Borrowed the picture again (from Col555 , thank you 
) to ask one more question:
How important is exact, flush mounting of your fin relative to board surface ?
With all this attention to the shape of the board, size and shape of the fin this small imperfections - like mounting protruding or should have significant effect on performance (?)
if that few milliliters protruding from the box make any difference at all ?
or sinking ?
which one is worse ...
Sometimes when filing new fin to fit I over-do it and fin will sink too deep.
How to fix it ?
I thought maybe to use old trick from snow skis ? pour parafine from candle and scrap access to perfect level ?
Too high, more sanding.
Too low, add epoxy with catalyst to build it back up.
Low is usually better, as you don't spin out if it's a little low.
High up to 1 mm is fine also, but of course, flush is best.
Select to expand quoteMacroscien said..
if that few milliliters protruding from the box make any difference at all ?
The boundary layer has built up by the fin box. Using this fellow's formula and a Re of about 5 X 10^6 it looks like it's about 1 mm thick back there. Flat is best, but if you go for less than 1 mm it should at least be sort of buried in the boundary layer.
Select to expand quoteIan K said..
Macroscien said..
if that few milliliters protruding from the box make any difference at all ?
The boundary layer has built up by the fin box. Using this fellow's formula and a Re of about 5 X 10^6 it looks like it's about 1 mm thick back there. Flat is best, but if you go for less than 1 mm it should at least be sort of buried in the boundary layer.
But that's a formula for a laminar boundary layer, it might be turbulent? Still guessing a bit. But if we knew a few mm made no difference we could save a lot of faffing about. Someone google "Transition to turbulence on a flat plate" and tell us what you find.
To test your claim I just performed simple experiment.
By loosening bolts I could now swivel fin up and down few milometers each way.
By pouring water from hose at high pressure I could now observe - see and hear how the stream react with protruding fin or sunken.
Maybe my jet stream on not exactly laminar but effect on any protrusion is spectacular .
Any, any protrusion even less then 1 mm will create loud audible sound and wide splash,
On another hand, a bit sunken fin doesn't seems to have any change at all.
I suggest anybody to perform similar experiment to see how dramatic small imperfection in fin placement may have.
How it translate to actual performance hard to say. If it cuts a knot or two from your 40 ktn effort ?
PS> I am guessing that using water blaster stream could exaggerate visual and sound effect, but still good illustrate/simulate the problem.
). This may explain why the edges on cut-outs don't seem to create much drag, and the fin head protrusions don't seem to make as much difference as say a piece of weed further down the fin. 
That makes sense, if it's turbulent the boundary layer grows more quickly, using your figures I got 1.1 cm. Did you divide by 0.75 or multiply? But either way it means the little bit of fin base that can poke out is well within the boundary layer, so instead of it smashing into the 40 knot free stream velocity, it's immersed in the water that's getting dragged along with the board, so the collision with that front edge might be more like 10 knots.
So if the first cm or so of a fin is in slower water is it optimum to use the same foil profile in that top cm? Maybe not?
( Of course If it was important and allowed for manufacturers wouldn't be able to cut 'em off to suit, producing several sizes of fins from the one starting point, ) 
Select to expand quoteIan K said..
That makes sense, if it's turbulent the boundary layer grows more quickly, using your figures I got 1.1 cm. Did you divide by 0.75 or multiply? But either way it means the little bit of fin base that can poke out is well within the boundary layer, so instead of it smashing into the 40 knot free stream velocity, it's immersed in the water that's getting dragged along with the board, so the collision with that front edge might be more like 10 knots.
So if the first cm or so of a fin is in slower water is it optimum to use the same foil profile in that top cm? Maybe not?
( Of course If it was important and allowed for manufacturers wouldn't be able to cut 'em off to suit, producing several sizes of fins from the one starting point, )
OK. Who is going to volunteer to make some moulded root fairings on their fins like on the aero wing in the pic to see if they can tell the difference?
It should be possible even with plasticine, but more permanent with some sort of epoxy filler putty. 
Now I came to think about it, I do remember from years ago some plastic moulded surfing fins made for 'A' (US) boxes with beautifully faired root moulding.
I would be pretty certain this has been modelled and tested in the aeronautical industry, but surely this has also been modelled by marine engineers?
Fangy is getting an ali deltoid fin made with root fairings, I think this was mainly to locate the front overhang accurately, instead of relying on the base taper, the fin sits on the fairing. Saves all that fitting, to stop weed doing a groucho marx at the front of the fin. But he did some research to find out it's affect on drag, and was surprised when it came up favorable, (well I was surprised, I think Fangy was) From memory it was a surfing site. But maybe Fangy will enlighten us.
Select to expand quotedecrepit said..
Fangy is getting an ali deltoid fin made with root fairings, I think this was mainly to locate the front overhang accurately, instead of relying on the base taper, the fin sits on the fairing. Saves all that fitting, to stop weed doing a groucho marx at the front of the fin. But he did some research to find out it's affect on drag, and was surprised when it came up favorable, (well I was surprised, I think Fangy was) From memory it was a surfing site. But maybe Fangy will enlighten us.
Prepare for enlightenment grasshopper. Ok the correct term is to 'fillet' the join. There is research out there on the effect of the fillet on lift/drag in hydrodynamic scenario. In a nutshell: the fillet adds surface area and therefore increases the drag slightly. The fillet resolves some of that drag into vertical lift in the case of a fin ( as opposed to the horizontal lift produced by the foil). This vertical lift is relatively efficiently produced when compared to tip twist. The vertical lift may be useful when using short or very stiff fins that do not flex at the tip. In real terms, a short 55degree fin rarely produces too much vertical lift, rather they tend to make the tail of the board feel pretty dead until you are really cranking, so a fillet maybe beneficial on a short steep rake fin. The surfboard industry noted that glassed on fins made the tail more lively when compared to boxed fins. Conversely, the fillet is probably of little benefit in longer fins with tip flex. The radius of the fillet is critical in getting maximum lift with the minimal amount of drag penalty. The windsurfing fin with fillet is a difficult beast to manufacture in a commercial sense. That in turn adds manufacturing cost. It is likely in vast majority of fin scenarios, the added cost and complexity is not justified. As Decrepit alluded to I am in the process of trying to get it to work, but in the meantime here is a preview of my 3D printed prototype with fillet visible.
Hey fangy, are you sure your vertical lift isn't negative? And the main thing about vertical lift at the fin, is how it affects board trim. Too much and the board rides too flat.
Select to expand quoteIan K said..
Did you divide by 0.75 or multiply?
My calcs for the 20ms-1 i.e. (~40kts solution):
thickness = .385/RE^.2/length x 100 (x100 --> back to cm)
thickness = .385/14940239^.2/.75x100 = 1.9cm
I had an attempt to calculate the speed INSIDE the turbulent boundary layer, at about 10%, say 2mm for fin protrusion but my brain had a meltdown. Turns out that calc is quite complicated but it's a very important calculation in CFD, google Y plus or dimensionless wall distance. Then I realised that knowing the velocity at a point INSIDE the turbulent boundary layer, was not going to tell me much about the additional drag it created. That's because you can't create another boundary layer inside the boundary layer, however the protrusions or 'roughness' do affect the shape and energy in the boundary layer.
Anecdotally though I don't believe these protrusions as long as they are well inside the boundary layer create significant drag. e.g. rivets on aircraft wings downstream of the spar, vortex generators, golf balls, and fast windsurfers with poorly fitted fins...
Select to expand quotedecrepit said..
So why are we polishing the bottom of our boards?
Makes them shiny 
Though seriously the front part of the board where the flow is laminar might show some benefit,
EDIT: I did some more calculations and the length of the board where the flow might remain laminar is tiny. So I will revert to my original suggestion that the main reason to polish them is to keep them shiny... More research for a windless day.
Select to expand quotedecrepit said..
So why are we polishing the bottom of our boards?
Ahh! That is the $64,000 question. I just knew we would get back to that one as soon as I saw the words 'boundary layer'!
Hang on guys, I need to stock up on popcorn........
3D printed fins! Wow, that is a breakthrough Fangy! 
What material are they printed with?
Select to expand quotedecrepit said..
Hey fangy, are you sure your vertical lift isn't negative? And the main thing about vertical lift at the fin, is how it affects board trim. Too much and the board rides too flat.
An except from the conclusions in the paper by Lavery et.al.
".... the presence of fillets can decrease the overall drag of the fin, even though the viscous drag is slightly increased due to the added surface area, the smoothing of the sharp edges between the fin and the board causes pressure distributions to be spread over a larger area. This ...drag decrease is small (10-12%) and even smaller (6- 7%) when compared to the total combined drag of the board and fin system... However, the presence of fillets was accompanied by an increase in a vertical lift force..."
As for board trim, at my weight, and the skinny tails I use, the board riding too flat is not a problem I have encountered yet
Select to expand quotesailquik said..
3D printed fins! Wow, that is a breakthrough Fangy!
What material are they printed with?
Apologies for the mini hijack Macroscien.
This one is plain old ABS and is as weak as weetbix. Its purpose is to serve as a prototype pattern for casting. I have my first cast Ally fin back from the foundry today. I need to do some serious polishing and then try it out. If that's a winner, I can move to production and hopefully get the cost down enough to allow the use of titanium.
Select to expand quotesailquik said..
3D printed fins! Wow, that is a breakthrough Fangy!
What material are they printed with?
I have just under 30kg of Carbon Fibre granules for injection moulding.
Maybe this can be made into the material they use for printing.
If anyone has an idea how to turn it into fins I would like to hear it or pass it on (at a cost as it is very exy)
Maybe injection moulding would be ok with this material instead of the old plastic like K4 use.
Select to expand quotefangman said..sailquik said..
3D printed fins! Wow, that is a breakthrough Fangy!
What material are they printed with?
Apologies for the mini hijack Macroscien.
This one is plain old ABS and is as weak as weetbix. Its purpose is to serve as a prototype pattern for casting. I have my first cast Ally fin back from the foundry today. I need to do some serious polishing and then try it out. If that's a winner, I can move to production and hopefully get the cost down enough to allow the use of titanium.
Fangy,
If aluminium works it can be injection moulded or injection cast I believe which gives a nice finish as opposed to casting.
Maybe look at different alloys to keep the weight down, magnesium alloy etc.
You may be looking for wear resistance & in that case ignore me
Select to expand quotefangman said..d
An except from the conclusions in the paper by Lavery et.al.
".... the presence of fillets can decrease the overall drag of the fin, even though the viscous drag is slightly increased due to the added surface area, the smoothing of the sharp edges between the fin and the board causes pressure distributions to be spread over a larger area. This ...drag decrease is small (10-12%) and even smaller (6- 7%) when compared to the total combined drag of the board and fin system... However, the presence of fillets was accompanied by an increase in a vertical lift force..."
As for board trim, at my weight, and the skinny tails I use, the board riding too flat is not a problem I have encountered yet
10 to 12 % doesn't sound like something to ignore! Is that paper on the web?
Fangy you may wish to try the fin BEFORE you polish it. Have seen some papers where bumpy surfaces reduced drag surprising the people doing the experiments. May act a bit like turbuncles do on the leading edge but further back along the chord, theories vary but some think it energises boundary layer and helps it stay attached and reduces separation bubble.
OK Ian :the link to the journal is here:
thereefjournal.com/files/7._Lavery_Foster_and_Carswell.pdf
I have been in touch with the author to ask his thoughts at higher Reynold's numbers, as extrapolating from the graphs in the above paper, to higher speeds than they computed, is not scientifically sound practice. As yet I have not heard back :-(
Aus808, you sort of guessed it. The whole reason for this little folly is to find a fin material that will stand up to the weed and not wear away and roughen the leading edge every time you have a groove in the smooth session. Decrepit made an awesome stainless steel leading edge fin, but the man hours required were immense. I am too lazy to do that! But I will certainly look at the casting methods - thank you for the info. Ultimately I want to get the fin to at least neutral buoyancy, so long term I am eyeing off a hollow titanium fin, if I can get the cost per unit down.
Yoyo any chance you could get me an author or paper title of that research? the only good stuff I could find related to flow over flat surfaces, not foils. At present I am sitting in the conventional foil theory camp; that is, the super smooth leading edge,with chord thickness a loooong way back to maintain laminar flow for as long as possible camp and I am always keen to challenge a theory!
SCREW science.
Just windsurf, learn from EXPRIERENCE, and you will be right every time.
Read the second post. That is EXPERIENCE, not science.
Was related to wings for Mars gliders. Using inflatable tubes (root to tip) . As you can imagine not very aerodynamic in the classical sense from leading edge to trailing edge but surprisingly worked quite well. The bumps acting as a trip. But this may only work at low Reynolds numbers. Your fin would probably work at Reynolds 2-3 million at the base at top speed and about 500,000- 700,000 at the tip
Read a NACA article last night that said there was less drag WITHOUT fillets. Same reason as above. Sharp contour caused trip => less drag (~ 1%)
Select to expand quoteyoyo said..
Was related to wings for Mars gliders. Using inflatable tubes (root to tip) . As you can imagine not very aerodynamic in the classical sense from leading edge to trailing edge but surprisingly worked quite well. The bumps acting as a trip. But this may only work at low Reynolds numbers. Your fin would probably work at Reynolds 2-3 million at the base at top speed and about 500,000- 700,000 at the tip
I have been wanting to have a sail that rigs as quick as this ..
Pretty cool demo, buzz light year would be proud. Those reynolds numbers are very low, probably appropriate for Mars atmosphere, and more likely to be found on a model glider or a golf ball... But demonstrates that sometimes creating a turbulent boundary layer earlier and with more energy is advantageous.
Select to expand quoteAUS 808 said..sailquik said..
3D printed fins! Wow, that is a breakthrough Fangy!
What material are they printed with?
I have just under 30kg of Carbon Fibre granules for injection moulding.
Maybe this can be made into the material they use for printing.
If anyone has an idea how to turn it into fins I would like to hear it or pass it on (at a cost as it is very exy)
Maybe injection moulding would be ok with this material instead of the old plastic like K4 use.
It's definitely new plastic we use. We also use the carbon filled plastic for some of the applications we mould for, but I've no desire to mould fins from it. Just read else where about aluminium fins. We made a billet Dural fin on the CNC way back, I can't remember the year but the fact it was in an early Electric Rock would date it, balanced against a 7m Warp speed with the corners cut off so it would fit my mast/boom and was just a cheap way to get planing on the lake when I couldn't afford bigger kit. Came about after my dad chatted to a customer that flew gliders.. can't remember the length but it was a lot longer than what you could buy at the time. 
