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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.
). 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, ) 
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.
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.
Though seriously the front part of the board where the flow is laminar might show some benefit,
3D printed fins! Wow, that is a breakthrough Fangy! 
What material are they printed with?
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%)
