Painting Foils

Even if your foils are designed to run laminar - and not all are, Laser for instance - they will only do so over a tiny percentage of the surface unless the finish is absolutely perfect, ie polished to a mirror finish. The rough surface induces turbulent flow very near the leading edge, so the lower drag of the laminar flow section is lost. Under turbulent flow, the surface is "scrubbed" by the fluid in the boundary layer, and this is where the higher drag of turbulent flow comes from. It goes without saying that the rougher the surface, the higher the scrubbing forces will be, and therefore the higher the drag.

If you think about it, if you slide off your centreboard, the water will too! Intuitively, it's clear that a highly polished surface gives less drag than a rough one. The real world situation is complex to explain, but we don't need to explain it. Trust your intuition! If you want the absolute maximum performance from your foils, keep them scratch and ding free, highly polished and perfectly clean.

This is all based on my hazy memory of my Physics degree from 20 years ago, but I also dug out Bethwaites book, and he did loads of practical experiments that seem to suggest that the performance gain is much greater than I thought it was, so maybe it's worth going to all the trouble after all.

And in case you're wondering, it was work all this out or watch "I'm a Has-Been".

Neil

Me and the missus had a long conversation about this the other night with my POV being that a rougher surface induces laminar flow and hers being that any laminar flow is killed by skin friction - actually it was more like hydrogen bonds or something but she a scientist and I'm an Contender sailor so I'd switched off long before I understood it.

The outcome was that a rough surface probably didn't help much and possibly absorbed more water anyway, so go with smooth. We breifly discussed polished surfaces by I ruled this out some months ago when I slid off a recently polished centreboard and threw a race with an inversion when I could have just about won it with a quick recovery .

Dennis is right, there are several disadvantages to a matt finish. Dirt sticks to matt finishes much more easily, so they need to be flatted regularly cos it's difficult to get them clean by washing. Skin friction is higher, although the difference is only really noticeable if you compare a matt foil with a perfect polished foil. I'd like to bet that no-one on this forum has any idea of the amount of time and elbow grease you need to put in to end up with a perfect foil. Let me give you an idea of the process

1. Sand with 240 dry to giva a key
2. Fill all dings/scratches with hard car body filler
3. Sand again with 360 - concentrate on the repairs first, use a block
4. Prime with 2-pack high-build primer - at least 2 coats sprayed, 15 minutes between coats
5. leave to harden 24 hours at room temperature - if you don't do this, the primer will sink into the sanding scratches after you put on the top coats
6. Guide coat - a very fine coat of matt black aerosol. Don't cover the foil, you're after a coat that is so thin you can still see the primer. Hold the can about 2ft from the surface, and move it quickly.
7. Wet flat with 400, using a block. Flat the whole foil evenly until ALL the black is gone. For the leading edge, fold the wet/dry over on itself several times to make a stiffish pad, and go easy!
8. Fill any pin-holes you can still see - now you see why you put on the guide-coat! - with primer using a small brush. Leave for 15 minutes.
9. Clean THOROUGHLY with a good quality spirit wipe, then tack-rag the surface to remove any last traces of sandimg dust
10. If you have sanded through the primer in any little spots you will need to spot-prime those
11. Guide-coat again!
12. Wet flat with 800, using a block as before.
13. Wash thoroughly with clean water - at least twice
14. Leave for at least 2 hours in a dry place. Airing cupboard is good!
15. Tack rag and spirit wipe again. Don't leave the spirit wipe to evaporate, take it off with a dry lint-free cloth
16. Spray 3 coats of 2-pack poly, 10 mins minimum between them. Cover the edges and tip first, leave for 10 mins, then cover the surfaces, then spray the 2nd and 3rd coat all in one
17. leave for 24 hours
18. Wet flat as before with 1200 until the entire foil is uniformly matt. This will take some time
19. Wet flat with 1500 or 2000 as before. You will feel the difference as you flat the foil. This is very important, as you won't be able to see the difference easily
20. When the whole foil is free of the 1200 scratches, polish with Farecla G3 until you get a mirror finish.

If you do it right, it should look like glass, literally! Now though, you have the problem of keeping it like that! Buy a new foil bag, cos the one you've got has sand ingrained in it, and make a cotton innner bag that you can wash regularly to keep the grit off! The gain in performance is small, but you might notice an improvement in pointing ability. Is it really worth all that time and effort, when you'd probably gain more from time on the water?

Have fun

Neil