FangyFin MkII Theory for insomniacs

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!

Sorry to be picky, as this is otherwise an excellent insight into what looks like a very well thought out and effective design.

I assume you actually mean 'Ventilation' where you use the word 'cavitation'.

Ventilation being the effect of air from the surface interacting with the fin below water level. Cavitation being the extreme low pressure vaporisation of water on the low pressure side of a fin and it is commonly speculated that it may start occurring at around 48-55 knots on windsurfing fins, depending on the fin design and load.

Have a look at this video from about the 3 min mark onwards for the next few minutes. Another example of an oldie but a goodie ;-)

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fangman said..
Have a look at this video from about the 3 min mark onwards for the next few minutes. Another example of an oldie but a goodie ;-)

Did you hear the bit on hydro foils when he mentioned above and below 50kts?
It doesn't sound like it a first when he's talking about low speed stuff, but that's what he means, low speed = not much over 50kts.

Fantastic video Fangy. That explains a lot!

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fangman said..

decrepit said..

fangman said..
Have a look at this video from about the 3 min mark onwards for the next few minutes. Another example of an oldie but a goodie ;-)

Did you hear the bit on hydro foils when he mentioned above and below 50kts?
It doesn't sound like it a first when he's talking about low speed stuff, but that's what he means, low speed = not much over 50kts.

Yes and I will be very surprised if this fin is good for forty knots as that is not its intended speed range. But it is also worth remembering that cavitation is inversely proportional to ambient pressure - the closer the fin is to the surface, the sooner it will occur. In one of the papers I read, the author suggested that cavitation can be induced to occur on foils at 15feet per second which is only 9 knots, so to resist cavitation until 48 knots would suggest an excellent foil design with low lift charactersitics.

Here is a bit of a random ramble:

Mal Wright's work with his Time Machine fins was based around producing a high lift to drag ratio and resisting cavitation to over 45 knots (TM45 series), 48 knots (TM48) and 50 Knots (TM50). It was based on his work with X-foil and lot of math about Reynolds numbers and stuff I have no understanding of. But he seemed not to be very far off. His TM45's were repeatedly proven to 45 knots and slightly over and I have had the pleasure of pushing his TM48 to a little over 47 knots. Unfortunately, I don't have a good example of his TM50 to test. Of course, all these fins are asymmetrical and quite high aspect. (and therefore no good in weedy conditions).

He was also sometimes heard expressing his opinion that at certain times we had 'fast water'. By this he meant that water conditions, probably to do with turbulence, allowed the fins to gain efficiency. It seemed to be quite a fickle thing but there were definitely times he had me convinced of it, when everything just seemed to 'click' into place and we just went noticeably faster in, seemingly, the same wind conditions.

I think that in some types of weedy water, we see something that appears to be like Mal's 'fast water'. For sure, the presence of weed in the water will work against turbulence just below the surface. And the glassy conditions it can produce could probably lead to less air introduced from the surface to affect the fin. Add to that the higher density of very saline water at Lake George and is it any wonder we see such efficient speeds there for the moderate wind strength.

It is interesting that some modern lower aspect Assy' fins are achieving the same speeds now and doing it with some more user friendly characteristics, especially, lower speed stability and lift. ie. One can push a lot harder on them at low speeds (10-15 knots) without them spinning out. Is it possible that just increasing the chord to thickness ratio with a wider lower aspect fin is reducing the cavitation threshold speed?

BTW. I would be quite surprised if a smaller, say 17 or 18cm version, of Fangys fin, would not be capable of 40 knots.

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sailquik said..

fangman said..

decrepit said..

fangman said..
Have a look at this video from about the 3 min mark onwards for the next few minutes. Another example of an oldie but a goodie ;-)

Did you hear the bit on hydro foils when he mentioned above and below 50kts?
It doesn't sound like it a first when he's talking about low speed stuff, but that's what he means, low speed = not much over 50kts.

Yes and I will be very surprised if this fin is good for forty knots as that is not its intended speed range. But it is also worth remembering that cavitation is inversely proportional to ambient pressure - the closer the fin is to the surface, the sooner it will occur. In one of the papers I read, the author suggested that cavitation can be induced to occur on foils at 15feet per second which is only 9 knots, so to resist cavitation until 48 knots would suggest an excellent foil design with low lift charactersitics.

Here is a bit of a random ramble:

Mal Wright's work with his Time Machine fins was based around producing a high lift to drag ratio and resisting cavitation to over 45 knots (TM45 series), 48 knots (TM48) and 50 Knots (TM50). It was based on his work with X-foil and lot of math about Reynolds numbers and stuff I have no understanding of. But he seemed not to be very far off. His TM45's were repeatedly proven to 45 knots and slightly over and I have had the pleasure of pushing his TM48 to a little over 47 knots. Unfortunately, I don't have a good example of his TM50 to test. Of course, all these fins are asymmetrical and quite high aspect. (and therefore no good in weedy conditions).

He was also sometimes heard expressing his opinion that at certain times we had 'fast water'. By this he meant that water conditions, probably to do with turbulence, allowed the fins to gain efficiency. It seemed to be quite a fickle thing but there were definitely times he had me convinced of it, when everything just seemed to 'click' into place and we just went noticeably faster in, seemingly, the same wind conditions.

I think that in some types of weedy water, we see something that appears to be like Mal's 'fast water'. For sure, the presence of weed in the water will work against turbulence just below the surface. And the glassy conditions it can produce could probably lead to less air introduced from the surface to affect the fin. Add to that the higher density of very saline water at Lake George and is it any wonder we see such efficient speeds there for the moderate wind strength.

It is interesting that some modern lower aspect Assy' fins are achieving the same speeds now and doing it with some more user friendly characteristics, especially, lower speed stability and lift. ie. One can push a lot harder on them at low speeds (10-15 knots) without them spinning out. Is it possible that just increasing the chord to thickness ratio with a wider lower aspect fin is reducing the cavitation threshold speed?

BTW. I would be quite surprised if a smaller, say 17 or 18cm version, of Fangys fin, would not be capable of 40 knots.

Yep, the fast water is a 'thing' in foil design. It is called Ncrit. The Ncrit value is a reflection of the turbulence in which the foil is operating. High Ncrit values equates to very little turbulence. All foils work better in high Ncrit values. Interestingly, in hydrofoils the surface smoothness is sometimes assigned an Ncrit value as well. The smoother the surface, the higher the Ncrit, the better the foil is expected to perform.

I totally agree with your observations too Daffy. Delta's here outside the weedbank in the chop are frustrating, but inside, in the smooth they are far more effective. I do not know the answer to your question about lower aspect foils and cavitation, but I did read that a long shallow 'roof' on the foil shape will help keep flow laminar for longer, so maybe there is some influence. My suspicion is, that it is far more complex than that, and way above my pay grade in understanding.

I am not sure about my fin being a 40 knot unit. Its seems to be fine in the mid thirties but I am concerned that the vertical thrust might lift the board too much at that speed. I suppose I will have to wait and see... What I would hope to see is better speeds and handling in lighter winds or choppier water than a conventional Delta provides. So far, this clearly appears to be the case, but the more people that try it and are happy, the more confident I can become.

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pepe47 said..
You might actually say that a surface finished with 400 grit w/d with it's microscopic bubbles is trying to emulate a polished surface?!

I don't know Pepe. I have read a bit about the surface smoothness debate. I have finished my fins to a high polish cos they look impressive in the dishwasher. I did read a fair bit on laminar flow and that seems to hang in better, the smoother the surface, so that is the reason I have gone hard. ( one paper I read suggested that insect splatter on the leading edge of wings was enough to trip the laminar flow! ) But I also think that the bulk of the drag is not from the fin, so the slight differences between finishes maybe immaterial. Similarly, the same research showed, its the bumps on the foil that are the problem, pits make very little difference. As far as the science of this is concerned, I dont 'know' the answers, I am simply regurgitating what I have read and putting my spin on it, and as the ladies in this household will happily tell you, I am very rarely correct :-)

Thank you for contributing your insight Wolfgang.

I apologise, but I am not clear on all the points you are making.
I note that you very effective fins all come with a very course sanded finish. Are you saying that this helps the fin to resist cavitation/ventilation?

As for the cutout, I agree it is very effective to prevent spin out. We found the same thing back in the 80's when we used to make the same shape cutout in our moulded plastic fins and it magically stopped them spinning out.

I don't understand why this feature, which was common over 30 years ago on a number of production fins, does not form part of more designs.

I am a very big fan of your double assy. LeeWoo Rake 30 designs. Very fast and very secure! Excellent all round fins!

Just as a corollary, with the increase in the use of wind turbines, there are a number of research papers that look at foil roughness and its effects on efficiency (=power generation) As the Reynolds number increases the surface finish needs to increase as well to maintain efficiency. This can present a challenge when you have insects and bird splatter on your turbine tips! This also relates to my fin as I am not trying to make the slipperiest/fastest fin. I am trying to make the most efficient fin for use in everyday conditions hence the desire to hang on to the laminar flow for as long as possible. Think more like a streamlined Landcruiser than a Ferrari.

Okay,I work only in carbon. Wolfgang

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fangman said..
"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."

I think I should edit this bit. Instead of using the bow wave, substitute, 'tight pressure gradient'. Water is incompressible, so the intention is that the water between the weed and fin surface will act as a barrier as there is not enough time for the water to be displaced and allow the weed to abrade the fin. Importantly; I did not find any research to back my claim, so this is just a slightly educated guess, nothing more.

I came to a similar conclusion just sailing in the weedy.
Because of the way the drag seems to reduce the faster you go, didn't make much sense until you factor in the pressure wave, that also increases with speed.

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racerX said..
>>>>>>
Not saying cavitation doesn't increase drag, nor am I saying it doesn't occur while normal Windsurfing, just that you would simply never know. >>>>>

Doesn't cavitation make a lot of noise? After all all that damage can't be all that quiet can it?

I thought you'd hear it.

OK, so could low speed cavitaion be the release of dissolved air?
That's going to happen at higher pressures and not "snap" back with any force, and may be the cause of trailing bubbles in the wake directly behind the fin.

And at high speed with much lower pressures that's when the vapour pressure of water is reached, and the noisy damaging stuff starts.

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racerX said..
Interesting topic.

Couple comments,

I don't believe cavitation is the cause off Spinout, I or some other nerd posted a paper that showed that cavitation had little effect on the stalling angle of attack. Will try and find it again.

Even in that video with foils clearly cavitating, there is no evidence of the foil stalling that I could see.

Not saying cavitation doesn't increase drag, nor am I saying it doesn't occur while normal Windsurfing, just that you would simply never know. Unless of course you read seabreeze

Also at the Reynolds numbers encountered on a fin I thought laminar flow could only be expected for a couple of cm, not long enough to cover typical cord lengths?

Also the elliptical plan form of the spitfire only applies for zero sweep, rake or sweep and low aspect ratio change this relationship.

But it sure looks fast, which is probably the most important

Thanks for the comments RacerX,
I would have to admit that I think the most beautiful things are curvy, eg.the best looking ladies do not have straight lines :-) I wanted to create a fin with as much efficient lift at low speed as possible.( It is not the fin for epic days). Thus I wanted to keep the flow laminar for as long as possible,(more efficient bottom end grunt) but fully accept it will be lost with increasing speed. Another part of the lift efficiency equation was lessening the rake, and the only way I could think to do that and deal with the weed was a swept elliptical planform. Before I committed, I went to have a look and see what I could find to support its use in a swept back configuration. NASA released a paper by Mineck and Vijgen, on a wind tunnel investigation of elliptical wings. They found that the best performing swept elliptical shape had the trailing edge arc perpendicular to the body. In another paper just published; Reductions in induced drag by the use of aft swept wing tips C. W. Burkett, lent suppport, "Recent experimental and computational studies have indicated that rearward curvature of a wing can reduce the induced drag factor to values less than that obtained from the unswept elliptical wing considered optimal in classical wing theory." So on that basis, I went with the curved leading edge.
I agree that cavitation isn't the cause of spin out, however the foil in the video is not a good model to study spin-out as it is not surface piercing. The video shows the growth of cavitation with increasing speed in an ideal water flow. However, when windsurfing using a surface piercing foil, I believe that bubbles are being introduced by chop/turbulence and the fin/board junction pressure gradient interferences are then fed into the low pressure on the foil surface, effectively supercharging that pressure system with air and allowing the foil to stall. I believe this is what Wolfgang showed with his clear bottom windsurf boards when testing fin design and cut outs.
Having bored everyone sh!tless with all that waffle, I am opened minded about the whole lot, so if anyone can point me in the direction of some good research papers to read, I would be stoked.

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racerX said..

fangman said..

racerX said..
Interesting topic.

Couple comments,

I don't believe cavitation is the cause off Spinout, I or some other nerd posted a paper that showed that cavitation had little effect on the stalling angle of attack. Will try and find it again.

Even in that video with foils clearly cavitating, there is no evidence of the foil stalling that I could see.

Not saying cavitation doesn't increase drag, nor am I saying it doesn't occur while normal Windsurfing, just that you would simply never know. Unless of course you read seabreeze

Also at the Reynolds numbers encountered on a fin I thought laminar flow could only be expected for a couple of cm, not long enough to cover typical cord lengths?

Also the elliptical plan form of the spitfire only applies for zero sweep, rake or sweep and low aspect ratio change this relationship.

But it sure looks fast, which is probably the most important

Thanks for the comments RacerX,
I would have to admit that I think the most beautiful things are curvy, eg.the best looking ladies do not have straight lines :-) I wanted to create a fin with as much efficient lift at low speed as possible.( It is not the fin for epic days). Thus I wanted to keep the flow laminar for as long as possible,(more efficient bottom end grunt) but fully accept it will be lost with increasing speed. Another part of the lift efficiency equation was lessening the rake, and the only way I could think to do that and deal with the weed was a swept elliptical planform. Before I committed, I went to have a look and see what I could find to support its use in a swept back configuration. NASA released a paper by Mineck and Vijgen, on a wind tunnel investigation of elliptical wings. They found that the best performing swept elliptical shape had the trailing edge arc perpendicular to the body. In another paper just published; Reductions in induced drag by the use of aft swept wing tips C. W. Burkett, lent suppport, "Recent experimental and computational studies have indicated that rearward curvature of a wing can reduce the induced drag factor to values less than that obtained from the unswept elliptical wing considered optimal in classical wing theory." So on that basis, I went with the curved leading edge.
I agree that cavitation isn't the cause of spin out, however the foil in the video is not a good model to study spin-out as it is not surface piercing. The video shows the growth of cavitation with increasing speed in an ideal water flow. However, when windsurfing using a surface piercing foil, I believe that bubbles are being introduced by chop/turbulence and the fin/board junction pressure gradient interferences are then fed into the low pressure on the foil surface, effectively supercharging that pressure system with air and allowing the foil to stall. I believe this is what Wolfgang showed with his clear bottom windsurf boards when testing fin design and cut outs.
Having bored everyone sh!tless with all that waffle, I am opened minded about the whole lot, so if anyone can point me in the direction of some good research papers to read, I would be stoked.

On the eclipse, I should provided more detail. The classical spitfire shape is for medium or high aspect ratio unswept foil. A foil of constant shape with no taper I.e. a rectangle will stall at the root, and the tip is larger than it needs be. A low aspect ratio swept fin will tend to stall at the tip but with delayed stall at the root.

One of the reasons sails with a bit of twist have less drag.

So nothing wrong with the curve it's more the optimum taper ratio.

Will post some links tomorrow.

I must admit my fin building extends to sanding out fin damage. But it's an fascinating topic

Cheers RacerX :-) and thanks in advance for any links you can dig up. In the research by the NASA guys I mentioned above, IF I have understood it correctly; concluded that, "...Increasing the curvature of the (..wing tip..) chord line makes the angle of zero lift more negative but has little effect on the drag coefficient at zero lift."
I had thought that making the angle of zero lift more negative, negated the problem of elliptical planform tip stall. Have I got this right?