# Science behind the Thraikill twin

Bill, I understand that you don’t like the theoretical threads, but this thread is supporting your fin setup!

I did some research on biplanes, very similar to the thraikill twin

Main conclusions are

• Softer stall angle, so very reliable in big waves!
• Better efficiency at low speeds, but inverse at high speeds. But since Reynolds numbers (a parameter used compare velocities in different fluids) in surfing is an order of magnitude lower than for airplanes, I assume we can consider all surfing at low velocity. So again great feature of this setup!
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The best source of info that I found on the design of biplanes: http://enu.kz/repository/2009/AIAA-2009-207.pdf

There is one image in particular that I’d like to lift out of it:

Note the following in figure 4:

• The top wing reduces the lift of the bottom wing
• The stall angle of the bottom wing is higher than the top wing! So the combination of both results in a softer stall.
• The difference in stall angle is increased by staggering the wings!

So if surfing a point break, it might be beneficial to place the wave side fin more forward than the shore side fin. Anyone tried that?

Hans,

It would be interesting to see what the graphics for flow and separation patterns of Mr. Thrailkill’s twin look like using the flow dynamics program/model you presented a while back for elliptical and other shapes…

Maybe someone can elucidate, but somewhere in the past I recall having a conversation to the effect that the fin size in relation to board size is too small for the foil on the fin to provide much actual lift from the foil, the lift comes from the surface of the fin meeting the oncoming water, like your hand outside the car window on the freeway.  The foil on the thrailkill twin then becomes a matter of streamlining.

Not really.      The camber on a 3/8th inch thick, single foil fin, is the same camber as a double foiled fin that is 3/4th inch thick.      That is a relativly high lift foil.       Theory aside, the proof is in the improved performance on the waves.      A number of folks have had that direct experience.

that’s funny Bill, I thought it was you that told me that, lol, I’m starting to slip a few gears…

Don’t dispair, Huck.     There IS an aspect, of what you described, involved in how a fin functions.        More than one thing is going on, in the dynamic environment of a wave.

True, about the low speed.      As fast as we think we are going on a wave, it’s still a low speed, in absolute terms.        The ‘‘stagger effect’’ occurs naturally, as a board travels across a waves face.     The stagger will vary, depending on the AoA into the water moving up the face of the wave.      I’m impressed that you picked up what is going on, with the setup.

Seems to me that whether you’re going right or left you need balanced fin function. I mean coming off

the top without sticking is real important. So here’s a question. If the fins are vertical they will give

an certain measure of engagement and release during the turning moment. Has putting about 2 to 3 degrees

of cant in each fin been tried? I seems to me that with foil experimentation and modest cant the set-up

could become even more effective. This is a projection, of course.

Having experimented with cant on several different types of boards and fin set-ups I have been

empressed by how a very few degrees of cant adjustment and moderate fin template change will have great affect

on board performance.

The experiments continue:

Stay Stoked, Rich

Thanks Bill!

The staggering however is independent of the AOA. If you compare figure 4 a & b, you can see that both apply for a hole range of AOA’s but still have a constant stagger angle of 0 and 30 degrees respectively.

I have an other nice insight from that same article. You said that you do not decrease the fin size for the fins individually compared to a single fin. This is very interesting, as the following picture illustrates that the wing span can be reduced more if the wings are further apart. But remember wings closer to each other have a positive effect on the combined stall angle.

So this shows indeed that fins close to each other should have roughly the same size as the single fin. While a twin fin on the rails can reduce the fin size to almost half the size.

However, the further the fins apart, the less the positive effect of the inside fin (~ upper wing) on the stall angle of the outside fin.

The statement that you can use more upright fins also makes sense. Sweep in single fins soften the stall by twisting and therefore reducing the AOA at the tip. And since the Thraikill fin setup softens the stall angle by the fin interaction, less sweep is needed. The more upright shape may result in a looser feeling.

This fin setup makes perfect sense to me. However I’d go for double foiled fins since the inside fin (~ top wing) is responsible for more than half the lift generated.

Wow. I don’t know how I missed this the first time it came around. Great info Hans.

@ Bill Thrailkill:

I don’t know if this is the proper place to ask this question as there is a surge of discussion on the “twingle fin” design across multiple threads, but what is the shortest board you’ve made with this arrangement?

I have a 5’8" with the twingle configuration. I love it and will probably be doing it on future boards (although there is added weight from the extra fin box). Also, everybody I show it to mentions staggering the toe and heel side fins just to ‘see what it does’. I haven’t ridden it enough to compare so I haven’t yet tried the staggered fin setup.

I would think that staggering the fins in this setup would create a slight pull to one side. Seems like the modern bi-planes have the top wing slightly ahead of the lower wing. Maybe it adds lift?

With the single foiled fins, switching the foils from flat side facing the other fin to facing away from the other fin would or should have an affect.

Bill if you are out there, using a 7" fin with an outline like the Smith/Parrish (Brewer), or a bonzer style, where would you place the leading edge of the fin? I may add a few boxes to a board I’m riding, but they would be the shorter boxes to keep weight down.

Aloha Harry,

What I have done, in your situation, is use the 8.5 inch FU box with the fin centrally placed.       I then position the fin/box combo, so that the base leading edge of the fin is about 11 to 11.5 inches up from the tail, to determine where to mount the box.      You will then have both fore and aft adjustment to fine tune the ride.      Another way is to set your boxes on either side of an existing single fin box, so that a fin in the center of the added box will be exactly where the existing single fin was located.      In this way you get a good comparison of the change in performance, of the paired fins.     Hope this is helpful.

The shortest I’ve put them in, is a 5’ 10’’ short board.

Thanks Bill. I really enjoyed talking with you the other day. I hope you had a good time while you were here.

Hi Harry,

I enjoyed our chat too.       Don’t forget to send me your mailing address.     The only negative on the visit, was the 6 inch Centipede that went up my pantleg during dinner, at The Beach House in Haleiwa.         Big ouch !       Surf was building back up, when we left on Thursday.       Predictions were for 15 to 20 foot waves on the North Shore.

I’ll PM you. I made these fins for the NPJr board, but I’m thinking about throwing a set of boxes into an existing single fin board I currently use a lot and see how they work closely spaced. The yellow and the blue fins are single foil the G10 fins are doubles. They range in size from 5" to 7" The blue fins are about 1/2" thick and made with a foam core glued to a glass backing then foiled and more glass added over the foam. They came out thicker because I wanted the outer layer strong. Together they would equal a 1" thick fin.

Bumped up for educational purposes.

Thank you very much!