Jacket
Well-Known Member
Rubbish. Pressure does devrease with velocity for water. If nothing else, Bernoulli's equations were derived from experiments carried out on water.
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Split centre boards
- Thread starter Sybarite
- Start date
DeeGee
Well-Known Member
Hmm, reserve judgement on that. Brendan's post is pretty informative, and seems to get round your objections.
My problem is that I am a mathemetician, not a physicist. There is a big difference between understanding the outcome of the application and solution of equations, including things like boundary conditions, analytically or peicewise, etc... but the formulation or choice of equation - and what has been abstracted away (by the physicist) in setting up the mathematical model - is something else again.
I can't think of many more appropriate threads for a sailing forum than this....
-))
<hr width=100% size=1>Black Sugar - the sweetest of all<P ID="edit"><FONT SIZE=-1>Edited by DeeGee on 24/10/2003 08:31 (server time).</FONT></P>
My problem is that I am a mathemetician, not a physicist. There is a big difference between understanding the outcome of the application and solution of equations, including things like boundary conditions, analytically or peicewise, etc... but the formulation or choice of equation - and what has been abstracted away (by the physicist) in setting up the mathematical model - is something else again.
I can't think of many more appropriate threads for a sailing forum than this....
-))<hr width=100% size=1>Black Sugar - the sweetest of all<P ID="edit"><FONT SIZE=-1>Edited by DeeGee on 24/10/2003 08:31 (server time).</FONT></P>
vyv_cox
Well-Known Member
Indeed it does, but you are talking about a different situation from a foil in moving water. The classical cavitation theory looks at water exiting a restriction and expanding into a larger space. Dependent upon various factors such as velocity, shape, temperature, vapour pressure (if not water) cavitation may or may not occur. My confusion lies with the open situation where the supply of water is unlimited and water can flow around the foil to balance pressure.
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S
Skyva_2
Guest
Look at this (Figure 3) for an illustration of hydrodynamic lift coefficients for various aspect ratio foils at different angles of attack. It refers to Marchaj's results.
http://marina.fortunecity.com/breakwater/274/1998/0601/index.htm#Underwater Board Shape
That's why my Merlin Rocket has a centre board like a Spitfire wing rather than a stubby board of the same area. It generates more hydrodynamic lift.
Compressibility has very little effect in this; the actual changes in pressure on a wing, a keel or a sail are small compared to ambient pressure, so presumably cavitation does not result ( your knowledge is way ahead of mine on that). The moving fluid therefore does not experience any significant change in density.
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http://marina.fortunecity.com/breakwater/274/1998/0601/index.htm#Underwater Board Shape
That's why my Merlin Rocket has a centre board like a Spitfire wing rather than a stubby board of the same area. It generates more hydrodynamic lift.
Compressibility has very little effect in this; the actual changes in pressure on a wing, a keel or a sail are small compared to ambient pressure, so presumably cavitation does not result ( your knowledge is way ahead of mine on that). The moving fluid therefore does not experience any significant change in density.
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vyv_cox
Well-Known Member
Thanks, that's useful.
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Peppermint
Well-Known Member
Re: Why not
Bernouli's Principle the basis for hydro and areodynamics proves that lift is possible.
How much lift you would get would depend on the area, section and profile of the board in relation to the inertia caused by the rest of the hulls lines and the rudder. It would also need a smooth stream of water without to much froth and bubble so that cavitation isn't sapping the benefit.
As generally you never get "owt for nowt" in physics I suspect that there's a trade off somewhere so that unless you went to extremes you wouldn't notice the difference.
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Bernouli's Principle the basis for hydro and areodynamics proves that lift is possible.
How much lift you would get would depend on the area, section and profile of the board in relation to the inertia caused by the rest of the hulls lines and the rudder. It would also need a smooth stream of water without to much froth and bubble so that cavitation isn't sapping the benefit.
As generally you never get "owt for nowt" in physics I suspect that there's a trade off somewhere so that unless you went to extremes you wouldn't notice the difference.
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Mirelle
N/A
It\'s been done.
Look carefully at the leeboards of any Dutch traditional sailing boat; they have camber like an aircraft wing, on the inboard (windward) side of the leeboard.
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Look carefully at the leeboards of any Dutch traditional sailing boat; they have camber like an aircraft wing, on the inboard (windward) side of the leeboard.
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