I've just picked up a fin that feels to have an unusually sharp leading edge. Checked all the old favourites, they're all different, but they all do appear blunter. ( Only going by feel, is there a way to easily measure small radii of curvature?)
Bethwaite in his book High Performance Sailing recommends 0.4 % of chord, that's a radius of 0.4 mm for a 100 mm chord. Maybe it's not as critical a parameter as Bethwaite suggests?

Now thinking about the manufacture, whether moulded or machined, the leading edge is going to be the hardest bit to produce accurately. It's where the mould splits, or it's the meeting point as the job is flipped while machining. So I suspect the leading edge is generally cleaned up in the factory by hand with a piece of sandpaper? In which case if the guy in the factory got distracted we should easily be able to finish the job ourselves. Any suggestions from the fin experts on how to go about hand finishing the leading edge?

A lot depends on the particular foil, most foil specs define the leading edge radius. So if you can figure out what foil it is, it's just a matter of finding the specs for it.

But the foils I'm using have 0 to very small radii, and I've noticed more commercial fins are starting to use similar foils. Obviously you don't want a sharp edge, if it is, a couple of passes with fine wet and dry is probably all you need to do. Just be careful of creating a sharp transition, in other words run the wet and dry over the foil not just the leading edge.

Interesting question. A good windsurfing fin shouldn't have a radius leading edge. A continuous sine-based curve I believe is probably better. The rest of the foil is not a series of radii stuck together so why assume we should make the leading edge part of a circle? As to how to make that I submit to the experts in manufacturing.

Locky?

Thanks Decrepit. Very interesting, that first foil has the upper and lower surfaces meeting at a theoretical point, but the second is a smooth continuous curve all the way around the LE. So some foils may just be blunted a little purely for safety. Sounds like it's not supercritical and a little variation out of the factory is understandable. I'll wait until we get a windy day to try it out before doing too much.

With a bit of googling I've come up with the leading edge stagnation point.

You can never get any real numbers in these theoretical articles of course, but if the stagnation point is more than a couple of tenths of a mm in front of the leading edge, then the sanding details we're talking about won't matter. However if the stagnation point is only 1/00 ths of a mm or less ahead of the LE then I'd think how we finished sanding the leading edge would have an effect on how the flow separates. Does anybody know how far the typical stagnation point is in front of a fin?

I suspect it's far enough in front not to make a huge difference. In our shallow water abrasive weed sailing, the leading edge gets rapidly chewed up, it has to get quite bad before it's noticeable.

Adriano, foil specs quote a leading edge radius unless it comes to a point. So I guess the radius is a very small section of a circle, as in the E836 above, it actually looks like a continuous curve, but the very tip is a small part of a circle, 1.8% of the thickness isn't very much.

I think it's a way to describe how blunt the leading edge is, depending if you want low drag or low stall characteristics.

Yeah fair enough. I guess it's a case where although in theory a sine curve is better, it does not materially matter if we use a radius in practice so long as the radius is minute. I guarantee you though, a penguin's wing or dolphin's fin will not have a radius on it anywhere.

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decrepit said..
I suspect it's far enough in front not to make a huge difference. In our shallow water abrasive weed sailing, the leading edge gets rapidly chewed up, it has to get quite bad before it's noticeable.

Might be time to drag out the old photo of the first fin to crack 50 knots. This photo and associated story appears to have been purged from the internet, but good old sea breeze archives keeps it in my images section.

The original story, as I remember, was that Martin Van Meurs trashed his fin on a shelly gritty sandbank. He pulled off the daggy bits hanging by carbon threads and headed back down the run on his way home, serendipitously catching the killer gust and posting the first ever gps 50 peak.





Later on the story, not a good story for fin manufacturers, got fuzzy. He trashed the fin "after" the record run. And the fin got quickly sent off to Boogie for repair. ( You'd think the whole rig would have been sent unwashed to a glass case in a windsurfing museum, along side a bronze statue of MVM). Myth or legend? Who knows for sure? Maybe we ought to get the myth busters on to it.

Never believed that story and still don't.

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Adriano said..
Never believed that story and still don't.

Well I was wrong in saying the fin's photo had been purged from the internet. It was last time I googled, but google's getting better, it now finds more original pics from 2007, on sea breeze of course. www.seabreeze.com.au/forums/Windsurfing/Gps/The-50-knot-fin/

Or is that the "google bubble" that we've now suddenly become aware of?

I just wonder how it works for our fins.

Do we need foil shape ( thick one ) ? or rather as thin as possibly ?

Assuming that we could prevent bending/ flexing our hypothetical/ideal fin:

a) if near 0,l mm will be ideal
or we need
b) proper foil / drop like shape ( thick at first and thinner at the end ) ?

For our consideration we could assume that out 0.1 will not bend laterally.

Think about angle of attack macro and what happens to the water flow over the fin.

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Macroscien said..
I just wonder how it works for our fins.

Do we need foil shape ( thick one ) ? or rather as thin as possibly ?

Assuming that we could prevent bending/ flexing our hypothetical/ideal fin:

a) if near 0,l mm will be ideal
or we need
b) proper foil / drop like shape ( thick at first and thinner at the end ) ?

For our consideration we could assume that out 0.1 will not bend laterally.

Whaaa.......

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decrepit said..
Think about angle of attack macro and what happens to the water flow over the fin.

I known, but I do try to imagine infinite thin fin (sound already funny) that is shaped into curve of the upper surface or our foil.
Obviously such fin will work only on one tack.
Definitely we could say that :
a) our fin resistance is proportional to the overall thickness of that fin ( that thicker the fin - the more resistance)
b) at bearing off we don't need that much lift from fin anyway
I just try to imagine razor sharp blade like fin ( that will cut your feet in no time) but in same time provide the least resistance in the water.

Maybe we could wear steel , butcher like socks and pant to prevent injury, but paper thin fin could be the answer for 60+ knt results ???

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sailquik said..

Macroscien said..
I just wonder how it works for our fins.

Do we need foil shape ( thick one ) ? or rather as thin as possibly ?

Assuming that we could prevent bending/ flexing our hypothetical/ideal fin:

a) if near 0,l mm will be ideal
or we need
b) proper foil / drop like shape ( thick at first and thinner at the end ) ?

For our consideration we could assume that out 0.1 will not bend laterally.

Whaaa.......

Do you know already how much resistance at speed above 50 ktn come from:
a) fin
b) board

??
I am guessing that most resistance still will be over fin, not the board
Am i right ?

Hey thanks for that link. It was a great blast from the past to read the rubbish learned wisdom I was touting in those long gone days!

Hmm.... come to think of it, not much has changed in my assessment of those fins.... But the TM V7 was always way to powerful for my modest stature to control. The TM 48 worked great for me at Luderitz though. I am sure the TM50 would have been even better but the only one I could find was not quite as stiff as the 48 so the 48 got the nod. The only reason I don't use them here at Sandy Point now is that there is so much more floating weed in the water. The 30 degree + fins don't catch it.

And then there is the 'minor' consideration that if you hit the bottom somewhere, the raked fins a a little more forgiving!

Now, if you have a close look at the damage on the fin in the pics in that link to the original thread you will see that it the damage to the fins is not nearly as bad as it looks in the above picture. It just goes to show that the boundary layer can cover a multitude of sins in a fin.

The take home message is: (take note Sammy ) Use that youbeaut' fin and push it for all its worth. Don't hold back because you think it might touch the bottom and get a few scratches. Fins can be fixed or replaced, Those magic moments on that magic day may never come again in exactly the same way. Live in the moment to the max! Here endeth the lesson.





" The older I get, the better I was"

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sailquik said..
. The only reason I don't use them here at Sandy Point now is that there is so much more floating weed in the water.

there is another adventsage of razor blade like fin: will cut all weeds in instant

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Macroscien said..

sailquik said..
. The only reason I don't use them here at Sandy Point now is that there is so much more floating weed in the water.

there is another adventsage of razor blade like fin: will cut all weeds in instant

Hmmm, you might be onto something there Macro. Minimal rake and maximum edge!

Just don't kick you fin or run over anyone!