study? naaaaah they use Deboichet. Or Techtonics. A few on C3. Sections are pretty much NACA63-010 or NACA64-010. These were designed to take advantage of laminar flow, but since they are finished with 400-600 wet n dry one wonders... Differences are in planform, and flex. These foils have a slight reflex to stabilise the moment coefficient so are quite forgiving when it comes to structural flex. They are state of the evolved art.

Reflex is an interesting beast. Its most effectively applied to increase the moment generated by the fin. Positive moment means that the fin tries to twist (referenced at 1/4 the distance from the leading edge to the trailing edge) such that the angle of attack increases with speed. ie the twisting moment increases with speed in the direction that increases the lift.

If you made the fin ultra stiff, it would not generate as much lift as a non-reflexed design of the same thickness and front profile. The reflex basically generates a "reverse" lift at the trailing edge, which balances out the more forward lift generated by the rest of the foil. Great for tail-less flying wings. If the plane dives (and speeds up as a result), the positive moment pulls up harder on the front of the wing and therefore tries to pull the plane out of the dive. If the moment coefficient were negative... stand back!

But the reverse lift counters the useful lift for roughly the same drag, so the L/D is generally lower in a reflexed design.

So its usefulness for fins? Its only useful if at the design angle of attack, the moment is lower than one wishes it to be. If used on soft G10, it makes a smaller fin generate lots of lift in the tip (due to the positive moment) making it feel big and powerful at lower speeds, but its unlikely to have good top end, unless it was needed to control the twist. There are many "standard" symmetric sections designed with a little bit of reflex, including the 64-010 used in several high performance commercial fins...

This new learning amazes me, Sir Bedevere. Explain again how sheep's
bladders may be employed to prevent earthquakes.

Hey guys, its not all a mysterious art. Applying some well established science goes a long way to explaining some of the mysteries and "magic" in fins. I used to follow the unwritten rule in the sport that "its too hard for science to explain", and listen to calls of "we tried applying science but it didn't work as good as trial and error" from industry "experts". But its been great to discover that the science established between the 1920s and 1960s goes a long way to explain whats going on with our setups.

Nebbian,
1)Its quite straightfoward to check if cavitation is an issue. First you need to estimate the side load on the fin. Then you estimate the AOA required to support that side load at the speed you are doing. From this you can calculate the pressure distribution over the foil (using a panel method such as eppler or Mark Drela's xfoil). If the suction side exceeds the Cp corresponding to atmospheric pressure for that speed, you have a vacuum... Of course cavitation assessment is a lot more complex than that, but it gives a close estimate of when to expect cavitation inception. Since we dont have a V12 pushing us along, it doesn't take much of a distortion of the virtual foil (due to cavitation bubbles changing the overall flow shape) to increase the drag significantly. Damage? We dont have enough power to cause explosive erosion. But little bubbles are enough to increase the drag to pop the fin out of the laminar drag bucket which got us to cavitation speed in the first place. So it acts like a speed limiter.

2)The reflex is built into the fins. Basically the rear is hollowed out concave. Wolfgang Lessacher makes an interesting fin with a reflex on each side but offset over the span. So the section is assymmetrical, but the camber shifts from negative to positive over the span, so it works on both tacks. He's a real character...

3) I heard of some guys in the UK trying to do something like that. Not sure if theres any real advantage though. It would be pretty tricky to get it all balanced...

4) hmmmm- dont hit rocks

All this stuff was discussed more like 50 years ago actually... "dude"

I guess a topic on fin selection would sail over the head of a kiterdude. Many people would be interested to know what it takes to go 47.3 knots and would be greatful for slowboat sharing part of his knowledge so freely even if they don't understand a lot of it. You don't see Antoine or Finian giving away their secrets.. although perhaps with their size and skill they don't need to refine the technical side as much as slowboat.

Get a life? I think slowboatboat has a pretty good like. Freelance work that allows him to go kayaking and windsurfing whenever the wind or water is good. He has sailed on the canal ( when Finian set his record)and Sandy Point in epic conditions . Done Le Defi (400+ competitors) and kayaked in the French Pyrenees and Austrian Alps. Sounds pretty nice to me. I often wonder about guys who walk around with baggies over their wetsuits mouthing english expressions like tosser and git.

i have completely enjoyed the debate. thanks nebbian, slowboat and all for having a say.

i always like to understand how the gear i pay heaps of money on works, so i can enjoy what i do and get the most out of the kit i have.

cheers guys...

I propose a dual

Slowboat vs anyone who chooses to challenge, you up Felcher

There will may be wind at dawn!

Hi guys Its great to see slowboat and others sharing their knowledge and experiences. Ive been playing around a little with fins mainly trial and error stuff.
Nebs I have to show you some of my fins you would prob laugh but hey they do seemed to work. Ive pretty much cut of the tip parallel with the board and just rounded the edges. I also attack the foil with a 4 inch grinder and moved the foil to about 45%.
Ive tried it a few times and it seems to work fine except if there's weed around forget about it.
Im no expert but just enjoy playing around and trying things.
ta Vando

quote:

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Originally posted by slowboat

nebbian, I dont think the tip finish is that important. Its in a part of the fin with the lowest lift. As for drag, it might make a slight difference in that its likely to remove any chance of laminar flow if its all scratched up, but again we sand our fins with 400-600 so... I dont worry too much about dings in the tip, as long as its not smashed to the point of having bits sticking out...
A quick look at the huge variety of different tip planforms on the market is also a good sign that its not that important.

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Hi Chris,

Since I know that are the fastest in Oz, I'll bow in respect. I do have one question - why is 400-600 so bad?

Having done studing of laminar (an turbulent) flow, and seen high-speed footage of flow over foils, I know that the water on the surface of the face doesn't actually move, ie: its the water about .5mm further away that is actually flowing.

In fact, a rough surface can (in some cases) actually encourage the water to stick to the fin surface, thus helping to reduce cavitation.

Thoughts?

quote:

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Originally posted by mineral1
www.seabreeze.com.au/gallery/gallery.asp?imageid=850

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That type of flex is quite normal for a formula fin. And is the exact reason why the formula boards have a "big tuttle" box in them.