Fin Efficiency

General Discussion
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I’ve been thinking a bit about fin efficiency lately. In other words, trying to reduce the “wing tip vortices” that occur at the tip of the fin. These vortices are created during a turn when the water under high pressure on one side of the fin “spills” over at the tip of the fin onto the low pressure side.

There are a number of ways to do this and create what is called a “bell shaped spanload”, in aeronautical terms, instead of the usual “parabolic spanload”. Reducing the amount of lift just at the tip of the fin will reduce the pressure differential at the tip and minimise the “spillover” and subsequent vortices.

One way is to twist the tip of the fin (only practical for side and not centre fins). For those who are interested you can read about this here on the following link - NASA research on the Prantdl-D wing.
https://www.nasa.gov/centers/armstrong/features/Prandtl-D_validating_new_wing_design.html

Another way is to incorporate a “flat winglet” at the tip of the fin (suitable for both side and centre fins). Rapidly reducing the fin area just at the tip is the way to achieve this. “Standard” template surfboard fins are not really adherent to this design principle. Designs preferred/created by some of the Swaylocks luminaries however seem more closely aligned to this philosophy. Wildy’s and Greg Griffin’s fin designs, as well as Bill Thrailkill’s preferred Brewer template are all examples.

For one reason or another this type of fin template seems to me to be little explored/exploited/available.

Cheyne Horan’s Starfin and the Wavegrinder designs addressed these wingtip vortices using angled winglets, but these are more difficult/costly to produce and also introduce some vertical forces which may be undesirable (depending on your point of view).

Have a look at the shape of a dolphins pectoral fin. It is a beautiful example of what I’m talking about. The tip of an albatross wing is another (in a different type of way though).

I have explored a little in this direction so far and found the results to be encouraging.

Maybe Wildy, Greg Griffin and Bill Thrailkill might care to share if this efficiency gain (less turbulence = less drag) is indeed the reasoning behind their designs/preferences for fin templates, or did they have other “things” in mind.

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Reduced tip style template on left (mild version). Standard template on right.

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Not one of the luminaries mentioned, not even close, but I tried the Wavegringer WG2 winglet fin in my single fin traditional longboards, all ready to skoff and dismiss it, but the first wave it was obvious the tail was locked in, and the board accelleration was quicker to its max speed, limited by tail rocker and soft rails in the tail. Super pivotal. There were times the fin seemed to dictate the line taken, and it would lock in and accellerate in certain parts of the wave, which required simply holding the line and slowly applying pressure before being able to really redirect. Basically, sometimes it tracks, while accellerating, a little bit like a car with no brakes going downhill. A bit thrilling, at times a bit reckless. Downturns into the pocket where where this was most noticeable. Weighing 215Lbs on average, I felt the WG2 is too flexy, as sometimes it would disconnect the rail when unweighting after a bottom turn, on my forehand. Backhand was not an issue.

After my sessions on th WG2, Going back to my traditional raked fin with the exaggerated tip was a letdown. I took it out and it has remained dry since. Its significantly higher surface area could not hold as well, and felt draggy and stiff in comparison, and i was a bit shocked. All this time… 30+ years of riding traditional longboards with ‘the bigger honking fin, the better’ mentality, and not knowing that it is such a limiting factor.

From there I Began trying Mr Mik’s high aspect ratio turbucle fins, and was again surprised. These were still quicker and looser than thay traditional raked fin, but did not have that blistering accelleration of the WG2. But they were also devoid of that tracking feeling of the WG2, and very forgiving. Happy go lucky fins that increased my desire to surf, and the amount of fun I had, and the waves made.

I have also tried the wavegrinder fins( fcs1 base) in my 6’11" which I consider my shortboard. I tried the bigger 5.7 inch deep fins on the rails, with the 4.7 inch center fin. I was not impressed. Dual foil rail fins seem to lack crispness and at times it seemed the winglets were simply at the wrong angle in realtion to my tail rocker and the rest of the board. I did like the smaller WG fin as a center fin, but could notice that same tendency as the larger WG2 in my LB, to track in certain parts of the wave face. The smaller fin felt just as big as a regular size thruster fin despite having some fraction of the surface area. I would like to try a smaller Wg winglet fin in my center probox, but returned a smaller high aspect center fin to it instead.

Using the smaller WG fins as rail fins, and there was not enough to push off of at slower speeds, they only seemed to engage once going faster, and they also lacked crispness when going form one rail to the other when going faster, and when I put my regular rail fins back in it felt right, again.

I am still not satisfied with my regualr rail fins on my shortboard. My going left rail fin is a TC Redline, and I really like it, Going right is the same template, plywood fin, with an ~80/20 foil. It lacks the crispness of the TC redline. I lost the righthand TC fin many years ago.

I really like the look of that brewer template fin you made, RDM. I have a fin panel waiting to be cut and foiled for new rail fins, and after your post last night, busted out the white cardboard and drew some templates, none of which I was happy with.

I am still torn though.

I would love to try some higher aspect turbucle fins on my shortboard. There is likely some speed threshold where the high aspect ratio and turbucles become draggier that what we are used to seeing in shortboard fins,. I do not think my traditional Longboard could attain those speeds ever, but shortboard certainly could, but that is a guess.

So it begs the quetion regarding fin efficiency. At what speed through water are we seeking that efficiency? The Wg fins in the shortboard did not wake up until some speed was generated, and once they did, I could not notice any speed or maneuverability gains, and they felt a bit squishy, and at times, tracky.

I can’t really hope to hand foil with any accuracy, turbucle fins, but their performance on my longboard has sparked my curiosity as to how they could perform on a shortboard, at least as a center fin.

They are incredibly forgiving when stalled. They do not instantly bust free when at their limit, and then nearly instantly reengage. I wonder what a top level surfer on a HPSB, could do with such a forgiving fin , which is also so speedy and happy go lucky, if those attributes translate into shortboard size fins and shortboarding. I really think this needs exploration.

The latest longboard fin I am hooked on is MrMiks Gullwhale fin. Super high aspect ratio and a pretty thin tip too. This fin has all the speed of the WG2, but no tracking. When it is stalled and loses grip, it does not lose all of its grip, and seems to be residing in a drag free pocket of its own making. Kind of a finless feeling, in that the tail is neither locked in, nor is there any feeling of drag. A few times I have had the lip crash right near the tail of my board going through the foamy wake of the previous wave, and the fin feels like it snapped off. Yet I maintain the angle before the lip crashed, and the speed, and yet the fin seems like it is gone, but the tail also does not simply slide out. Very unusual feeling when this occurs.

Anyway, I am wanting to make some rail fins for my 6’11" and the traditional fin, the black one on the right, no longer soothes any part of the visual cortex of my brain. That more Brewerish template fin you made, does, and I look forward to having some near my toes or heel in some head high plus waves, and the sooner the better.

Taking clues from Biomimickry, I am liking the shape of the pectoral fins of silky sharks and similar. High aspect ratio, narrow tips. Some videos of them on youtube, when they are exhiting their aggressive posture with pectoral fins almost straight downward, one can see definite similarities to the forces we subject our surfboard fins to. Much more so than the dorsal fin of a dolphin, or the tailfin of a bluefin tuna.

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 For me, I had several ''other things'' in mind, as it relates to fin choice.
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The Spitfire fin championed by Solosurfer for many years on this forum I would also consider as having a rapidly reducing fin area at the tip conducive to minimising wingtip vortices. Those here who have surfed with that fin can certainly attest to its speed through turns and lack of drag.

Wrcsixeight, your experience with surfing the Wavegrinder thruster fin set up is not surprising. Having three sets of vertical forces would always lead at some point to them “fighting” each other and you feeling the results under your feet. With the single fin Wavegrinder and Cheyne’s Starfin at least your only dealing with one point of that vertical force. Even the Starfin can be tricky to deal with if you’re not concentrating. It can literally “throw” you off if you’re not deliberate with your intentions.

There is no real speed at which the effect of a reduced wingtip vortex design will “kick in”. Higher angles of attack increase the strength of the vortices so typically they are actually stronger at lower speeds. So surfing a fin which is designed to reduce this drag will be faster through the turns rather than appreciably faster at its “top end” in a straight line. Having said that, the angles of attack generated even when just surfing “down the line” are much greater than most realise.

Here’s an example of a “raked wingtip” that Boeing use to reduce wingtip vortices.

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I had not thought about the 3 Wavegrinder winglets fighting each other. I was attributing my negaitve opinion to the fact the rail fins have no flat side and are 50/50 foils. I’ve been meaning to try my SB as a twinny with just the larger WG fins, but when I rode it with the bigs as twins and the small as a thruster, I very much did not like it and left decent waves prematurely rather than trying to figure it out or adjusting them foe or aft in the probox in the lineup .

The spitfire fins are certanly interesting. High aspect, narrow tip.

I have not tried the winged keel/star fin, but its winglets are much larger than the Wavegrider WG2. The unweighting after a frontside bottom turn weirdness I’ve been attributing to excessive flex, could be the small winglet itself not happy with the motion, or some combination of both. The WG2 fin, in a deliberate tail stomp pocket stall, waiting for the wave to feel the inside bar, seems to very much dislike this. It seems happiest at full throttle.

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Just eliminate the tip…

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Oh no! That’s giving me flashbacks to the Swaylocks days of Roy Stuart.

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Indeed. I did some simulations for him to illustrate that tunnel fins DO GENERATE a tip vortex.
You don’t need a tip to generate one.

check this out: https://twitter.com/finfoil/status/630697112825688068



About vorticity:

vorticity is NOT EQUAL to turbulence.

A fully laminar vortex is possible.
The smaller the vortex, the more likely it is to introduce turbulence, and indeed turbulence leads to losses.

Vorticity (and circulation) are required for lift generation, see: https://en.wikipedia.org/wiki/Circulation_(fluid_dynamics)

Reducing the tip vortex actually means spreading it over the wing span so that the resulting vortex becomes bigger and therefore less turbulent.

Next to that, there are a lot more things to consider that will need you to sacrifice efficiency in order to improve surfing performance (e.g. stall resistance, moment force, break resistance, …)

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A couple useful links to define the terminology:
https://www.seabreeze.com.au/forums/Windsurfing/General/Interesting-read-on-Fins?page=1

https://www.grc.nasa.gov/www/k-12/airplane/wrights/results.html

When you talk efficiency you have to define what environment you want to be efficient in. There’s always a compromise to be made. That first link has some good comments related to this:
“At higher speeds thinner profiles normally show a higher efficiency. Hence thinner profiles are better suited at higher speeds - but not necessarily for reaching them. There’s a cut-off point at around 9% (of chord ÷ thickness). Below 9% would typically be used for speed. 9% or higher for slalom use. It could be a good idea to develop fins with >12% ratios specifically for very low speeds, like first time use etc . The result will be a high-lift fin at very low speed.”

Crazy how far ahead of their time the Wright Brothers were. I only post that link for this little nugget of wisdom:
“In 1900 and 1901, the brothers had closely approximated Lilienthal’s airfoil shape, but had a very different wing planform which generated very different wing performance from Lilienthal’s published data.”

The twist you mentioned is called “washout”. Halcyon makes a lot of fins with that feature for just the reasons you mentioned. He also makes a lot of fins with rapidly decreasing tip area. I think Halcyon would be a great reference for this discussion in regards to fin efficiency.
http://forum.surfer.com/forum/ubbthreads.php?ubb=showflat&Number=1873670

Super interesting topic that I think has a lot of valid directions to go in. I’m personally curious in playing with that thickness number and seeing what relative “speed” I surf at my local beachbreak. My assumption is that relatively thicker foils, more towards the 12% thickness would benefit more typical shortboard manuevers in average under head high surf. Think aerobatic planes vs. passenger airliners. One is optimized for directional instability and the other for long distance efficiency.

Those efficiency gains by the Boeing wing tip shapes are in the low single digit %'s - https://www.boeing.com/commercial/aeromagazine/articles/qtr_03_09/article_03_1.html, best-case scenario, I wonder how much you’ll feel an efficiency gain in a surfboard fin of single digit % improvements?

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Hans - I stand corrected. I trust your computer models far more than my own half baked theories!

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When I build fins I always use a 12-15% thickness to chord ratio. Wood/foam cores are really the only way to do so and keep the weight down however.
Commercially available fins are all pretty much 10% or under.
Of course side fins are only half a foil so their ideal thickness to chord ratio should be half of that of a centre fin. In a standard thruster set of fins the side fin foil is about a 10% ratio whilst the rear is about a 5% ratio. So whilst the side fins have an acceptable foil thickness for their chord length, the centre fin does not.
Those rear side fins of Halcyons in your link are eerily similar to the aforementioned dolphin pectoral fins. He should expect to be hearing from their lawyers very soon.

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From NASA:
“elliptical wing planform has the lowest amount of induced drag and all other wing shapes have higher induced drag than an elliptical wing.”
https://www.grc.nasa.gov/www/k-12/airplane/induced.html

The Spitfire wing was elliptical to reduce induced drag – NACA 2200 sieries, “13 per cent of the chord at the root and 6 per cent at the tip.”
https://www.aerosociety.com/media/4953/the-aerodynamics-of-the-spitfire.pdf
https://www.aerosociety.com/media/4843/the-spitfire-wing-planform-a-suggestion.pdf

Interesting year-old images from Hans:
https://www.swaylocks.com/forums/elliptic-vs-square-foil-outlines

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In my mind going with a 12-15% thickness/chord ratio would allow you to reduce the overall area of the fin?
Have you played with this at all. The assumption being that the thicker foil creates more lift (and drag) but by doing so you could achieve similar performance with less total fin area (less wetted area).

I made a few sets of fins a long time ago to test this theory, but could never figure out how to mold a Futures base onto them. I decreased the overall size by 5% for each 2% increase in thickness (as close as I could approximate by hand measuring with a micrometer).

Halcyon was/is very much into biomimicry in his fins. Love seeing his fins, they were always so unique.

Back to your initial post topic, I think there is quite a lot of area to explore efficiency in fins, the trick is actually measuring it, and figuring out if it really makes a discernible difference in actual performance.

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I basicly did a direct comparison. Calculated the area of my old fin and made the new one with straight lines (fins in the avatar)
The straight edges on the trailing edge definitely produced more performance which I believe in part to be a lot less turbulence. It felt cleaner in the water.
Also effecting performance was the reduction in area of the fin tip allowing a deeper fin to pivot more easily…not sure about the washout effect, but it felt smooth in transition at any speed.

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Meanwhile, while I and the rest of the “Swaylocks Mafia” sip cocktails on our private luxury yacht that was paid for with our collective fin fortune - courtesy of ideas stolen from Mr Roy… consider that ‘hoop fins’ (aka ‘tunnel fins’) have actually been around awhile.


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Pay attention class, the voice of considerable experience is speaking.
Much respect, Wildy.

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I only ever fly in aircraft that meet my demanding expectations for wing tip vortice minimisation.

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Thanks for the explanation of your design process and findings Wildy.
When I began to look/think a little more deeply about this subject it was the fins shown in your avator that struck me as being an example of a planshape that made more sense than just about everything else out there - just enough rake for drive, straight lines for cleaner entry/release, highest pressure point line on the foil always moving rearward, and rapidly reducing tip area to minimise fin tip vortice creation.

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So why do fins get thinner towards the tip?
From the discussion on these pages it seems that thickness should be maintained all the way to the tip.
Or is it too hard to keep a high (say 12%) thickness to chord ratio in the fin tip on a fin raked on a continuous curve?