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Old 21st May 2009, 09:40 PM   #11
Farlo
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Well I read it again and the author gives no evidence that boundary layer is thicker on a wet-sanded surface. Moreover where is that thickness measured from? Top or bottom? Regarding friction drag, matt is of course rougher than polished but does it make a sensible difference? We're not talking about mountains here. The question is more about wet or not, and how the hull will glide on a mix of air bubbles and water. But go polish everything. I'm quite curious to hear if you go any faster. Hello Tiesda, do you have an idea?

Last edited by Farlo; 21st May 2009 at 09:59 PM.
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Old 21st May 2009, 10:11 PM   #12
mike
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FYI on board bottoms:

http://www.boardlady.com/fast.htm

She says don't polish.
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Old 21st May 2009, 10:33 PM   #13
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To quote Marchaj in aero and hydrodynamics of sailing - page 342

"In general conclusion it appears that the roughness of foils surface is always harmful, and the labour expended on perfecting the finish on the forward third of the chord of a rudder or fin keel is never likely to be wasted."

before this he comments that polishing the leading edge is a good thing but that you can pretty much do as you please with the rear third of the chord and it makes not much difference. He compares a highly polished surface with a surface sprayed with 180 carbundurum.

so thats bethwaite and marchaj and a guy from lockheed martin.... versus some comments from internet forum posters..........
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Old 21st May 2009, 11:49 PM   #14
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does it make any difference?
the bottom line is that it does not.
the bottom line is martin van meurs doing 50 knots at southend on a fin that a few minutes before he had run into a mussel bed with- it was scratched and battered and hanging and yet he did 50 knots on it.

If you have a polished fin you can then keep it clean easily, with a wipe over- if you wet sand it it will pick up dirt and crap easily, and hence you will forever be having to wet sand it- so polished is probably easier to look after. But do whatever it probably makes little difference.

The lockheed martin guy talks about the hobie tri foiler and cavitation- i think he is mistaken to call this cavitation, i think its ventilation. He made that mistake so he might easiliy have made others.
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Old 21st May 2009, 11:52 PM   #15
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For those with any R&D in their blood-

Easy test
Take a good fin that works for you, and wax one side and not the other. Try it and see GPS and all. Let us know what you find.
Remove the wax with Acetone.
Better yet polish and then wax. But then you may need to resand
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Old 22nd May 2009, 12:05 AM   #16
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The bottom line is, first tune the sailor. If you do the test yourself, the difference will be 99,99% chance due to yourself and not the polished or sanded fin, board. So R&D conclusion will be that the variance will be bigger on the sailor then the sanding or waxing issue.
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Old 22nd May 2009, 12:38 AM   #17
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leysenkr can you explain your theory about vortices?
I thought the sharkskin idea was to reduce vortices disturbing the boundary layer, not to create them.

also the lift and drag of varying surface roughness foils is easily tested in a tank- its been done and referenced by Bethwaite who says that polished is better.

what more do you want?
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Old 22nd May 2009, 02:18 AM   #18
Farlo
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A planning hull does not exactly behave as a foil in a tank. Even analogies with two-phases flow in pipes can be misleading because boundaries are not stable. You will notice that most pro-model boards have matt finish. You may call it marketing BS but I can assure you that if you polish your guitar you won't play faster.
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Old 22nd May 2009, 02:26 AM   #19
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A smooth surface is good and for this example the reference. Some specific roughness might be better at a certain speed. But on another speed, this exact roughness will slow down more than a smooth surface so it will be difficult to gain more speed. You need the exact speed to obtain the optimal gliding for a certain roughness.

Polished might be better in that specific test. But the sailor will influence the endspeed much more than a polished or non polished something.
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Old 22nd May 2009, 04:19 PM   #20
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Production costs for polished finish are higher.(Especially with fins) Difference is perhaps that small that for 99% of sailors its irrelevant but knocks 50 or so off board.

Perhaps Martin Van Meurs might have done 50 .5 knots had he had a polished fin ??? Who knows ???

Evidence is there . It definitely suggests polished is best. Theories are wrong though ! (Sometimes)
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