FangyFin MkII Theory for insomniacs

10 minutes of free airport wifi...got to keep this short. (..as if that's gonna happen :-P)

I keep forgetting the design criteria I set myself was to design a fin that was able to provide more lift, get planning sooner, and not spin out in chop. I think because we are all 'speed sailors', we inherently want to make stuff go faster. I am totally guilty of this and so have to remind myself of the core principle, Smiles on dials on average days, not so much the epic days. I also absolutely agree with Decrepit in that I probably don't have it all correct, after all, the whole design has never been CFD analysed and is simply a result of some semi educated guesswork and a lot of copying other people's ideas.

So back to the questions at hand. The fin has no similarities with a Delta, other than the overhang forward of the box. Delta's rely on an attached leading edge vortex to generate lift. Their foil is not a Eppler or NACA, and the area behind the max thickness is used to generate secondary vortices for further lift. Because they don't rely on the foil shape as much, they are finer in profile. Deltas have large surface area drag, but low foil drag.

In contrast, the FF relies heavily on foil shape for lift. The fillet tames the low pressure side interference pressure waves between the perpendicular surfaces of fin and board. This is at the expense of peak lift generation. That is, to reduce the energy loss from turbulence at the junction, there is a trade off, and less peak lift is produced. However, the fin is more stable and tolerant of adverse pressure fluctuations and will not ventilate easily (spin out) The peak lift loss is countered with a fatter foil and rounded leading edge. NASA's supercritical foil shapes are designed transition through each flow state with the least amount of losses, and I took inspiration from them.

I have run out of time. I will post more when I can.

The FF has a marked taper from base to tip when viewed front-on. The fineness at the tip is also increased compared to a fin made of G10 because of the aluminium material strength. When rotating the fin from 'dead-on' to approx 15 degrees ( i.e angle of attack), the apparent frontal taper and surface area presented to the flow, does not change markedly. I felt that if the drag profile of the fin did not change rapidly with angle of attack, the fin would be more stable and less susceptible to ventilation.

Secondarily, a stronger, stable spanwise flow is created. This is in itself is 'draggy', but it is hoped the resulting upward thrust will decrease board surface area drag as quickly as possible and enhance early planing - like foiling but with a self limiting effect. Because the design intent was not to produce a speed fin, but a user friendly everyday super weedy, I considered the increased drag an acceptable compromise. Swindy has done lots of experiments thinning the foil thickness, and had produced fins with noticeably less drag as a result.

The fillet also contributes to the vertical lift, but to a much lesser extent.The FF has a Wolfgang Lessacher cut-out at the rear of the base. Wolfgang used a clear plexiglassed board to observe flow behaviour around the fin whilst sailing at different. The conclusion of all his experimental designs, was the cut-out. It's purpose is shunt ventilated flow on the high pressure side through to the low pressure side of the fin (lifting side) before it has a chance to interact with the trailing edge and initiate ventilation.

In contrast, the FF concave fillet shape works on the low pressure side of the fin, so the intent was to combine the two features to provide further ventilation resistance.A convex fillet - bulb shaped - is also effective but works mostly on the high pressure side of the foil. Given the Lessacher cut out was already in place to deal with unsteady flows on this side, I chose the concave fillet rather instead.

The centre of effort location was a compromise with the amount of overhang. With my weight and height and skinny tailed vintage boards, pushing up the tail of the board at low -moderate speeds is a desirable feature. As has been noted previously, this is may not be the case at high speeds, but once again, high speed handling was not the design intent, and as such a C of E placed a little further back was not considered detrimental.

Now for those still awake, did I miss anything?

If I remember my physics well enough it's down to F=ma.
The upward force is a result of the mass of water ( for arguments sake is relatively constant) being accelerated out and down by the foil shape. ( The taper of the fin from base to tip results in a pressure differential and the flow being partially deflected downward) The resultant force is therefore partially resolved in the upward direction, ( Newton again - for every action there is an equal oposite reaction) giving rise to the 'vertical lift' effect of the fin.

yep

Everyone asleep now? If not, let's talk about angular momentum and vortex formation....crap, I am putting myself to sleep now....

I think Yoyo is referring to my post where I mentioned a very exaggerated taper so it's virtually a hump.

I got a present in the mail today. Four tiny FF18's. If you want a DIY version ($150) drop me a line. If you want a finished one you will need to be patient, as fin finishing time is scarce at the moment
I have posted a picture of the FF18 next to an FF24 to give some sense of scale. It is close to half the surface area of the FF24. I would expect that it is probably a match for < 5.5 metre sails, but I will confirm that when one of the crash test pilots has had a spin.

Looking at how small the 18 looks reminds me of my experience with the MUF Delta XTs. I got a 24 that worked very well in a Falcon 99 with a 7.0. But when I tried an 18 in an Isonic 72, it felt way too small, despite the much lower width and smaller sail. May be quite different with a FF 18 and flat water, though. Can't wait to try one .. only 4 more months!

I used the Swoody 18 a few weeks ago, and it worked well, except there was too much chop for it.
I'm only 65Kg and used it with my new 48cm board and 5.4 koncept. I'm sure in a good seabreeze at a flat Fangys it will be ideal.

Peter, I hope the weed does it for you in December, it'll blow your mind! It's just surreal sailing at 30+kts on a mirror.

Next ti,e we meet, we must remember to compare the. Swindy has reduced the area significantly as well as thickness, so they may not be much different.
Any way if we get a good day, I'm willing to test them side by side.

Wow!
Put me down for one of those 18's Fangy!
No hurry. The snow is keeping me well occupied.
In around Dec/Feb I will be feeling the need for Fanging on the Flat again!

Tectonics new fin for 2019 looks very familiar. Imitation is the greatest form of flattery perhaps
The durability of a fine fillet made of G10 will be interesting to monitor long term because at $286 AUD plus freight it would be a bummer to snap it off!