
Posts: 3089
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Mon Jun 24, 2013 5:29 pm    Post subject: Reply with quote

Define

could I improve its performance significantly ?

I have a FOX fin , store bought, that is cut down(same as you describe) to make a small weed fin, I have never had the urge to shape the bottom.

A bevel of sort s at the leading edge may smooth the flow, I can't surmise that you could hurt anything, but significantly enhance ?

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LeeD



Joined: 12 Jun 2008
Posts: 1048

PostPosted: Mon Jun 24, 2013 6:02 pm    Post subject: Reply with quote

Shouldn't you try it?
If it spins out, you need some sanding and possibly reshaping at the tip.
If it's hopeless slow, you need to clean up the leading edge, making it more blady than blunt.
If it works peachy clean, don't mess with it.
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rangerider



Joined: 19 Jul 2009
Posts: 161

PostPosted: Mon Jun 24, 2013 9:31 pm    Post subject: Reply with quote

I would try it as is. If you get a lot of vibration / harmonics then I would try to bevel it. Why take the chance of making it perform worse without trying it as is first
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joethewindsufa



Joined: 10 Oct 2010
Posts: 204
Location: Montréal

PostPosted: Tue Jun 25, 2013 4:21 pm    Post subject: Reply with quote

sounds like a "hack job" to me Sad

people do not seem to respect their fins and the R&D that went into them

with MarineTex and sand paper fins can easily be brought back to or close to original shape

i buy fins and repair them, butt NO hack jobs

some SB beginner fins are very square. NOT formula nor slalom

think you are better off getting a real fin than wasting time with that one Sad
butt what do i know Smile
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LeeD



Joined: 12 Jun 2008
Posts: 1048

PostPosted: Tue Jun 25, 2013 6:02 pm    Post subject: Reply with quote

Old aerodynamics theory I first heard from watching LeMan's sports's car races. "If you can't foil it down to a pin, cut it off, blunt and clean"
Worked for nose to tail as well as roof's and undercarriages of the cars going over 170mph.
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xander.arch



Joined: 23 Apr 2009
Posts: 189

PostPosted: Tue Jun 25, 2013 6:27 pm    Post subject: Reply with quote

I'll preface my thinking with "I have no technical knowledge of any kind of fluid dynamics." I am however a windsurfer and I think we all have a certain feel for flow.

I can see how a cut wing makes sense on a craft where the wing has relatively little movement along the long axis. On a windsurfer this would be the up and down direction which of course there is alot of as we slice through and over chop. For this reason I'm going to argue that the flat bottom will create turbulance, inneficiency, drag, and maybe lead to cavitation. If you have nice grinder/sanding skills I think you should certainly try reshaping the tip. If you have no sensitivity for detailed sanding then you may actually make it worse. So it kind of depends on your skills.

I also think that the tapered tip on fins acts as a kind of shock absorber to the lever effect that the fin has on the board. Basically, the foil at the tip is doing nothing more than allowing the water to release cleanly off the fin as the fin moves up and down. This release allows you to more easily put the board on rail and give the ability to find multiple angles of attack as you turn. I'm guessing that if a fin where shaped as a perfectly consistent foil the lever of the fin would really resist any position other than perfectly vertical.

Alright - that's enough. Let's hear from somebody who knows fluid dynamics.
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SeaSpray



Joined: 14 Mar 2012
Posts: 32

PostPosted: Tue Jun 25, 2013 7:51 pm    Post subject: Reply with quote

I've always wanted to put a curb feeler on the bottom of my fins so I quit running them into sand bars and shells. I wreck fins in no time.


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windchamp2



Joined: 02 Jun 2010
Posts: 24

PostPosted: Thu Jun 27, 2013 3:54 pm    Post subject: Reply with quote

A 85cm fin cut down to 70cm will have more area at the base compared to a regular 70cm fin, Its designed that way to add more low end power to the fin. It will not be as fast as a fin with a normal tip but it will get you up planning and sustain you through the lulls for longer
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