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NeilP View Drop Down
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Post Options Post Options   Quote NeilP Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 06 at 12:27pm

Rupert - the difference between the feel of a matt and gloss surface is down to the area of contact with your hand. A sanded surface is made up of valleys and peaks. Your hand only contacts the peaks, so the frictional force will be less. When it's running in a fluid, the fliud is in contact with the entire surface. This is a bit of a "flat earth" argument, all the science is on the side of the highly-polished foil.

Tom, I think you're missing the distinction between "smooth" and "fair". If a surface is perfectly smooth, it will be reflective, and therefore shiny.

Neil

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Post Options Post Options   Quote timnoyce Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 06 at 4:47pm
It is all clearly documented in Frank Bethwaites High Performance Sailing! Does this really mean that there are some forum users without this tome?! Its not too late to add it to the Christmas list!
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Post Options Post Options   Quote Granite Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 06 at 6:22pm
A gloss painted surface is quite bumpy especially if it is brushed on.
To get best performance you should flat it back with wet and dry and cutting paste and end up with the perfectly polished surface.

The mat is fast argument is because wet and dry with 800 to 1200 grit is much better than shiny un sanded gloss paint. Fully polished is better than 1200 grit but not by that much
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 06 at 7:06pm

The other theory I have heard is the idea that the matt finish traps a layer of water and keeps it still, and then the water is flowing round itself, not round the foil, so resistance is reduced still further. Never been sure about that one, but one of the America's cup teams put alot of money into the idea a while ago, and 3M came up with a stick on sheet which reduced skin friction. I believe it got banned.

 

Personally, I polish the boat and rudder as it is easier to clean the dinghy park mud off, which I'm pretty sure is slow...

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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 06 at 9:42pm
I've sanded off my pimples - they all contained water!  What do you reckon - is this osmosis?
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Post Options Post Options   Quote NeilP Quote  Post ReplyReply Direct Link To This Post Posted: 05 Dec 06 at 9:46am

Redback, it's almost certainly a case of the foil not being completely dry when you painted it. If you primed it first the water may have been absorbed by the primer, and it then breaks down the adhesion between the primer and the top coat.

Solution is to sand the whole foil back to gel-coat if it has it, or to epoxy if not, and then dry it thoroughly and start again. Good luck.

Neil

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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 05 Dec 06 at 11:15am
Originally posted by Rupert

The other theory I have heard is the idea that the matt finish traps a layer of water and keeps it still,


The good old surface tension theory. Complete ***** as the foil is underwater of course...

Originally posted by Rupert

but one of the America's cup teams put alot of money into the idea a while ago, and 3M came up with a stick on sheet which reduced skin friction. I believe it got banned.


Riblets. Yeah, but what you have to know about that stuff is that it was a specially textured surface which had to be aligned precisely wth the local water flow on the boat and only worked within a very limited speed range. As the old 12 metres only worked within a limited speed range it did work on them. It was a world and a half away from a rough surface...
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