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Old 11th January 2009, 12:53 AM   #21
Erik Loots
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Quote:
Originally Posted by andretsin View Post
Ok, now i searched in google and found this link:
http://www.bluejacketboats.com/planing_boat_theory1.htm
here you can see how the centre of presure is in the front part. I didn't read the text.
An other question: Momentum exchange is for sure involved in the behavior of wings. Do you thing an airplane can fly just without throwing the air downwards?? Even easier: i hope you agree the wing of an airplane works like the pelle of the propeller of an helicopter. Does the helicopter trough air downwards? Even more, just look at the flow of the air in next picture:
http://www.desktopaero.com/appliedae...s/image372.jpg
And last think. I have heard lot of times that an aeordynamic profile can produce lift in zero angle attack. The problem is how do you define which is the chord of the profile. If you define it coincident to the flat bottom of the profile then yes, the aerodymic profiles seem to be the miraculous shapes wich can produce lift at zero angle. But if you define it as the line which goes from the point where the flow is divided in the leading edge until the vertex of the trailing edge then is not so miraculous. Because what we thought it was zero angle of attack with this new deffinition is 3 to 5 degrees of attack. And of course, in this case the air is divided in two in that point at the leading edge and meeting again in the trailing edge wich is in lower position, so the air is forced to travel from up to down, so there is momentum exchange.
Erik, i'm also student of Naval Engineering
Thank you for the good explanation!!! The pictures make it very clear to me.
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Old 11th January 2009, 03:41 AM   #22
Floyd
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The extent to which a wing utilises momentum exchange is totally irrelevant.(Obviuosly to some extent they do) But a planing surface is not a wing ;has no upper surface;consequently any references to Bernouli/Venturi/etc are totally flawed.
A board operates totally with momentum exchange.
The original post asked wether cut outs increase or decrease drag. Starboard are suggesting they reduce drag. I suspect they increase it but they offer advantages which out weigh this increase. ???

If cutouts decreased drag surely they would have appeared in other water sports years ago.Eg. Speed boats, ski`s.commercial planing craft etc.

There is lots of practical evidence to suggest centre of lift of planing surfce is geometric centre; but again this is pretty irrelevant to original question.
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Old 11th January 2009, 05:29 AM   #23
Waiting4wind
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All interesting stuff. I wonder how much of typical board design is science and how much is art. I would suspect most is the latter as the core hydrodynamic design principles are already in place. Would be interesting to hear from Starboard's perspective.
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Old 12th January 2009, 03:16 PM   #24
andretsin
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Floyd,
everything i wrote was to say that you reduce drag with cutouts because you reduce wetted area. Other boats don't use cutouts because they just don't have more wetted surface than needed. For example if there is too much wetted surface in the stern of a speedboat they just cut all the stern in straight line, without extrange cutouts, because they don't have the problem of having there any rear foot of the pilot an neigther the fin. I wrote this in my first post, please read it again.Same for any other boat.
Bernoulli, venturi and all this stuff are not formulas only for wings, but for any fuid in any condition (pipes, waves, rivers, wings, propellers, pumps, planing boats, blood in bodies, etc.)
The author of the link i put about planing didn't disagree in anymoment about the point that the centre of effort is concentrated in the first part of the surface. He didn't like some small details about the pressure distribution about the ones we still didn't start to talk. The idea of trying to guess where is the centre of gravity and buoyancy of my windsurfing board is too complicated to demonstrate anything, because everything is changing at every wave and there are also the sail forces wich are very difficult to evaluate. For the results to be valid you can't accept errors bigger than 10cm and I thing this is too few for a simple check out.

Last edited by andretsin; 12th January 2009 at 03:41 PM.
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Old 12th January 2009, 05:10 PM   #25
Floyd
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Andretsin
But thats not the case.
Board will ride on an ever decreasing wetted area is it increases speed.(Migrating toward rear) At any given speed to support load there needs to be adequate lift.(Just enough)The cutouts simply make sure that wetted area does not migrate too far back.(ie to help prevent control problems/tail lift/tail walking etc) For this cutouts do work. (And they make it possible to have a wide palning surface for high leverage for fin)
BUT I think you will find drag correlates quite highly with total edge length.(Not perpendicular length) So saw tooth/cut outs/wingers etc increase this total edge length and hence actually increase drag. If this were not the case fins would have saw tooth arrangement on trailing edge. (as would speed boats etc)

I`m not arguing against cutouts;just find starboards explanation (re-reducing drag) is rather against theory I have researched.

Totally agree that science does not give all answers and often things that work fly against current theory.This may well be case with cut outs.Its the line between art and science .
Many shapers make fantastic boards with little if any knowledge of hydrodynamics. But starboard are giving a scientific theory which IMO contradicts current "theories".
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