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Old 8th November 2009, 10:07 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, 06:02 PM   #102
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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 06:59 PM.
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Old 29th January 2016, 05:45 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, 04:32 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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Old 19th February 2016, 08:55 PM   #105
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IF you kept all of your units consistent AND did the calculations correctly, THEN I don't see attempting to delay onset of turbulent boundary layer with polish finish making a measurable difference compared to smooth-sanded finish.

IF you did make a mistake in those calculations but later found the actual Re # offered reasonable possibility of avoiding turbulent boundary layer, OR substantially reducing the area over which the flow was turbulent, THEN you might move on to experiment with dragging a smooth-sanded (and properly weighted) hull behind your motor boat at speed with a scale mounted inline as part of the tow rope to measure the total drag, and then repeating the experiment with that same hull in a finely-polished condition.

In no case, would I expect wax to make a difference as 'that' is another version snake oil and will not overcome the 'zero-slip' boundary condition. However, there have been other techniques developed to manipulate the boundary layer including suction (pumps/perforations used to remove/reduce the boundary layer thickness) and pumping polymer liquid into the boundary layer but I know even less about these ;0

Last edited by Me Again; 19th February 2016 at 09:05 PM.
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Old 22nd February 2016, 03:08 PM   #106
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Welcome You Again, I guess you are not new in this post. With fins and the like profiles you may expect turbulence to be delayed significantly, but a windsurf hull is more or less flat in the rear bottom so the transition may occur at Re = 500000 - or after one inch - correct me if I'm wrong. Whether or not it is detrimental to the speed is another (endless) story.
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Old 24th February 2016, 10:31 AM   #107
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Yes, it's Me Again and once again - forgotten password,...

I believe there is a huge difference between the amount of drag created by laminar vs turbulent flow. the actual difference is research-able and calculate-able. same thing with estimating a dimension for 'how rough is rough'. in an experiment, calculating the expected results should come before actual testing.

I haven't done any of this type stuff for many years but it is fairly straightforward fluid dynamics. I think a person wouldn't need an engineering or physics degree to get through it, but may be more likely to mis-use a formula or physics principle without being practiced in this sort of thing.
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Old 24th February 2016, 10:40 AM   #108
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but yes, I think you are correct insofar as the total drag of the real world windsurfer could very well be so overwhelming compared to the board's skin friction/drag from laminar vs turbulent flow that it might not be a 'real' issue. I don't know if that would be the case but we are talking about pushing the envelope of performance so to speak and all avenues of optimization should at least be considered.

the experiment of dragging a hull behind a boat with inline scale attempts to provide a way to isolate the overall drag of the board from other issues such as sailor skill, rig, etc..

although this is a topic that interests me, I don't have free time to pursue it any further than reading a couple threads like this
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Old 25th February 2016, 10:09 PM   #109
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google '3m riblets'. there's a nice paper (pdf) with description of experiment with plate in water flow that is interesting
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