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C249 View Drop Down
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Joined: 30 Aug 05
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Posts: 7 		Post Options Post Options   Quote C249 Quote  Post ReplyReply Direct Link To This Post Topic: Speed Bumps
    Posted: 27 Sep 05 at 11:33pm
Originally posted by 49erGBR735HSC

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.


It's not hard at all according to Marchaj - he says it's basic physics because there is (IIRC) an inter-molecular attraction between the atoms of the water molecules and the atoms of the hull (even if they are of different materials). He says the water will ALWAYS stick to the hull to some extent.

I assume the difference between the way water sticks to various surface finishes is just an indication of the varying strength of the molecular bond versus the force of gravity/surface tension etc - anyone know?
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a_stevo View Drop Down
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Posts: 72 		Post Options Post Options   Quote a_stevo Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 05 at 12:18am

At the heart of fluid mechanics/dynamics is boundary layer theory.One of the very first principles of boundary layer theory is the "no slip condition" which uses an assumption that the water/or anyother fluid at the surface ( a sail or hull in this case) has zero relative velocity.

The differences in drag due to surface imperfections is due to the boundary layer being increased in thickness. which means that you drag along more water with you. Also the surface irregularities cause turbulence which can trip the boundary layer into a turbulent flow. Turbulent flow is generally a lower drag state as the velocity profile has greater speed at lower levels when compared to laminar theories. As a result the smoothness of your hull and foils are much more critical when going slow.
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