I never would have thought the flat sides would have been outside. Interesting. Just the opposite of what I would have thought. One of your protégées up here just snapped his on an outside bar at a local break. Though provoking Bill.
I understand your thinking Bill. Re-direction, stalls etc. I remember when you showed me the small fin that also goes in the 10 1/2" box. Can’t remember if you had it installed forward or rear of the large fin. My problem is my style of surfing. I never stall on cutbacks or forward re-direction out of a cutback. Single fin longboard, Two Plus One or a thruster. I’m a big guy but not necessarily a “Power Surfer”. I finesse it a bit and use my ankles a lot. So I have never felt a need to correct or improve on something that is not a problem for myself. Obviously your ideas have merit and are confirmed by the fact that others either think like you do or are copying those ideas. Lowel
I too believe this setup works better with flatter on the inside.
A flatter side increases the tendency of leading edge separation (hard stall), but the other fin will counter this tendency so you can get away with flatter on the inside.
But because the outer sides are generating the majority of the lift, I wouldn’t make them flatter as this just induces more leading edge separation, the exact thing you try to counter with this setup.
But that’s just my 2 cents, Bill has been riding this setup for decades.
Aloha Hans,
The leading edge of the foil of each fin, for best efficiency, MUST have a blunt radius. (IMO) Not the sharp leading edge, seen on most commercial offerings. The ‘‘Twingle’’ setup works well, with either single foiled, or double foiled fins. Attached water flow, during very high AoA maneuvers, is the intended goal of that fin setup. If you have that, then all the other benefits will follow.
Functioning like a canard, it is placed forward of the fin. The stall I refer to, is the disrupter water flow on the fin, not a rider induced stall maneuver of the surfboard.
I fully agree on the blunt leading edge for subsonic flows.
A fin that is flat on one side has no advantage at all it’s purely done for cheaper production.
With flatter on the inside I mean that if you have a decent 70/30 foil, my understanding would advice using the 30 inside and 70 outside, while you advice the opposite.
I have no data or experience like you to back this claim this is purely base on my understanding of hydrodynamics like I explained, while you have decades of experience with this setup so I’m not trying to argue, I’m just giving my opinion.
Sharp leading edges make no sense, we fully agree on that statement 
In one of his earlier posts on the setup, Mr Thrailkill said that his take on it was that the inboard foils opposed each other from the short distance forcing the flow against each other to contribute to the effect.
Yes, but you don’t need to create a venturi for this, a venturi has no net effect on the lift if you consider both foils, it only adds drag.
To suppress separation, a flat plate would be good enough, so no need to make the inside foil thicker.
I’m not referring to a rider induced stall either. Clearly though for myself; I would gain nothing from a double fin setup of your design type. It falls into the old "if it works don’t fix it " line of thought. Lowel
Hans, a reading of post #1, in the thread titled ‘‘Bill Thrailkill you owe me a new surfboard’’ describes well what happens, when a Twingle setup is added to an existing surfboard. The increased drag that you referenced above, seems to have made the board go faster, than when surfed as a single fin. I mean, with the added drag of that ‘‘venture effect’’, the rider reports increased speed and maneuverability. How did that happen? What did I do wrong?
There can be many reasons why the single fin is slower e.g. bad foil, too little fin area or too much flex leading to partial stalling in turns, … the list is endless.
You should compare apples with apples. You should compare the same fins with thick foil on inside vs on outside (it’s an easy swap).
The drag induced by the venturi is not the problem, that’s quite small. The problem is the leading edge stall appearing on the flatter outside.