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Isis View Drop Down
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Posts: 2753 		Post Options Post Options   Quote Isis Quote  Post ReplyReply Direct Link To This Post Topic: Speed Bumps
    Posted: 25 Sep 05 at 8:51pm
^that was the principle behind the matt paint, to trap a layer of microscopic bubbles and therefore reduce the wetted surface area of the hull
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Ian29937 View Drop Down
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Posts: 409 		Post Options Post Options   Quote Ian29937 Quote  Post ReplyReply Direct Link To This Post Posted: 25 Sep 05 at 8:52pm

The matt hull finish was to crate the same effect, trapping a thin layer of water to reduce the drag.  Reducing drag is much less important with sails which work on lift generated by pressure difference.....

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49erGBR735HSC View Drop Down
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Posts: 1991 		Post Options Post Options   Quote 49erGBR735HSC Quote  Post ReplyReply Direct Link To This Post Posted: 25 Sep 05 at 8:56pm
If you are trapping water against a surface, doesn't that ultimately slow you down. It would be really hard to trap a continuous skin of water round the boat, it might work in instances but I don't think it would stand well against a boat with a constant quick fast moving flow round it. I think Bethwaite experimented with it at one point too.
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JimC View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 1:37am
Originally posted by jeffers

Wasn't there something trick about the Aus or NZ boat quite a few years
back when they went for a 'sharkskin' finish on the hull


The sharksin was on Conners 12 at the Freo Amcup. It only worked when very carefully placed and over a very narrow speed range. Fortunately 12 metres, unlike dinghies, only really work within a pretty narrow speed range.

Such things are illegal now under the racing rules:-


53 SKIN FRICTION
A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer.


So if you see someneputting dimples on or whatever you can protest them!
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Stefan Lloyd View Drop Down
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Posts: 1599 		Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 6:31am

Originally posted by Ian29937

Reducing drag is much less important with sails which work on lift generated by pressure difference

Creating lift also always creates drag. Prandtl's theorem says so. Impress your friends by dropping that into the conversation. 
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Ian29937 View Drop Down
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Posts: 409 		Post Options Post Options   Quote Ian29937 Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 9:06am
Originally posted by Stefan Lloyd

Originally posted by Ian29937

Reducing drag is much less important with sails which work on lift generated by pressure difference

Creating lift also always creates drag. Prandtl's theorem says so. Impress your friends by dropping that into the conversation. 

Obviously true, but it is the lift which propels the boat.

As I said the drag is 'less important' than creating the optimum sail shape to generate the maximum lift, to go faster.  If speed creases do this, and they clearly do for many classes in certain conditions, then this is the way to go. 

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Stefan Lloyd View Drop Down
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Posts: 1599 		Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 9:26am

Originally posted by Ian29937

If speed creases do this, and they clearly do for many classes in certain conditions, then this is the way to go.

I agree with that. 
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timnoyce View Drop Down
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Posts: 1991 		Post Options Post Options   Quote timnoyce Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 5:09pm
right as the other speed bump topic seems to have gone more technical i thought i might try and sway this one towards another type of speed bump which you seem to find down at the sailing club.... the ones in the car park!!! WHY oh WHY do sailing clubs all have speed bumps?! Do they have a secret link with boat repairers, manufacturers and MOT men? When i am trailing my boat i take that as reason enough to drive slowly, i do not need some idiot to put in some mini mountains to shake apart my beloved piece of kit as it is brutalised by the trailer!
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Rupert View Drop Down
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 7:03pm
We decided against at Whitefriars when we redid the road, I'm glad to say...we now just have the problem of people going at silly speeds down it. There are even tyre skid marks by the gate where people have wheel spun. Can't win, really!
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NickA View Drop Down
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Posts: 784 		Post Options Post Options   Quote NickA Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 05 at 8:54pm


53 SKIN FRICTION
A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer.  !!!!!!!

So my aged boat would be illegal if I'd scratched it on purpose, but OK since I did it by accident???

Only one thing to say to you lot.  Trip turbulators.
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