Hypothesis - for foils create a Hydrophilic finish (water loving) to enhance laminar flow by wet sanding (1500 or higher). For board bottoms use McLube Hullkote, a Hydrophobic finish (water hating) to increase release through turbulent flow.
McLube Hullkote www.mclubemarine.com/hullkote/
Haven't found a Hydrophilic product...
Some videos on the subject
Last time I repaired some scratches in my foil wing and sprayed with clear coat I used it before finish sanding and it felt really draggy. Using a hose I noticed the water was beading off (hydrophobic), then I sanded with wet and dry so water was no longer beading off, and it felt way less draggy when I next tested it. Which aligns with your foil hypothesis, albeit non scientific.
Polished is slower..... so so much testing on that.
Mclube are trying to sell you something..... 
Sanded is faster. However i doubt anyone can feel the difference between 800 and 1200 or 1500 on a foil
I went down this rabbit hole prepping race fins... hydrophilic is faster and gives better control.
Select to expand quoteMark _australia said..
Polished is slower..... so so much testing on that.
Mclube are trying to sell you something.....
Sanded is faster. However i doubt anyone can feel the difference between 800 and 1200 or 1500 on a foil
Boardlady used to recommend just 600 on the bottom - she used to be a naval architect (building racing yachts) before her board repair career. But echoing everyone else. Hydrophillic 100%. Even a bit of something greasy (hydrophobic) on a foil like a tef-gel fingerprint is noticeable (ventilation near surface so much worse). If water beads up, start sanding/cleaning. Anything much above 800 is for your OCD as Mark says. I think one of the major foil brands does just 800.
For fins and foils, having water attached not only reduces drag but also inhibits separation making spin out and ventilation much less likely.
As for sanding, she says circles. Some foil makers suggest parallel with water flow, others perpendicular.
boardlady.com/fast.htm
I've seen research on drag for ships, etc indicating that a surface roughness of the equivalent of 200 grit sandpaper the lowest drag. It may be at our higher speeds that smoother is better and I'm not going to be the first to sand my board with 200 but it goes to show that slick and shiny doesn't really work in water at our speeds. (I've also read an anecdote from a high level foiler that sanding scratches out of a wing with 200- it was the only thing he had with him - in the car park made his foil behave better and still allowed him to hit 35ish kts.)
Mike Z recommends wet sanding at 1000, 2000 at 45degrees and polishing with a 3000 trizact pad.
Mike has done loads of testing over the years...
Here is a better example on foil instead of car ;)
The right part is sended with 1200 paper.
There is less drag in hidrophobic surfaces as seen on the experiment of the video. The issue here is that is not convenient such a slipery surface on foils as the laminar flow can separate from the foil creating cavitation or ventilation. All the struggle is just to prevent that, not because is faster.
The same question for the board is the opposite, I see advantages in polishing the board or applying some sort of hidrophobic coating. A simple laquer spray does the job. Having a sticky board does not help in my opinion.
regards
Apart from the fact that he is talking about ventilation not cavitation it is spot on. A hydrophobic finish on a foil is prone to flow separation. If this happens the foil will ventilate, lose all lift and you will CRASH. I wont get into the debate on board bottoms but on foils it must be hydrophilic.
Select to expand quotemr love said..
Apart from the fact that he is talking about ventilation not cavitation it is spot on. A hydrophobic finish on a foil is prone to flow separation. If this happens the foil will ventilate, lose all lift and you will CRASH. I wont get into the debate on board bottoms but on foils it must be hydrophilic.
+1 Even a few oily/greasy/tefgelly fingerprints on a foil wing is enough to make it a crash monster. Learned that one the hard way.
Nice topic,
Is sanding only relevant for high aspect foils?
Or do foilers with low/mid aspect foils also benefit from scratching their parts?
If you want an easy way of testing, do what I did (once), apply rainex to all wetted areas, realy go to town on it all, that **** repells water like nobodies bussiness.
Great for windshields in the rain, windsurfers on the other hand were at the other end of the spectrum of greatness.
Lots of detergent to get it all off
Select to expand quoteCornholio said..
Nice topic,
Is sanding only relevant for high aspect foils?
Or do foilers with low/mid aspect foils also benefit from scratching their parts?
Because I'm built that way (just enough OCD to keep it interesting), I would. Big, thick wings are draggy enough. They don't need any more encouragement to go slowly. I don't think you need to do the "PWA" treatment but anywhere from 800-1000 is fine. I'd even venture 600 is fine for a big, slow wing.
I wonder if the negative effect of hydrophobic coating is from bubble collection. Air bubbles are hydrophobic, water is not, so any air bubbles in the water would have a tendency to stick to the surface. On the low pressure side, they'd also get larger, proportional to the pressure decrease. Even a very small bubble stuck to the surface would create turbulent low, which would increase drag, and also reduce the chance that the bubble is "washed off", as would be expected from laminar flow.
In the general forum, the issue of cavitation at high speeds came up, and two experts who know much more about this than I ever will said that "cavitation" at fins is due to the collection and expansion of bubbles and dissolved gas in the water, rather than from conversion of water from the liquid to vapor state due to (very) low pressure. Maybe similar things are at play with respect to coating. Some surface roughness from sanding, as opposed to polishing, could also reduce the adhesion of bubbles, by reducing the contact area.
I agree on sanding foils, but what about the board bottom? Is there any consensus about it? In my foil board it feels sticky when pumping out to fly so I?m making an experiment by spraying laquer on the bottom. I had only tried it once and I liked it better but I'm not sure at all..
Sanded bottoms do increase control, but as the priority in foil boards is just to fly soon I wonder if it is a good idea
Select to expand quoteveton said..
I agree on sanding foils, but what about the board bottom? Is there any consensus about it? In my foil board it feels sticky when pumping out to fly so I?m making an experiment by spraying laquer on the bottom. I had only tried it once and I liked it better but I'm not sure at all..
Sanded bottoms do increase control, but as the priority in foil boards is just to fly soon I wonder if it is a good idea
Sanded, imho, but the need to go super fine is much less. You want the board to slide well at low speed to get off the water quickly. Given the low speeds that the bottom is exposed to, that would suggest that 600-800 (probably even lower) is sufficient. The board length is way too long to worry about laminar flow, you simply want a relatively low turbulence boundary layer.
This is an interesting reading:
forums.sailinganarchy.com/threads/glossy-vs-wetsanded-underwater-surfaces.154037/
My point is that in foils the priority is to keep the flow attached at the price of an increased drag, so sanding has a point. Seems hat is not the case of the hull
Br
Select to expand quoteveton said..
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My point is that in foils the priority is to keep the flow attached at the price of an increased drag, so sanding has a point.
The only thing I saw mentioned in that thread that increase drag are turbulators - the opposite of a finely sanded surface. Did I miss something? We are all sanding to reduce drag and improve other performance characteristics including reduced chance of ventilation/spin out.
For 95% of us, getting off the water means accelerating the board to the point where the foil produces enough lift. Even wingers who started off with the concept that they would pump/bounce their boards off the water at low speeds (and, thus, had very short boards with angled tails) found that longer, faster hulls (DW) promoted better light air performance with HA wings. Also, having the board slide easily on touchdown is preferable, imho. You do have an interesting take as far as release. Certainly something to consider and find where the trade-offs are.
