FangyFin MkII Theory for insomniacs

sailquik said..
Questions Mr Fangman:

Have you seen any evidence of corrosive pitting on you oldest fins? Do you anticipate that this could occur? Do you think anodising would be a good idea?

Reason I ask is I have an fantastic aluminium assy speed fin made by Mal Wright way back in the mid 80's and it shows signs of pitting, even though it has been painted with 3d primer paint. I have to get it out every now and then and refinish it to prevent it progressing. It may well be a different grade of alloy of course.

decrepit said..
Wow 30m to sell out, there must be heaps of needy sailors stalking this thread.

decrepit said..
I have a feeling that the weed will keep most of the fin polished, maybe just the rear of the fin will need a bit of attention?

FangyFin FAQ


What size sail?
I have some really rough estimates based on my experience and that of fellow sailors:
20cm < 6m ( I have used a 6.6 okay, and Swindy used a 7.1!)
24cm 28 cm < 10. (My biggest sail is a 9.5 Race and it was fine)

fangman said..
With the success of the FangyMkII seeming to be something other than my imagination, I thought I had better start a new thread and explain my thinking for all the souls who have trouble sleeping. Nothing in this fin is original. To paraphrase the famous quote, I simply stood on the shoulders of giants to peer a little further in the direction of improvements that could be made in fins designed for shallow and weedy water. Thank you to all those fin gurus out there who were email stalked or pestered by me. I would particularly like to acknowledge the help of Wolfgang Lesssacher who was generous with his time, sharing his vast experience and knowledge. This fin is an amalgamation of the wealth of knowledge that was shared and in a small part, a tribute to the boffins in the windsurfing community.



My starting point was to consider the fin and board as one in a hydrodynamic system, rather than concentrating separately on what makes a board fast and what makes a fin fast. The final design is not a supercavitating design and I am not expecting it to be really fast. What I wanted to create, was a fin that worked in moderate breezes to provide enough lateral lift to allow good upwind sailing and then perform predictably and safely on the downhill in heavy weed, shallow water and small chop. At the moment the upwind legs and spin outs on a conventional 55 degree fin are frustrating at times, I wanted to solve that problem. So the brief I gave myself, was a short, high lift fin that was reasonably slippery.

After a lot of researching, reading scientific papers, watching videos of fluid dynamics, my solution was the MkII. A fin designed for the everyday conditions, rather than the epic days, but with increased hydrodynamic design efficiencies to allow comparatively good speeds in less wind. All that follows are my conclusions and they maybe considered alt-facts only. The designs have not been tested scientifically, so this is an opinion piece. There is a good chance your opinion may be different.

The increase in efficiency is primarily from reduction of energy loss from cavitation. My solution was to try to keep the flow laminar for as long as possible and reduce sources of cavitation. In considering the board and fin as one, I looked for research showing the interaction between a foil and a flat surface. Happily the US military, did a bit with their hydrofoils. There are some videos showing that the junction of the foil and its support, cavitates very easily and early at the leading edge. This is made worse with decreasing depth. A windsurfing fin is immersed, but only just, so it presents the worse case scenario for cavitation. Their research showed that a blended fillet, 6% radius, of the chord was the most effective way to reduce cavitation. So the fillet on my fin was born.





The extension of the fillet to the front serves also as very effective weed catcher stopping weed getting underneath fin. The broad base stops pressure marks/ damage to the board surface where fin touches.
The cut out is a result of Wolfgang Lessacher's work and observations of fin behavior in real time on the water through a plexiglass surface panel. I used this in conjunction with the aforementioned research papers on hydrofoil behaviour that clearly showed that the junction of the board and fin created interference pressure and as a result cavitation starts very early on the leading edge. This cavitation plume, plus any air entrapped between the board and the water may join with ever present trailing edge surface cavitation. The resultant supercharged cavitation then rapidly progresses forward across the foil and engulfs the entire surface. This is spin out. Wolfgang showed that, with the fin at an angle of attack 10 -15 degrees, the cut out swept this plume to leeward before it had an opportunity to attach to the trailing edge.



A front view of the Fangy MkII shows a large taper from base to tip. This allows very strong and hollow aluminium design. However its main purpose is elsewhere. Research shows that only 15% of the total drag is from the fin. A much larger portion of the drag comes from board contact with the water. The fin acts like a very sharp v-hull speedboat, so as speed increases there is a small amount of span-wise thrust pushing down the fin, which in turn pushes the board up and reduces the draggy wetted area sooner. This effect is self limiting. The fin might have more drag than a conventional shape, but its ability to lift the board and reduce the much greater wetted area drag more than compensates for this.


The leading edge itself, is very highly polished but blunt. It is highly polished to remove as many cavitation triggers as possible( this applies to the entire foil surface). It is blunt for several reasons: The first is to create a bow wave ahead of the fin. The intention is to try and push the weed out of the way with water pressure rather than let it abrade the fin itself. The second is to create more lateral lift at lower speeds allowing transition to planing to occur sooner. The third is to provide a speed boat bow effect and create thrust down the fin to push the board up. The effect being earlier planing because of reduced wetted surface drag. Consequently, it is expected that making the fin finer in entry will not improve its performance, rather it will be detrimental. There is one piece of research that shows the fillet does contribute to this effect also, albeit only in a small amount.

The elliptical outline is theoretically the most efficient wing shape. The leading edge is shallow ellipse, the trailing edge a deeper ellipse. Inspiration was drawn from what is regarded as the most beautiful wing design ever, the Supermarine Spitfire. The rake varies from 50-55degrees over the ellipse. Extensive crash testing has shown this is the minimum rake that weed-bergs can be safely tackled. And finally, the foil has a long roof with mid point a long way back to encourage efficient laminar flow for as long as possible.

So all that remains now is the tweaking of design and trying to get the foundry to cast them hollow. Stay tuned for updates. Oh and congratulations for staying awake until the end!