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Old 23rd May 2009, 07:09 PM   #31
Farlo
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"Sanding is all about giving more friction for the water to stick to". Finally we get there. But the difference in friction drag between a matt or polished fin is of third order, while keeping water stuck is essential. Now if you want to polish the first third of the chord and sand the rest... have fun, but you will spend less time on water.
By the way, do you really polish your board and fins (and guitar)?

Last edited by Farlo; 23rd May 2009 at 07:39 PM.
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Old 23rd May 2009, 07:57 PM   #32
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"The viscous nature of air or water causes it to "stick" to the surface over which it flows; thus the velocity directly on the surface is zero for any velocity of the main air or water stream. Put into our terms, as our hull speeds through static water (water with no velocity) at 10 kts, the water molecule right next to our hull “sticks” to the hull and is “dragged” along at the same 10 kts. Proceeding above the surface, the velocity gradually builds up to free stream velocity (the velocity of the stream if the surface wasn’t present at all) at some distance above the surface. In our sailing case, where it is the surface that’s moving through the fluid, as one looks at molecules of water in increasing increments of distance from the hull, they gradually go from 10 kts, to 0 kts.. This area between the surface and the point where velocity reaches that of the free air stream is called the boundary layer. The reaction to the retardation of the flow velocity within the boundary layer is called skin friction drag.The thicker the boundary, the more drag."

Quote from link

Salient point

"The thicker the boundary the more drag"

The rougher the surface the thicker the boundary !!!

Surfaces should be polished.

Nex time you board a plane have a look at wing`s surface. Shiny ! Smooth not rough!!!

Suppose they`ve got it wrong too !!!
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Old 23rd May 2009, 08:41 PM   #33
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"In extensive tank testing for world-class racing boats, it was established 40-some years ago that a glossy surface has substantially more drag than a matte one. In tests I was involved in for an America’s Cup boat, we found that simply sanding a glossy bottom with 600 grit paper, reduced the surface friction by about 5% at ½ hull speed, i.e. at about 4.5 knots. Instead of sticking to the glossy bottom, the water molecules would be “tripped up” by the minute ridges left by the sandpaper. This slight turbulence would reduce the thickness of the film of water being moved along with the vessel (the “boundary layer”), and thereby reduce the overall drag.

Surfboards are traditionally finished super-glossy-shiny. Windsurfers, on the other hand, have long since followed the lead of sailboat racers and taken the gloss off the bottoms of their craft."

Windsurfers got it wrong.

When theory doesn't help, it's time to practice.
See you later, I've got something to sand.

Last edited by Farlo; 23rd May 2009 at 08:49 PM.
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Old 23rd May 2009, 10:02 PM   #34
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Quote:
Originally Posted by Farlo View Post
"Sanding is all about giving more friction for the water to stick to". Finally we get there. But the difference in friction drag between a matt or polished fin is of third order, while keeping water stuck is essential. Now if you want to polish the first third of the chord and sand the rest... have fun, but you will spend less time on water.
By the way, do you really polish your board and fins (and guitar)?
finally we get to where YOU think it is but the truth is less well understood
another interesting read here

http://www.sailtoronto.com/racetraining/bottom.pdf

note it is NOT thought that sanding is a method of keeping a layer of water attached to the hull. It is thought that this is the case whatever the surface finish, polished or sanded.

further read this
https://secure.seabreeze.com.au/Foru...TOPIC_ID=43204

interesting, basically the surface finish is going to be dependant on the section of the foil and the conditions you are sailing in. If its a turbulent flow section, sanding seems to be in order- if its a laminar flow section in smoother water then polish away. Again interestingly there doesnt seem to be a logical argument for wet sanding a hull in preference to polishing a hull.....
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Old 24th May 2009, 04:43 AM   #35
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Except that "America's Cup sailors are wet sanding their hulls to 600 grit finish" [Sailing World]. They probably got it wrong too. A carefully wet sanded hull can be as smooth hydrodynamically as a polished one, with no sensible difference in friction drag. Anyway we won't convince each other. Let's hear what others have to say.

Last edited by Farlo; 24th May 2009 at 04:49 AM.
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Old 24th May 2009, 02:58 PM   #36
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Except that "America's Cup sailors are wet sanding their hulls to 600 grit finish" [Sailing World]. They probably got it wrong too. A carefully wet sanded hull can be as smooth hydrodynamically as a polished one, with no sensible difference in friction drag. Anyway we won't convince each other. Let's hear what others have to say.
americas cup boats dont go as quick as a windsurfer, permissable roughness will be different and as you rightly pointed out earlier, windsurfers in general are planing hulls.
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Old 25th May 2009, 02:33 PM   #37
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Personally, i want water to flow over the board with as minimal resistance as possible, so i polish the board.
The fin however, i want water to stick to it to reduce spinout. For this reason, the fin gets sanded with a fine wet sand
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Old 26th May 2009, 04:37 AM   #38
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Originally Posted by Klint View Post
Here is what Carbonart advice you to do every now and then with your board to maintain top performance. I wonder if even the top PWA sailors do this, or do they hand their boards in to some bottom finish guru ?

http://www.carbonartwindsurf.com/Per...ce/Maintenance
Hi,

I really do not understand the CarbonArt. I was on Defiwind in France where they presented the whole quivery of slalom and speed boards and none of them was polished.

My opinion is definetely not polish the fin, and if you would like to than only on a certain area. And about the hull...I am for wet sanding. The reason is simple...polishing might work as well, might even be better or about the same...I do not actually care. The point is that it is too difficult to keep it in that state...the same for sanded surface...the advantage on roughened surface is that the small dirt or fat does have a smaller effect than on a pollished surface...that's what I think about it.

PS: Please tell me about one production board that is delivered with polished bottom...I would like to now...and do not mention CA...because thay are definetely sanded!!!

Ciao Michal.
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Old 26th May 2009, 12:42 PM   #39
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Originally Posted by mim View Post
Hi,

I really do not understand the CarbonArt. I was on Defiwind in France where they presented the whole quivery of slalom and speed boards and none of them was polished.

My opinion is definetely not polish the fin, and if you would like to than only on a certain area. And about the hull...I am for wet sanding. The reason is simple...polishing might work as well, might even be better or about the same...I do not actually care. The point is that it is too difficult to keep it in that state...the same for sanded surface...the advantage on roughened surface is that the small dirt or fat does have a smaller effect than on a pollished surface...that's what I think about it.

PS: Please tell me about one production board that is delivered with polished bottom...I would like to now...and do not mention CA...because thay are definetely sanded!!!

Ciao Michal.
Carbon art boards used to come with a high gloss bottom my new CA speed boards have a sanded finish.
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Old 26th May 2009, 02:13 PM   #40
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Carbon art boards used to come with a high gloss bottom my new CA speed boards have a sanded finish.
That is what I saw in France.

PS: a pitty that i did not know it while i was there I would ask the guys why they changed from pollished back to wet-sanded surface...
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