Offset toe ins. Twinzers quads

[stoneburner]

I suspect the twin positioning is related to the delayed stall of the bi-plane wing effect. I would speculate that the stagger is related to keeping both fins close to the rail while separating them.

Googles AI summary of past Sways threads about the bi-wing effect.
(Mostly the result of Hans input.)

“Swaylocks, an online forum for surfboard shapers and enthusiasts, has discussed the “biplane effect” in the context of surfboard fin design, particularly in relation to twin fin setups

Here’s a breakdown of what the “biplane effect” means on Swaylocks, especially in relation to twin fins:

• Stall Delay & Softer Stall: Discussions on Swaylocks mention that the biplane effect can soften the stall characteristic of fins, meaning one fin stalls later than the other. This allows for thinner and stiffer upright fins while still providing forgiveness.
  *Another thread on Swaylocks further elaborates, stating that the top wing reduces the lift of the bottom wing, while the stall angle of the bottom wing is higher than the top wing, resulting in a softer stall when both are combined.
• Fin Interaction & Stall Angle: When fins are close to each other, they have a positive effect on the combined stall angle, meaning the setup is less prone to stalling. This allows for the use of more upright fins, as the interaction between them softens the stall, reducing the need for sweep (which typically softens the stall by twisting the fin).
• Potential for Reduced Fin Size (with qualifications): Discussions on Swaylocks suggest that while fins close together might need to be roughly the same size as a single fin, twin fins placed further apart (closer to the rails) might allow for a reduction in individual fin size (potentially almost half the size). However, this effect is less pronounced the further apart the fins are.
• Flow Control and Stall Delay: The biplane configuration is recognized as a stall-delaying, passive flow control method. Scientific studies, referenced on Swaylocks, confirm that biplane configurations can significantly delay stall angles for certain setups.

In essence, the discussions on Swaylocks indicate that the biplane effect in twin fin setups can enhance surfboard performance by providing:

• Increased Forgiveness: Due to the softer stall characteristics.
• Potential for Different Fin Designs: Allowing for thinner, stiffer, and more upright fins.
• Improved Flow Control: Delaying stall and potentially allowing for smaller fin sizes.“

Science behind the Thraikill twin

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.

Thrailkill's Twingle Flow Visualizations

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. [[{“fid”:“116958”,“view_mode”:“default”,“fields”:{“format”:“default”,“alignment”:“”,“field_file_image_alt_text[und][0][value]”:false,“field_file_image_title_text[und][0][value]”:false},“type”:“media”,“field_deltas”:{“2”:{“format”:“default”,“alignment”:“”,“field_file_image_alt_text[un…

Fin Efficiency

That’s hard to say, it all depends on what you mean with “poor thickness to chord ratio”. The twist becomes more important for thinner profiles as they tend to stall abruptly due to “leading edge stall”, where thicker profiles stall more softly by default (“trailing edge stall”) and therefore can be made more stiff and less raked. All those effects work together, it’s very hard to make a general rule which effect will dominate in general. A nice example of a different balance between all thos…