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Old 8th November 2009, 11:07 PM   #101
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And to continue this unanswered question

WHICH WAY SHOULD YOU SAND- PERPENDICULAR TO THE BOARD OR PARALELL??
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Old 9th November 2009, 07:02 PM   #102
Farlo
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Some twenty years ago Olivier Augé, one of the top speed French windsurfer, advised to sand horizontally near the head and turn vertically near the tip. This was supposed to optimize water flow and probably coped with the knowledge and technology of that time (remember the speed record was around 30 Knts). Frankly I wonder if sanding direction makes any sensible difference when the grit is fine enough. Select and Deboichet now sand their fins horizontally from head to tip, so the current (logical?) option seems to be parallel to the board.

Last edited by Farlo; 12th November 2009 at 07:59 PM.
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Old 29th January 2016, 06:45 PM   #103
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2 different things going on as regards fins and bottom of board!

in all cases, even including polished surface, it a zero-slip boundary condition should be assumed. The velocity of the flow at the surface = 0. Various snake oil salesmen will lie and try to confuse unwary buyers but it is true. However, roughness is a different issue and does matter.

1st fins - fins have a low pressure side and strong possibility of separation (spinout) which is more or less disastrous outcome as regards going fast.

though laminar flow has much less friction that turbulent flow, a turbulent flow may have better chance of staying attached to the low-pressure surface as it goes 'around the bend'. apparently the extra energy in the flow helps out in this case. since spinout is disastrous, turbulent flow is the lesser of two evils.

various arrangements of roughness have been used in real life to 'kick off' the turbulent flow specifically for increasing the ability to avoid separation from the lower pressure side of foils.

the bottom of the board is a different story than fins because it is all pressure side (no low pressure side like on fins). so separation of flow is not the issue. here, we might hope for much less friction from laminar as opposed to turbulent flow. The Reynold's Number of the flow will usually tell give you an idea of whetherthe flow may be turbulent or not. Two important aspects of the Reynolds number (assuming sea water in all cases for example) is the speed, and the distance over which the flow occurs.

Assuming smooth surface, the initial part of the flow over the surface will be laminar but may change to turbulent over the distance, depending on speed. On a very long ship for example, even if the first meter or so was laminar, overall, you may as well calculate skin friction based on the rest of the ship's surface having turbulent flow.

It is not an exact Reynolds number where a flow must become turbulent but quite a range. A big factor in determining how high or low a Reynolds number yu can have without becoming turbulent is the surface smoothness.

The effect of smoothness is also relative, depending on the boundary layer's thickness. The faster the flow, the thinner the boundary layer becomes and the less avoidable turbulence becomes as roughness 'kicks off' the turbulent flow which then continues along the surface.

the right questions are :

what is the Reynolds Number for flow over the bottom of a windsurf board?

is there reasonable expectation of laminar flow over all or a significant portion of the bottom?

how rough is 'rough' for this particular flow?
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Old 2nd February 2016, 05:32 PM   #104
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With the condition Re < 500000.and a speed of 40 knots (20 m/s) a water flow would become turbulent after 2.5 cm (~one inch). This doesn't leave too much hope for a laminar flow. Or is it wrong somewhere?
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