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2nd February 2016 05:32 PM
Farlo 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?
29th January 2016 06:45 PM
Nose Itall 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?
9th November 2009 07:02 PM
Farlo 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.
8th November 2009 11:07 PM
Unregistered And to continue this unanswered question

WHICH WAY SHOULD YOU SAND- PERPENDICULAR TO THE BOARD OR PARALELL??
7th November 2009 08:18 PM
sailquik Xfoil can be used to calculate cavitation speeds for windsurfing foils. These calculations are as I understand it correlated with actual tank tests. The speeds calculated correlate very closely with those 'hit the wall' experiences some top speed sailors have had and which can be observed in their trackfiles. (eg. Chris Lockwood in the canal a couple of years ago).
There is no sudden loss of control when cavitation starts to affect a windsurfing fin. Spin-out does not occur. Ventilation is a different phenomenon. There might possibly be some vibration effects but if you can feel that when rattling over corrugated chop at 45+ knots you are indeed a very sensitive new age man. ;-)
Xfoil calculations have shown that AOA of windsurfing fins at 45-50 knots are indeed very small and in the range of 1 degree. Assy foils have even less AOA.
The amount of force required to propel a windsurfer at 50 knots is actually quite small. Most of that force we feel is sideways force. The resultant force is only a fraction of that. I have decided it should be fairly easy to experimentally find the order of that force simply by towing my speedboard behind a boat with a spring scale attached to the tow rope. A project for the next calm day.
So we don't usually have an excess of power to 'push through' any extra drag created by the onset of cavitation, however minor. Damage to the fin surface may not be apparent because the inception of cavitation is probably not severe enough to do that damage. Damage requires power and we don't have any to spare. It is all used up by the increase in drag caused with the incipient cavitation which makes the 'brick wall'.
Those of us that speed sail here at Sandy Point are quite familiar with the feeling that you have hit a 'wall' in some situations. It's not board drag though because on the next run after one of the many variables or conditions have changed, you suddenly find that 'wall' gone. Same board, same fin, different conditions. Maybe the wind had swung a bit more broad and increased so one can now unload the fin slightly. Some of us think we can tell the difference when water turbulance conditions change. There are many things we can probably never quantify but we can get a 'feel' for them with experience and objective clear thinking from careful observations. There are about a dozen guys here who probably get more time over 40 knots, often in quite marginal conditions (less than 30 knots of wind) than just about anywhere else in the world. Some patterns definitely emerge and many of them confirm the theory from the science.
Sanded v's polished: Working with extremely accurate and optimized speed fins we have tried both. The jury is still out which means the difference is most likely minimal, or at least very hard to pin down in an objective way due to constantly varying conditions. Most of the sailors here go to great lengths to carefully prepare their fins to be as true to design, fair and blemish free as possible. Even those that 'sand' their fins use very fine wet and dry paper (1000-1200 grit) I have had some PB's on mirror finish polished fins and have also done the same after grinding the tip through the sand on the bottom at high speed. (I try to avoid that though if possible! ;-)) Often, there are combinations of factors like the water state, wind angle and the big one: wind strength that over ride the dulled, less than theoretically optimized fin for a PB run. Who knows if we may have gone 2 knots faster on that run if we hadn't 'naturally wet and dry'd' the fin though??
6th November 2009 04:16 AM
Farlo Just seen nanostructured surfaces that are completely non wettable. I wonder what the effect can be on lift/drag. It must be different because air get trapped into the structure so there is virtually no boundary layer.
8th June 2009 05:28 AM
Erik Loots
Quote:
Originally Posted by Unregistered View Post

He also suggested thicker boundary layer "might" delay onset of cavitation/aeration but wasn`t sure !!!???
Intresting... it would confirm the feeling I have got about the finish of a foil...
8th June 2009 12:17 AM
Unregistered Talking to bloke reckoning to be an hydrodynamist. (I know him from uni days)
Anyhow his take on it was ;
Polished less drag but !!!
You aren`t comparing like with like once a fin is "rough".
Rougher builds up thicker boandary layer ; dependant on speed and viscosity of fluid. The thicker boundary layer effectively increases thickness of fin throughout its chordlength.This makes fin potentially less susceptable to changes in angle of attack and more tolerant of higher angles of attack.(If surface flow is maintained)In effect it behaves more like a thicker (more rounded leading edge)fin.

He also suggested thicker boundary layer "might" delay onset of cavitation/aeration but wasn`t sure !!!???

I`ll phone him back and ask about additives as in last post !! Plenty of additives in coast around us already though !!!

Perhaps we`re all right !!!!
7th June 2009 11:46 PM
Unregistered To whom it may concern

The laminar flow of polymer solutions is exploited by animals such as fish and dolphins, who exude viscous solutions from their skin to aid flow over their bodies while swimming. It has been used in yacht racing by owners who want to gain a speed advantage by pumping a polymer solution such as low molecular weight polyoxyethylene in water, over the wetted surface of the hull. It is however, a problem for mixing of polymers, because turbulence is needed to distribute fine filler (for example) through the material. Inventions such as the "cavity transfer mixer" have been developed to produce multiple folds into a moving melt so as to improve mixing efficiency. The device can be fitted onto extruders to aid mixing.

The example above leads to another possibility:

Microgrind the wetted area ( board or fin ) then spray it with a semi viscous solution such as Icex
7th June 2009 09:44 PM
Erik Loots I was on tectonics which I still really like, one day I tried a select fin from a testrider. I felt in difficult conditions the Select giving a more constant performance. Tectonics sometimes threw me of on higher speed when wind suddenly increased. Now I see big names in the Netherlands using the tectonics and they do crash more often (compared to me)

I asked in my local shop about those select fins, they offered me a nice deal for select fins.

The best thing about select they can make anything. With tectonics you have basicly only one very good speedfin. I tried a few days ago the new Select S09 fin, what a performance!! definitly a winning slalomfin, very good steering!. You don't hear much but even some worldcuppers use &got some select fins.

Yes some select fins make a noise, but if you look to all other aspects it is a very fast foil. about 50% of my foils I finish myself, its not needed in most cases. It gives a good feeling, when the fin is totally checked

Cheers,

Erik
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