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decrepit said..
My fins have a constant thickness to chord ratio, usually 9%, from base to tip, but thinking about it. -----
Plane wings normally reduce the ratio at the tip, because they want the stall to start at the tip. That way it can be controlled, possibly similar with a fin. If the tip stalls, you loose a bit of grip that can be felt, and stopped by pulling the back foot in. If it stalls at the base, then you'll have ventilation running up the whole fin and instant spin out.

That makes sense, does every body agree???

May have to do some subtle fin mods.

No, or at least maybe

Rake or sweep changes the relationship. A raked fin will stall less abruptly and at greater angle of attack than an equivalent upright fin. The span wise loading changes as you rake or sweep a foil (or sail for that matter) similar things happen if you change the aspect ratio. IMHO it so damn complex that the only way is to experiment and see what works best. i.e. you may improve one aspect but you make something worse at the same time.

Don't overthink it

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decrepit said..
My fins have a constant thickness to chord ratio, usually 9%, from base to tip, but thinking about it. -----
Plane wings normally reduce the ratio at the tip, because they want the stall to start at the tip. That way it can be controlled, possibly similar with a fin. If the tip stalls, you loose a bit of grip that can be felt, and stopped by pulling the back foot in. If it stalls at the base, then you'll have ventilation running up the whole fin and instant spin out.

That makes sense, does every body agree???

May have to do some subtle fin mods.

I have a beautifully made carbon Curtis 48 cm fin . So sexy it hurts .It cost me quite a bit , second hand , from USA .It's a very upright pointer with a squared off end . The end has not been chopped . The chord to thickness ratio is constant all the way to the tip . It is a very stiff fin and has a blunt looking leading edge . I want to love it but unfortunately I don't . When it lets go , it let's go . No warning . Because of this , I can't confidently push and get into it . When it spins out you have to go downwind almost 90 deg or virtually stop . I used to think because it's so stiff , but now , mabee because of no thinning at the tip ???
Its now a bloody expensive tool for setting finboxes .

Around NorCal, those dark gray Curtis fins have a bad reputation, qspinout and basecracks.
On 75-90 cm boards, Debotchet and SystemB seem to never spin out and durable beyond one season.
G-10 Curtis hold well, but slow due to thicker foils.

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LeeD said..
Around NorCal, those dark gray Curtis fins have a bad reputation, qspinout and basecracks.
On 75-90 cm boards, Debotchet and SystemB seem to never spin out and durable beyond one season.
G-10 Curtis hold well, but slow due to thicker foils.

It has a high gloss woven carbon finish . Incredibly light , so presuming a foam core . Sexiest fin I've ever seen .
Interestingly this fin retains a 12.5% chord to thickness ratio all the way to the top . 110mm wide at base . So it should be super grippy , but its not .
I don't know how fast it is because I'm too scared to push it .
So there goes the theory of fat being grippy .

Just to be safe , on fin build Mk2 , I might start at 9.5% . Check its grippyness . If its got heaps , its easy to make it a bit thinner .

my guess is the leading edge, the radius, doesn't look to blend into the foil very well. To keep the water flow attached, you need no sudden deviation to the flow.
So a fat fin with a good leading edge profile, can bend the water round the foil gradually, whereas a thinner foil has to have a tighter curve. But it's all about that smooth transition, a thick foil with a sharp transition, will have no advantage. So here's a chance to experiment, see if you can fix the curtis.
Also wet and drying the gloss away may help, a small boundary layer can help keep the flow attached.
First thing to try.

+1 ^. I can't remember the academic so as to be able to quote him properly, but he said because of the very high Reynolds numbers fins are subject to, the flow around them should be considered to be pipelines rather than streamlines. The inertia forces are so dominant that any attempt to change the flow direction should be as gentle, gradual and smooth as possible. Sharp leading edges will cause a separation bubble on the leading edge as soon as the angle of attack exceeds the ability of the 'pipeline' to stay attached, which gets worse the faster you go. Turbulent water (unsteady flows) will have the same effect. The separation bubble will join with the trailing edge low pressure and sheet cavitation will occur - instant spin out, with no warning.
So I agree with Decrep and Racer, keep it like me : simple and rounded.

Ps , the edge you guys are looking at in the last pic is the trailing edge.
The front edge , ( right of pic ) , looks beautifully blended .
Its hard to take a good pic , but the profile looks good and the leading edge is about 2.5mm diameter .
It doesn't make sense why it lets go like it does , I'm leaning toward the fact that it doesn't thin out at the tip .???
For a long fin it has no flex . Its as solid as a rock.

I have one of those fins too, lying around somewhere.
I think they are simply too stiff, they simply feel a bit dead in the water with no feedback. A softer fin will not only bend but also twist. This changes the angle of attack along the fins, and there will be a warning before the whole fin spins out.

I agree on the leading edge discussion, a blunt edge will spin-out later compared to a sharp edge. But my Deboichet and Z-fins are possibly sharper (less drag) and do not have the spin-out issue.

Definitely try wet rubbing the gloss and printing off it. I have a brand new fin(different brand) that was glossy and with a print on it, I was about to get rid of it because it would just let go with no warning and no obvious reason. This has fixed its problems.

Eta, I notice my more expensive fins (tribal and a Z fin) have no gloss and no printing

Maybe the trailing edge it too thick as well. That's going to cause a big low pressure area, just what you need to suck air up from the tail.

Home made fin Mk2
Same design as above fin , but a size smaller .
27cm high , 15 mm thick , 9% chord thickness.

Chunk of 16mm thick G11

Your fairing / radiusing on that is beautiful is not easy to get inside curves correct

Can't wait to see it finished this time...
Q: Not having ever made a fin that isn't heavily raked, how do work out where to position the fin over the box base?

Just a light sand with 600 grit to get rid of the shine .
As to how I positioned the fin to base . About two thirds of fin area behind the centre line . ,Beer in hand , I stick my thumb out in front of me , close one eye , tounge half stuck out . About there is good
It feels in the right spot on the first one ,mast stays in the same position another fluke . Fluke is my friend

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Imax1 said..
Just a light sand with 600 grit to get rid of the shine .
As to how I positioned the fin to base . About two thirds of fin area behind the centre line . ,Beer in hand , I stick my thumb out in front of me , close one eye , tounge half stuck out . About there is good
It feels in the right spot on the first one ,mast stays in the same position another fluke . Fluke is my friend

Haha! +1 on that

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Imax1 said..
Just a light sand with 600 grit to get rid of the shine .
As to how I positioned the fin to base . About two thirds of fin area behind the centre line . ,Beer in hand , I stick my thumb out in front of me , close one eye , tounge half stuck out . About there is good
It feels in the right spot on the first one ,mast stays in the same position another fluke . Fluke is my friend

What brand beer? Sometimes the devil's in the details.

Love your work, too.

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Paducah said..

Imax1 said..
Just a light sand with 600 grit to get rid of the shine .
As to how I positioned the fin to base . About two thirds of fin area behind the centre line . ,Beer in hand , I stick my thumb out in front of me , close one eye , tounge half stuck out . About there is good
It feels in the right spot on the first one ,mast stays in the same position another fluke . Fluke is my friend

What brand beer? Sometimes the devil's in the details.

Love your work, too.

Boags works good , XXXX does NOT . For super delicate work I suggest Balter xpa .

Completely finished ....... fangman . I know , its ugly
Area 330cm squared.

I'm sure that will work really well. My pre fangy normal weedies are about the same aspect ratio without the rear cutout and flange, but work very well. I'm of the opinion that the flange is less important with more upright fins. But I haven't got around to testing that theory on my 45s

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decrepit said..
.... I'm of the opinion that the flange is less important with more upright fins. But I haven't got around to testing that theory on my 45s

Stop mucking about and do some testing Decrep! When one of the sailors here put a fillet on his pointer and it changed the fin from nasty to a favourite, I decided it was time to hit the books again. One of the things I learnt was the momentum of the surface layer is critical in keeping the fin surface sealed from air ingress. The greater the momentum, the less likely ventilation will occur. (This does not apply to cavitation) 'Squeezing' the surface layer with a fillet may be helping guard against ventilation. Also, given momentum is Mass x Velocity it would seem to imply the faster you sail the more protection against ventilation.( but once again, not cavitation).
Oh and Imax, I think your beer goggles are buggered. The fin looks sweet to me.

OK, so where do we want maximum "squeeze" at leading edge or at point of lowest pressure?
If the later is the case, the fillet foil should probably be offset towards the back of the fin, in relation to the blade foil?

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decrepit said..
OK, so where do we want maximum "squeeze" at leading edge or at point of lowest pressure?

I am not entirely sure on that point. A few points to muddy things up a bit: the performance of full fences versus leading edge fences doesn't seem to be consistent across different researchers. This may be because I have not read enough, some of the papers have outdated methodology/analysis and the extrapolation from models to full size is not valid.
There appears to be consensus,( but not total agreement) that areas of marked low momentum => low vapour pressure must exist on the foil surface before ventilation will occur. The sources of low momentum are the tip vortex, the leading edge separation and the trailing edge separation. A strong tip vortex can persist and break the surface many metres behind the fin tip allowing air to travel up the centre of the vortex ( an area of low momentum). It would seem that measures taken to avoid cavitating will therefore inhibit ventilation.
Of some concern was that one researcher was able to trip the foil into ventilation by allowing water droplets to hit the surface of the water ahead of the foil. The other dragged the tip of a ball point pen in the flow upstream and this had the same effect. Not all researchers were able to verify that disturbance in the surface seal appears to be this critical.
All in all, if you throw in the fence effect of the board and turbulence from chop etc I am not confident about applying their findings to windsurfing without a lot more reading and some further qualifications.

Yeah , I just put the fillet on this fin because it worked on the fin before , which I copied off a fin that worked .
I'll leave all the tech stuff to you nerds .
My little brain tells me , when it comes to flow , curves are better than angles .
I suppose it's relative to the fillet size . Simply pushing straight through water , the bigger the fillet , the thicker the object is , creating more drag . Having a sharp angle would also have to create drag . A very small fillet would have to reduce drag . If a larger fillet increases grip , a smaller fin can be used . Is that why a smaller Fangy style fin is way grippier than a large Delta ?
I know a 24 cm Fangy at two thirds the area , has more grip than a 30 cm Delta , probably more to do with thickness . Still , I'm a fillet fan .