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 Post subject: Polish hydrofoil wings?
PostPosted: Sat Jan 03, 2015 8:02 am 
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I just got my MHL lift with race wings and after 5 sessions or so the tips of the front wing has gotten a bit scraped by the normal use, can I polish the tips in order to get the finish in the carbon it had before?
I ride in onshore winds with a sandy bottom.

Cheers


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Sat Jan 03, 2015 9:21 am 
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Location: Denmark
Yes you can :D

Just use extremely fine wet sand paper first, and it is better than new.

It will be matte now, but if you would like it shiny and glossy, use marine polish or similar - works great :thumb:

8) Peter

Ps: l thought you were asking about hydrofoils made in Poland, when I saw the headline :rollgrin:


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Sun Jan 04, 2015 9:39 am 
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Peter_Frank wrote:
Ps: l thought you were asking about hydrofoils made in Poland, when I saw the headline :rollgrin:

Me too :lol:
However we should see "made in Poland" foils this year :D


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Sun Jan 04, 2015 5:15 pm 
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haha sorry about that, didn't realize it when posting...

Hope to see some Polish foils being made though!


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Sun Jan 04, 2015 7:13 pm 
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Maybe a blue blooded Nobile KBHF for 2015?
R H


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Tue Jan 06, 2015 5:40 am 
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theres an interesting chapter in "High Performance Sailing" by Frank Bethwaite who was a friggin genius with weather and fluid dynamics. From their testing he found that there was a 30-some % drag reduction on foils when polished vs. just sanded. There was not much difference when compared over a large surface like a boat bottom because complete laminar flow is impossible over such a span, but over foils it was dramatic


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Tue Jan 06, 2015 8:32 am 
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ThistleSailorKC wrote:
theres an interesting chapter in "High Performance Sailing" by Frank Bethwaite who was a friggin genius with weather and fluid dynamics. From their testing he found that there was a 30-some % drag reduction on foils when polished vs. just sanded. There was not much difference when compared over a large surface like a boat bottom because complete laminar flow is impossible over such a span, but over foils it was dramatic


Interesting indeed, although I dont think, in fact I am sure this is not correct...

30% something is definitely not.

Many believe it is the opposite way, that sanded are faster than polished (you see a lot of pro racers going all matte).

But IMO the difference is so small that hardly or not noticeable, since there is no general consensus about what the better choice is.

To make things even more difficult - then you might have laminar flow at very low AOA yes, but when going upwind or downwind at higher AOA, it will be turbulent again, and in fact you might get better and more constant lift with a sanded wing here, thus better performance.

Not so simple, and might be the reason we dont (yet) see any general consensus.

It has in fact been discussed for the last 30 years or longer, also on windsurf hulls and fins and similar - still without having any trustworthy conclusions (to my knowledge).

You might find info on the internet saying either one or the other quite "convincingly" - but be careful, as often it is nothing but claims or erroneously tests, or because the laminar/turbulent paradox fools many :o

At least when some claims a HUGE difference, it is wrong :naughty:

8) Peter


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Tue Jan 06, 2015 9:15 am 
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Location: France
Info from "THE BOARD LADY" who has fixed some 6,000 boards.
http://www.boardlady.com/fast.htm

"Water is pretty sticky: dip a paddle into the ocean and it comes up with much water attached. Move a board through the ocean – be it surf, windsurf, stand-up, or kitesurf -, and it will drag a substantial amount of water along, which, as it eventually cannot hold on to the bottom any more, emerges behind in the form of a wake. The energy that you generated with your sail, kite or paddle, went into accelerating not just you and your board, but this rather substantial mass of water as well.

To go faster, then, we need to decrease the amount of water “sticking” to your board – “reducing the drag” in techno-lingo, since the best wake is no wake!

In extensive tank testing for world-class racing boats, it was established 40-some years ago that a glossy surface has substantially more drag than a matte one. In tests I was involved in for an America’s Cup boat, we found that simply sanding a glossy bottom with 600 grit paper, reduced the surface friction by about 5% at ½ hull speed, i.e. at about 4.5 knots. Instead of sticking to the glossy bottom, the water molecules would be “tripped up” by the minute ridges left by the sandpaper. This slight turbulence would reduce the thickness of the film of water being moved along with the vessel (the “boundary layer”), and thereby reduce the overall drag.

Surfboards are traditionally finished super-glossy-shiny. Windsurfers, on the other hand, have long since followed the lead of sailboat racers and taken the gloss off the bottoms of their craft.

If you wonder if your bottom is too shiny, throw a cup of water at it: if the water beads, like it will on a freshly waxed car, then the surface tension is high, and therefore its resistance going through the water is high.

To change it to a low-drag surface, sand it in a circular motion with 600 grit wet & dry paper, until water thrown at it runs off in sheets – WITHOUT ANY BEADS forming.

Instead of wet & dry sandpaper, you can also use a “Purple Pad”, a Scotch brand synthetic wool pad designed to burnish metal and take off rust. Again, a circular motion is optimal.

To keep this fast surface fast, wash it now and then with soapy water; and lightly apply the Purple-Pad when the bottom appears yellow.

How big a difference will this make? After a bottom repair a while back, I decided to Purple-Pad the entire bottom of a windsurfing board. The owner noticed with a raised eyebrow, but did not comment on it when he picked up the board. However, he was back 4 hrs later, demanding to know what exactly I had done to his board: a friend, who had always easily passed him before, was no longer able to even keep up with him.

Or this recently from Shawn C: “I purchased an old Hifly 265 poly board that I used a couple of times and really enjoyed. I noticed that the board had a few small gouges on the bottom so I thought that I would sand them down using wet/dry 600 grit as you suggested. I then mistakenly waxed the bottom and had horrible performance on the water.”

I know of someone who painted a KBHF T bar with a super gloss shinny product and it then flew much worse so he sanded it with 600 g and back to normal.

R H


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Wed Jan 07, 2015 3:04 pm 
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Peter_Frank wrote:
ThistleSailorKC wrote:
theres an interesting chapter in "High Performance Sailing" by Frank Bethwaite who was a friggin genius with weather and fluid dynamics. From their testing he found that there was a 30-some % drag reduction on foils when polished vs. just sanded. There was not much difference when compared over a large surface like a boat bottom because complete laminar flow is impossible over such a span, but over foils it was dramatic


Interesting indeed, although I dont think, in fact I am sure this is not correct...

30% something is definitely not.

Many believe it is the opposite way, that sanded are faster than polished (you see a lot of pro racers going all matte).

But IMO the difference is so small that hardly or not noticeable, since there is no general consensus about what the better choice is.
......
8) Peter

if we are voting, i'd like to side with ThisleSailor on this one.
when you consider the specifics of a kite hydrofoil , the detail suggest favorable conditions for laminar flow and significanly lower drag coefficient than other surface craft.

1) low reynolds number (even at 30kts)

2) short chord lengths (minimize spread of turbulent spots from contaminants in the water)

3)fully submerged foils and mostly submerged struts ( removed from surface turbulence and contamination from air/bubbles)

4) low induced drag at low AOA ( as James has tried to beat into our heads, they go very fast and angle of attack is very low with negligible changes )

5) viable use of laminar flow profiles with good boundary -layer stability over design speeds and AOA ( eppler and speer )

the skin friction drag of a surface with a laminar boundary layer can be 10X less than comparable tubulent BL.

for a device like a kite hydrofoil under design condition i believe drag is mostly dominated by skin friction, with minimal lift-induced drag, wave drag ,other pressure and interferance drag.

a 30% improvement over total drag if laminar conditions are achieved seems quite reasonable.

regards,
-bill


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 Post subject: Re: Polish hydrofoil wings?
PostPosted: Wed Jan 07, 2015 3:48 pm 
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For a boat hull where laminar flow can not be maintained, a lightly sanded surface is best to reduce the drag of water attaching to the hull surface.

For a foil wing, a polished surface is best to actually assist in keeping the laminar flow intact. The drag (Friction) caused by the polished surface helps keep the flow from separating.

The best setup would be highly polished front and rear wing with a lightly sanded mast and fuselage.
You could also lightly sand the trailing edge of the wings to help flow separation as it leave the wing.


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