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49erGBR735HSC View Drop Down
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Post Options Post Options   Quote 49erGBR735HSC Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 06 at 11:43am

Spent 2 days fairing foils once getting them exactly perfect, well as far as I could judge only for the crew on the first outing to ding the bottom by slamming the foil down in shallow water. Got a more careful crew now and just make sure the foils look smooth and feel smooth because sod's law says the longer you spend finishing something the quicker it will get broken .

Dennis Watson 49er GBR735
Helensburgh S.C
Boat Insurance from Noble Marine

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Matt Jackson View Drop Down
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Post Options Post Options   Quote Matt Jackson Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 06 at 1:00pm

Me and the missus had a long conversation about this the other night with my POV being that a rougher surface induces laminar flow and hers being that any laminar flow is killed by skin friction - actually it was more like hydrogen bonds or something but she a scientist and I'm an Contender sailor so I'd switched off long before I understood it.

The outcome was that a rough surface probably didn't help much and possibly absorbed more water anyway, so go with smooth. We breifly discussed polished surfaces by I ruled this out some months ago when I slid off a recently polished centreboard and threw a race with an inversion when I could have just about won it with a quick recovery .

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English Dave View Drop Down
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Post Options Post Options   Quote English Dave Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 06 at 2:20pm
Originally posted by Matt Jackson

Me and the missus had a long conversation about this the other night with my POV being that a rougher surface induces laminar flow and hers being that any laminar flow is killed by skin friction - actually it was more like hydrogen bonds or something but she a scientist and I'm an Contender sailor so I'd switched off long before I understood it.

Nothing on the telly then?

English Dave
Ballyholme Yacht Club

(You'd think I'd be better at it by now)

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Post Options Post Options   Quote NeilP Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 06 at 10:58am

Even if your foils are designed to run laminar - and not all are, Laser for instance - they will only do so over a tiny percentage of the surface unless the finish is absolutely perfect, ie polished to a mirror finish. The rough surface induces turbulent flow very near the leading edge, so the lower drag of the laminar flow section is lost. Under turbulent flow, the surface is "scrubbed" by the fluid in the boundary layer, and this is where the higher drag of turbulent flow comes from. It goes without saying that the rougher the surface, the higher the scrubbing forces will be, and therefore the higher the drag.

If you think about it, if you slide off your centreboard, the water will too! Intuitively, it's clear that a highly polished surface gives less drag than a rough one. The real world situation is complex to explain, but we don't need to explain it. Trust your intuition! If you want the absolute maximum performance from your foils, keep them scratch and ding free, highly polished and perfectly clean.

This is all based on my hazy memory of my Physics degree from 20 years ago, but I also dug out Bethwaites book, and he did loads of practical experiments that seem to suggest that the performance gain is much greater than I thought it was, so maybe it's worth going to all the trouble after all.

And in case you're wondering, it was work all this out or watch "I'm a Has-Been".

Neil

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Post Options Post Options   Quote richardhenderso Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 06 at 4:24pm

Intrigued by discussion on smooth v matt.

Golf balls are covered in hundreds of little indentations which apparently make for faster progress through the air.  Wouldn't this work with water?

Richard
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Post Options Post Options   Quote tgruitt Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 06 at 5:04pm
I still disagree, of course a matt finish foil is perfectly smooth, you still wet and dry it as much and put just as much work into it! its not completely matt, just not polished shiny. Where is Andy P to back me up....

Edited by tgruitt
Needs to sail more...
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49erGBR735HSC View Drop Down
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Post Options Post Options   Quote 49erGBR735HSC Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 06 at 8:09pm
Originally posted by richardhenderso

Intrigued by discussion on smooth v matt.

Golf balls are covered in hundreds of little indentations which apparently make for faster progress through the air.  Wouldn't this work with water?

From what I can remember, the indentations reduce form drag and also help the ball to spin, which in turn the spin creates lift which makes it's progress through the air more efficent. You can't spin foils on boats, so the theory won't work.

Rough vs smooth is perfectly displayed if you put a surface in a wind tunnel. The flow round the smoother foils is quicker, therefore, the foil has less friction and moves through the water quicker. If you want more lift from a foil, you're better concentrating on the aspect ratio and thickness of the chord of the foil. Surface friction increases drag and slows boats down.

Dennis Watson 49er GBR735
Helensburgh S.C
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redback View Drop Down
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 06 at 9:14pm
I was horrified to find my rudder had tiny little pimples on it.  What am I doing wrong.  Clearly it needs a repaint?
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 06 at 10:23pm

I think the matt finish arguement holds true compared to a gloss paint finish, rather than V's a polished surface. If you run your hand over gloss paint, you can feel it "stick". 1200 grit it and your hand slides much more easily. Polish it, and it is better still, but not by anything like as much.

As for the "you'd do better spending more time sailing" argument, sand and polish the boat and foils after the sun has gone down, at home in the "warmth" (I wish) of the garage. Can't sail then anyway.

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Post Options Post Options   Quote Prince Buster Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 06 at 10:51pm



Edited by Prince Buster
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