Sanding Foils - 100 grit?

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Sanding Foils - 100 grit?

French speed sailor reaches into the car, pulls out the 100 and finds serenity Shok (translations by google) His max foil speed 34+ knots.

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Oula, I don't know if it's worth a dedicated post Biggrin

When I sand the stuff it's always in the direction of flow. During a session I had a few setbacks, it must be said that the conditions were very difficult with streams of current, choppy waves and strong irregular wind.

Instead of adjusting my gear again (without knowing if it came from the settings), I took the sanding option. I only had 100 in my truck. I lightly sanded all the mat and my wings. After that, I did not have a single "drop" with support that seemed healthier to me.
Since that day, I have been paying close attention to the surface condition of my foil. In my opinion, it can be rather rough in finish but as long as it does
I have the impression that the faster we go, the more the foil is constrained and the more choppy the state of the water, the more important the quality of the surface is to keep the streaks of water as laminar as possible. What I am sure is that I did not feel less slippage with a coarse sanding. On the other hand when the chop taps on my cross mat, I gained serenity.

source: https://www.windsurfing33.com/forum/viewtopic.php?f=42&t=120482&p=860398#p860398

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Re: Sanding Foils - 100 grit?

Could be that his foil was covered with dirt, grease, slime, etc before he sanded it, and was better after he removed it. Had he gone to a higher grade he might have been better off. In "High Performance Sailing" Bethwaite is pretty adamant that best performance for foils (in his case meaning fins, rudders or centerboards) comes from a highly polished surface. (Work your way up to 1200 grit then polish.) He even did an experiment to prove it. He also says the hull should be highly polished.

In practice, I doubt any of this matters much to anyone who is not racing for a living.

What happens in a black hole stays in a black hole.

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Re: Sanding Foils - 100 grit?

I'm going to go with the assumption that the guy who's doing 37 mph on open water on a foil is probably taking decent care of his gear. Biggrin
He's the guy at the top of the Brest Speed list.
IMAGE(<a href="https://nsa40.casimages.com/img/2019/12/10/191210014656238793.jpg" rel="nofollow">https://nsa40.casimages.com/img/2019/12/10/191210014656238793.jpg</a>)

You are absolutely right - oil, etc on a foil will spoil the flow over it. Who knows? Maybe he just had a Croque-monsieur before going out. Biggrin

Some people are definitely in the "polish" camp. Bart Kramer later mentions in the same thread that a lot of pros do 1000 on the wing to retain some flow and 3000 on the mast and fuse to make it totally slippery. All done in the direction of the flow. There is some debate whether creating a turbulent boundary (akin to vortex generators or "tripping tapes") helps.

Enjoy: https://athene-forschung.unibw.de/doc/124194/124194.pdf Biggrin

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Re: Sanding Foils - 100 grit?

But we still don't know how fast he might have been had he used 400, 600, 800, 1200 and polish....Besides if he so careful about his gear why did he only bring 100 grit?

What happens in a black hole stays in a black hole.

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Re: Sanding Foils - 100 grit?
Quote:
... when the chop taps on my cross mat, I gained serenity.
Good Just like me!
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Re: Sanding Foils - 100 grit?
Randy wrote:

But we still don't know how fast he might have been had he used 400, 600, 800, 1200 and polish....Besides if he so careful about his gear why did he only bring 100 grit?

First, if you told me a week ago to sand my foils with 100 grit to improve performance, I'd have told you you were nuts. This is the experience of a semi-pro rider who in early December was at the top of his local standings with the current world record holder and PWA champion standing only third. It's entirely possible that he could have done better with 400 or 800. On the other hand, he hit 34 kts which precious few do. He certainly felt like it didn't hold him back. I'm sure he would tell you that there are a lot of data points between 100 grit and 1200 or even 2000 that should be explored. It's noteworthy, again, in that it didn't absolutely destroy his performance like most of us (including me) would have assumed. That's why I felt it was noteworthy.

I'm sure he already had a prepped foil. What he was talking about is the behaviour of the foil in the (very) rough conditions. The behaviour was more docile allowing him to push harder. Certainly laminar flow is the lowest drag in a tank but laminar flow is also much more susceptible to disruption. If it separates, it's much higher in drag than a turbulent flow. Banging around in the chop of Brest Harbor, it's possible that there's too much flow disruption.

I can only speculate why he only had 100 grit available. When it happens to me, it's because I've used up my supply of 400, 800 and 1200 on previous outings. Biggrin

It's an interesting experience that probably suggests there's more to it than we may be aware.

Here's the Brest crew speed sailing close to 40 knots last spring in open water. Julien Bernard is around 2:25.

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Re: Sanding Foils - 100 grit?

Could have been a Bob Beamon moment. Or a dolphin smoothing path in front of him. Or that perfect gust of wind......I admit, I am just being argumentative. It will be interesting to see if he continues to use his 100 grit. So do you plan to start sanding your foils with 100 grit?

What happens in a black hole stays in a black hole.

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Re: Sanding Foils - 100 grit?
Randy wrote:

Could have been a Bob Beamon moment. Or a dolphin smoothing path in front of him. Or that perfect gust of wind......I admit, I am just being argumentative. It will be interesting to see if he continues to use his 100 grit. So do you plan to start sanding your foils with 100 grit?

I've pondered 600-800. My Tillo which is polished down to 2000 seems more prone to ventilating than the AFS which looks 800ish. At my speeds, I don't think it would matter.Board Lady used to recommend, iirc, 600 for board bottoms and not to go glossy.

Maybe you just need some time on the water? Or did you just spend the entire weekend with 3000 grit paper on your board and foil; and ended up not having time to get out? Biggrin

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Re: Sanding Foils - 100 grit?

No sandpaper for me. One interesting side note - the guy I bought my second batch of foil gear from was obsessive about tefgel. Made a point telling me to use it all the time and that he used it all the time. When the gear arrived noticed it was all sticky on wings, tail etc. Spent a lot time washing tefgel off with Dawn. (I figured it washes oils spills off of birds....). Stuff seems to work fine now. Perhaps I should have tried 100 grit.....

What happens in a black hole stays in a black hole.

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Re: Sanding Foils - 100 grit?
Randy wrote:

...Perhaps I should have tried 100 grit.....

Yeah, that'll get the dishes clean... Lol

Back in Scouts, we used to use river gravel to scrub the bottoms of our pots. Maybe about 30 grit? Shok

btw, Tef-Gel is food safe. At least the guy sold you a foil you could eat off of. All it takes is one spring session at Old Fed for that to go away.

--- The Arrogant Jerk: Crabby and irritable since 1998.

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Re: Sanding Foils - 100 grit?
Randy wrote:

No sandpaper for me. One interesting side note - the guy I bought my second batch of foil gear from was obsessive about tefgel. Made a point telling me to use it all the time and that he used it all the time. When the gear arrived noticed it was all sticky on wings, tail etc. Spent a lot time washing tefgel off with Dawn. (I figured it washes oils spills off of birds....). Stuff seems to work fine now. Perhaps I should have tried 100 grit.....

Yeah, I probably have some tef-gel to wipe clean. Blush
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Re: Sanding Foils - 100 grit?

This was linked a couple of years ago on Moses's FB page: https://www.riftboards.com/sanding-your-hydrofoil/

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Re: Sanding Foils - 100 grit?

I hope you only have to do all that work if you own a carbon foil......

What happens in a black hole stays in a black hole.

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