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Old 18th May 2009, 02:03 AM   #1
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Cool Polished or microground ??

Should bottom of boards (and surfaces of fins) be polished (shiny) or "mat" finish ??

Think there`s a few schools of thought. Seen arguments for both. Anybody knowdefinitive answer ?? Evidence ???
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Old 18th May 2009, 04:53 AM   #2
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I don't know, but I've polished my Drake R13 70 NR S Flex and I'm having more spinouts problems after that.
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Old 18th May 2009, 04:31 PM   #3
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Matt finish makes water wet the hull, which in turn reduces friction. Several products have been proposed to reduce friction, but none works really better than water itself. Also they tend to increase weight while sanding (one popular way to get matt finish) does not. You can see that a surface is properly wetted when water forms a thin continuous film on it rather than individual drops. A well wetted fin will resist spin-out better. However be carefull not to alter the shape when sanding your board or fin.

Last edited by Farlo; 20th May 2009 at 05:42 PM.
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Old 20th May 2009, 10:58 PM   #4
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Quote:
Originally Posted by Farlo View Post
Matt finish makes water wet the hull, which in turn reduces friction. Several products have been proposed to reduce friction, but none works really better than water itself. Also they tend to increase weight while sanding (one popular way to get matt finish) does not. You can see that a surface is properly wetted when water forms a thin continuous film on it rather than individual drops. A well wetted fin will resist spin-out better. However be carefull not to alter the shape when sanding your board or fin.
hogwash.

read this - " http://www.mothboat.com/CMBA/Building/notwax.htm "
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Old 21st May 2009, 03:20 AM   #5
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No evidence that boundary layer is thicker on wet-sanded hull. Moreover when planning most of this doesn't apply. And you'd better keep water stuck to your fin, friction drag being second order. Does Lockeed Martin also sell wax?

Last edited by Farlo; 21st May 2009 at 04:35 AM.
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Old 21st May 2009, 02:03 PM   #6
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Quote:
Originally Posted by Farlo View Post
No evidence that boundary layer is thicker on wet-sanded hull. Moreover when planning most of this doesn't apply. And you'd better keep water stuck to your fin, friction drag being second order. Does Lockeed Martin also sell wax?
go back and read it again , it debunks your comments.
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Old 21st May 2009, 02:37 PM   #7
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Bethwaite in "high performance sailing" writes about a test( page 233) he did with 2 rudder blades with NACA 0009 sections. they were prepared identically to 1200 grit finish then one was buffed to mirror. The dull (sanded) one apparently generated 23% less cross flow lift force. Apparently this tallies with expectations from theory.
This then rather suggests that polished is better

can anyone explain how the drag will be lesser over a sanded rougher surface than over a polished one?
I can believe that there is a permissable level of roughness depending on the boundary layer thickness, which would mean once sanded to a certain smoothness there isnt much benefit in going smoother(ie polished) but to say that polished is worse than sanded? i havnt seen any evidence of this.
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Old 21st May 2009, 05:19 PM   #8
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Irregularity causes vortexes like a swimsuit with sharkskin. Vortexes causes less friction than smooth surfaces. Smooth surfaces cause a vacuum and this is the friction in the water. But irregularity doesn't mean that every irregularity is good. What is smooth? What looks like smooth can look very rough under a microscope.
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Old 21st May 2009, 06:38 PM   #9
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how do smooth surfaces cause vacuum?
and what surface finish causes the right vortex's to reduce friction?
and how does the presence of vortices reduce the friction and where on the foil? leading edge, somewhere after seperation bubble?
how about polished leading edge to delay seperation, then sanded somewhere after to aid reattachment of the seperation bubble?
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Old 21st May 2009, 06:44 PM   #10
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Its a fantastic answer (the link from earlier post) If its read properly it does answer the question.
I`m polishing fins and bottm of board. !!!
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