# Thrailkill's Twingle Flow Visualizations

So here’s the image I posted earlier.

A twingle set-up of 7" fins, spaced 2" off center, foiled inside, flat outside. Water flow along a plane halfway down the fins, so 3.5"under the board, at 3 m/s and 15 degrees AoA.

Quick note on the colors. The colors represent the velocity, so if you look at the right side of the images you see the unpertubed flow in green (3 m/s). Going to yellow, orange and red the flow is accelerated, going toward light blue and deep blue the flow is slowed. So the red-orange areas represent lift (accelerated flow, less pressure), while deep blue is flow that is almost still or even turning around as you can see in the turbulent stalled region.

I need to process the other images to make them lighter, tomorrow. Mandatory disclaimer, these are fast simulations, they give an approximate idea of what’s going on, reality is more complex.

Bonus Image, same situation at 20 degrees AoA. The wake is more turbulent and the pair generates less lift than at 15 degrees but remarkably the flow is still mostly attached on the outside fin.

For comparison here’s only one fin (single foiled) in the same conditions as above, 15 then 20 degrees AoA.

At 15 deg the flow is already starting to detach, from pretty close to the leading edge, but the fin is still probably holding.

At 20 deg it is just a drag machine.

Reminder, the map is not the territory, I do these to trigger the imagination. Also they are still images of a highly dynamic system, when the flow is turbulent like in the stalled regions, it actually switches between several states. And that is in a virtual water tank with steady flow ! When you add a living wave and the rider’s inputs, the whole thing is beyond calculations for the forseable future. Which is good.

I have regrouped so single and twingle of the same angles can be compared.

The 20 degree angle seems to demonstrate a more significant improvement for the lower fin in the twingle.  However at 15 degrees, for the upper twingle fin, flow separation is worse than if the camber is placed externally at 15 degrees.

However, to do this right, we need to look at the same parameter for twingles with (symmetric) double-foiled fins and twingles with (asymmetric) single-foil cambers placed externally to one another.  Paired with their single fin conterparts at the same angle and flow parameters.

My first thought on some of this is that the twingle may have a bit of a “wing slat” component to it as much as a biplane effect.

As I mentioned in the last post, to be thorough, a comparative analysis should be done for flow with these twingle configurations. (Twin blue symmetric foils and twin green asymmetric foils [with outside camber].)

Foils are proportionally correct for 3/8" thickness, 6" chord length at base and 2" separation.

Is it possible to do the simulation with the foiled side out?

I use assymetrically foiled (one side flat) and symmetrically foiled fins in my Thrailkill twingles, a 7 footer and an 8 footer. Sometimes I use them with the flat side out and sometimes I use them with the foiled side out. I also have 2 NPJr Duo style boards that I use with the same fins. Be interesting to see what differences there are between those fins with the 6" spacing.

I have assymetrical foiled fins for the Bahne style center box in 5", 6" and 7". Been using the 5" fins, but I haven’t ridden those boards for several months.

Ok so firstly, I must say again that I do not have the computer power to give fine results, like the difference between foil orientation or small changes, like fin spacing for example. These kind of studies would need a computer beast with 10 overclocked processors and it would need to run for days. And that would still be a “garage scientist” set-up. I am drooling over some CFD stations like that, but at \$10k there are many other things I’d like to buy. And even though I would love to have one, I would still only use the results as inspirations or hints to work in certain directions (like I do with my current set-up). Nothing can replace reality.

I have read dozens and dozens of scientific publications on aero and hydro-dynamics and 99% percent of them conclude this way : “Our results in this study SEEM TO INDICATE, that so and so effect is taking place, FURTHER INVESTIGATION will be needed to determine so and so.”

I love science but I recognise that for complex problems it is just one part of the process of progress, the others being intuition and real life testing. I posted the above images because they show how the flow is forced to stay attached in the bi-plane configuration. The bi-plane configuration has long been known to be a stall delaying passive flow control method, just like tubercles btw.

Here’s the conclusion of a 2015 bi-plane study : “Experiments were performed to investigate aerodynamics of two-wing configurations at a low Reynolds number of 100,000 for a range of values of stagger and gap. These measurements have shown that two-wing configurations are a viable method of overcoming the challenges of low Reynolds number flight. Lift increases and stall angle is delayed significantly for certain configurations. Aerodynamic benefits are observed in the post-stall regime of the single wing.”

Yes obviously, we do not need a computer to know that an upside down assymetric foil is less efficient. But the twingle system is not an aircraft wing. I think that in this case we should rely on actual surfing and if Bill says that the foil inside/flat outside configuration is better I believe him and I try to understand why. Some other users of the twingle might prefer double foiled or foil outside better. Science is not going to change their minds.

Here’s where I think the secret sauce is: With the foiled side out or with symetrical foils we have more lift but less of the venturi effect. Keep in mind that an assymetric foil has zero lift at a negative angle, say minus 3 for example. Then try to imagine what happens between 0 and 3 degrees AoA for all 3 possible foil configurations.

I could but as I said above I do not think I can produce meaningful results. Showing the delayed stall is the best I can do, differences between foils will be lost in the noise with this software and my weak computer. If you could spare a minute, I would love to hear your impressions of the differences between the two configurations, that would be more valuable than my cartoons.

I think the question is, to what extent have symmetric foils been tested?

The Venturi effect can be manipulated with symmetric foils by varying chord thickness and separation distance between the twin foils.

BTW I am a scientist.  And for presentations and publications, “seem to” and “appear to” are known as “weasel words.”

There is something particular about Bill’s prefered configuration, that the two others don’t have, it is more obvious between 0 to 3 degrees AoA.

What is happening between 0 to 3 degrees for symmetric foils?

Thanks for the response.

For me, I haven’t felt too much difference with the fin orientation on the Twingle, or with symmetrically foiled fins. The last time I rode the Twingle I think I had the fins with the flat side in and I didn’t notice anything negative. The Duos are more sensitive with the fins, wrong location, size, type and you can tell immediately. The Twingle doesn’t have that problem, or I just don’t notice it as much.

The board below was my first attempt. It had 3 boxes and I rode this one with 2 fins and with 1 fin. I didn’t notice any big improvement, but I didn’t notice anything negative either. This board doesn’t have a leash attachment, so I never push it too far. A few years ago I decided to reshape the board because it had a delam, and it was a little wide to carry under my arm, I also found that there was a leak between the center box, so I removed it. The last time I used this board I had a really good time, it was in smaller surf, maybe waist to head high.

The other Twingle board is more of a big wave board, so I ride it in overhead plus surf, it works fine.

May a better computer come you way, WAO!

Thanks for sharing, very interesting.

Yes, and we should all wear masks.  Or not.

I’m late to the party, but please allow me to give my input.

Below I’ll talk about top and bottom fin as if those are biplane wings, it just simplifies the explanation and corresponds to how the image is laid out.

This image perfectly explains why I believe the fins should be symmetric or even assymetric towards the outside instead of inside.

The idea behind having them foiled on the inside is to create a venturi, this venturi induces an extra low pressure region between the fins. But because both fins are identical to the inside (mirrored), the force on the inside surfaces is exactly the same for top and bottom fin, resulting in zero contribution to the total lift by the venturi. So the verturi only increases drag. (And then I even ignore the possibility of cavitation when the pressure become so low the water starts to form bubbles, this can happen with speed record windsurf fins, but probably wont be an issue here unless you make the venturi to extreme.)

A nice foil profile is needed to keep flow attached, that’s why you see leading edge stall (the most abrupt type of stall) on the top fin, with a nice foiled top side it would not stall as fast and in case of trailing edge stall, it would stall much softer.

Talking about the bottom fin, the presence of the top fin helps in keeping the flow attached, this is known as the “biplane effect”. Therefore, you can get away with a flatter top side of the bottom fin (reducing the venturi), although a symmetic profile would still keep the flow attached to the bottom fin longer.

The key to the thrailkill twin is that the top fin stalls earlier than the bottom fin, softening the total stall characteristic. However, this does not mean you have to make the top fin stall more abruptly because you’ll counter the beneficial effect.

When the top fin stalls (as illustrated in the image), you still see there is lift generated by the combinanation. The downforce by the top fin, low-medium, is less than the upforce by the bottom fin, low-high. This is the thrailkill twin or “biplane effect” in practice.

Another crucial point is not to use flex fins. The purpose of flex is to soften the hard stall of thin profiled fins, this job is taken over by the biplane effect. Flex can interfere with the biplane effect in suboptimal ways.

The easiest way to see if this set up works is ride one.  The most common ride report (2) that I get is that this system is tried and then abandoned.  If it worked well, you’d see more of them.  The only ones I have seen in public were on boards that were made by Guru followers here on Sways.

I agree. However designs are often abandoned because they have been used in combination with the wrong board, fins, …

Often due to a lack of understanding on how to make it work.

A good example is the trifin on sailboards, which has been abandoned for years due to construction complexity and the struggle to make them work together, get it wrong and the board will drag like hell, and yes this is rider experience. But when you get the setup right it works fantastic. Nowadays the sailboard trifin is back but still only few really nail the setup. And guess what, it’s those that understand the physics that are able to get them right. The witchcraft shaper is one of those who nails them and has kept doing them when everybody abandoned them and still he is the only one succeeding in making them work with large side fins, he understands the physics better than anyone: https://witchcraft.nu/boards/trifin/

Single fins are easy to build and hard to do wrong, thrusters on surfboards are also easy because you surf the rail (not flat like sailboards), quads are harder. The thrailkill twin is very hard to get right, that’s why it’s abandoned. If we keep doing the easy stuff, this sport would not progress.

There are other examples - Bonzers are mostly limited to the Campbell Bros and their associates, the Duo is limited to Neal Purchase Jr,  the Zinger is limited to ACE/Eaton, just to name a couple.

I don’t think popularity or the lack thereof indicates to functionality.    Much of what sells is the result of aesthetics and marketing, including the effects of pro surfing.

Would the 3-fin setup been every bit as popular with novices who lack the skills to surf them if we had called it the tri-fin instead of the Thruster?

Good observations.  Not saying it can’t work.  Most any kind of fin setup can work.  But if that setup isn’t pretty revolutionary it is usually abanbonded.  Neal Purchase and the Campbell Brothers have been pretty revolutionary.  When I see two full sized fin boxes that close together with full sized fins installed.  I see nothing but Drag.  But I’m not gonna keep stating the obvious, nor am I going to buy into propaganda.

I think another way a design feature doesn’t catch on is when the style of surfing its aimed at doesn’t catch on.  I watch Neal Purchase on his Duo and TO ME those lines he’s drawing don’t motivate me.   It’s like the Bonzers or the channel bottom thrusters; you need a heavy rear foot to take advantage of those features.   Watching a thruster rider attempt to pump a single fin and then complain that they’ve got no drive is another example.   Etc, etc.

I know.        It’s comical, eh?       Another reason many don’t build them, is cost.      Exra FU glass fin, approx. \$65 dollars.      Extra FU fin box, approx. \$ 9 dollars.

Hello Gdaddy; so what would you say to the thruster rider in that case? Not drive but more intrinsic speed?

Regarding design. A bonzer daily rider 5 10 that I used has 2700 waves ridden (yes I counted its) but has a contemporary shape not an old style shape. I am more of a light surfer than heavy back foot one but the board worked pretty good after the first 80 waves or so (rear foot too forward was the error) I think the guys doing fish do the same error, shaping old shapes and use keel fins…

I will try to see this guy Purchase surfing those boards but I assume old style shapes however, great surfer…but put those shapes under a normal guy and most will return to the contemporary shape with a thruster (or may be quad if the board has more area) set up saying that the “retro” stuff is just that: retro.

-------also I have been observing that the market for the thruster et all (read, CI; lost, Pizel etc) now are widening ALL the boards. Included toobs boards. Most between 19 1/2 to 21. Seems that the fellas are eating too much junky food.