Author Topic: glossy surface has substantially more drag than a matte one.  (Read 24574 times)

SEA

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glossy surface has substantially more drag than a matte one.
« on: April 22, 2010, 08:25:28 PM »
This article is from the board lady and I am now seeing why Naish uses an unfinished bottom , or matte finish on the bottom.  What do you guys think ??? I'm wet sanding my bottom ... the board that is :)
 
Check out the pictures at the bottom



By Eva M. Hollman

Water is pretty sticky: dip a paddle into the ocean and it comes up with much water attached. Move a board through the ocean – be it surf, windsurf, stand-up, or kitesurf -, and it will drag a substantial amount of water along, which, as it eventually cannot hold on to the bottom any more, emerges behind in the form of a wake. The energy that you generated with your sail, kite or paddle, went into accelerating not just you and your board, but this rather substantial mass of water as well.

To go faster, then, we need to decrease the amount of water “sticking” to your board – “reducing the drag” in techno-lingo, since the best wake is no wake!

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.

If you wonder if your bottom is too shiny, throw a cup of water at it: if the water beads, like it will on a freshly waxed car, then the surface tension is high, and therefore its resistance going through the water is high.

To change it to a low-drag surface, sand it in a circular motion with 600 grit wet & dry paper, until water thrown at it runs off in sheets – WITHOUT ANY BEADS forming.

Instead of wet & dry sandpaper, you can also use a “Purple Pad”, a Scotch brand synthetic wool pad designed to burnish metal and take off rust. Again, a circular motion is optimal.

To keep this fast surface fast, wash it now and then with soapy water; and lightly apply the Purple-Pad when the bottom appears yellow.
   


How big a difference will this make? After a bottom repair a while back, I decided to Purple-Pad the entire bottom of a windsurfing board. The owner noticed with a raised eyebrow, but did not comment on it when he picked up the board. However, he was back 4 hrs later, demanding to know what exactly I had done to his board: a friend, who had always easily passed him before, was no longer able to even keep up with him.

Or this recently from Shawn C: “I purchased an old Hifly 265 poly board that I used a couple of times and really enjoyed.  I noticed that the board had a few small gouges on the bottom so I thought that I would sand them down using wet/dry 600 grit as you suggested.  I then mistakenly waxed the bottom and had horrible performance on the water.”


SEA

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Re: glossy surface has substantially more drag than a matte one.
« Reply #1 on: April 22, 2010, 08:30:07 PM »
Sorry guys the pictures are opposite in the thread they should be the beading water is Slow the Sheeting water is Fast


Pureadrenalin

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Re: glossy surface has substantially more drag than a matte one.
« Reply #2 on: April 22, 2010, 09:01:25 PM »
I like the sanded finish maybe cause I'm dull ;D. Checkout Doug's quiver majority sanded.

SEA

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Re: glossy surface has substantially more drag than a matte one.
« Reply #3 on: April 22, 2010, 09:17:51 PM »
I guess Doug is on it ??? 

corgi

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Re: glossy surface has substantially more drag than a matte one.
« Reply #4 on: April 22, 2010, 09:30:40 PM »
Interesting tip, thanks SEA, I'm all for making my board faster.  I just have a hard time sanding my board being its only 2 months old.  If I do, is handing sanding good enough or would using an orbital sander be more efficient?  ???

bigdom

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Re: glossy surface has substantially more drag than a matte one.
« Reply #5 on: April 23, 2010, 02:18:51 AM »
sharkskin is even faster

SEA

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Re: glossy surface has substantially more drag than a matte one.
« Reply #6 on: April 23, 2010, 05:30:56 AM »
Interesting tip, thanks SEA, I'm all for making my board faster.  I just have a hard time sanding my board being its only 2 months old.  If I do, is handing sanding good enough or would using an orbital sander be more efficient?  ???


I would definitely NOT use an orbital sander !!! You'd want to just sand by hand a little at a time till you achieve the finish you are looking for.
 
In Eva's article it says...... "To change it to a low-drag surface, sand it in a circular motion with 600 grit wet & dry paper, until water thrown at it runs off in sheets – WITHOUT ANY BEADS forming.

Instead of wet & dry sandpaper, you can also use a “Purple Pad”, a Scotch brand synthetic wool pad designed to burnish metal and take off rust. Again, a circular motion is optimal."


When I got back my Naish 14 glide from being repaired this week, Clay Carson remarked how the bottom of the board had a matte finish on it and that he had to wet sand it to make the repair like the rest of the bottom. However he  said he had to follow the sanding lines of the rest of the board. He said it appears that Naish finish sands the bottom of the 14 and 17 glides  with a straight up and down stroke ( from tail to nose). So I don't know which is better a straight up and down sanding stroke or a circular one like Eva recommends. I guess we will have to experiment ???

stillwater

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Re: glossy surface has substantially more drag than a matte one.
« Reply #7 on: April 23, 2010, 06:23:38 AM »
I have wondered why my Starboards were matte finished. I don't think I will be waxing them in hopes of making them faster. Thanks for the insight.

DavidJohn

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Re: glossy surface has substantially more drag than a matte one.
« Reply #8 on: April 23, 2010, 06:40:08 AM »
The bottom of my new Naish 17 and also the new 14' Javelin are sanded.. but not just sanded.. they're directionally sanded.. We used to do it on our slalom and speed windsurfing boards.

DJ

1tuberider

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Re: glossy surface has substantially more drag than a matte one.
« Reply #9 on: April 23, 2010, 06:55:05 AM »
Some race boats have blueprinted hulls.  I know on my whitewater racer bottom maintenance was important for top speed. Hulls are flipped over and low spots filled in and no hook in the tail. 

If you have dents or other imperfections I would address them before the sanding process.

I also believe there are applications that can be painted on that will give similar results.  Drift boats can get a coating of glovit to not only seal the bottom but to also make it easier to paddle by decreasing resistance. 

What are the top racers using?  Some may be going beyond sanding and may be keeping it a secret.


chipmonk

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Re: glossy surface has substantially more drag than a matte one.
« Reply #10 on: April 23, 2010, 10:37:28 AM »
Hey, in your pictures it looks like that Rusty is a molded board. Is that true ? Will this work to wetsand the paint on a molded ?


JustPaddle

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Re: glossy surface has substantially more drag than a matte one.
« Reply #11 on: April 23, 2010, 03:46:32 PM »
A downside of the matte finish on the Naish boards is that my friends 14ft Glide has black marks from the Dakine tiedown straps.

DavidJohn

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Re: glossy surface has substantially more drag than a matte one.
« Reply #12 on: April 23, 2010, 04:41:25 PM »
A downside of the matte finish on the Naish boards is that my friends 14ft Glide has black marks from the Dakine tiedown straps.

Mine did the same so I swapped to the blue Kialoa straps and now I don't get any marks at all.

btw.. These are the best things for removing marks from boards.. http://www.chux.com.au/images/library/Image/hero/hard_surface_2pk.jpg

DJ


Lobes

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Re: glossy surface has substantially more drag than a matte one.
« Reply #13 on: April 23, 2010, 06:57:02 PM »
This is all fascinating and to be honest I would have assumed the opposite, that glossy was faster.

I too would like to know  more about the performance difference regarding applying matte with a circular sanding motion or a straight up and down.

heave

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Re: glossy surface has substantially more drag than a matte one.
« Reply #14 on: April 23, 2010, 09:24:54 PM »
I usually just do 400 grit wetsanded with linear flow.  Its mostly a .005 mm granular roughness that matters.
« Last Edit: April 23, 2010, 09:37:16 PM by heave »