Flat bottom foils are better

This:
is almost twice as good as this:


That's an observation. Some theories about why at boardsurfr.blogspot.com/2024/06/better-foils.html

"good" is relative to what you want to achieve.

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FranP said..
You are wrong. It depends on what do you want to achieve in a foil and the optimal combination between speed, lift, planning, roll agility, etc.

Sure, if you want more drag and more pull from the had wing at a given speed, then get a less efficient foil. That works well for beginners, especially those coming from sports where they are used to a lot of pull, like windsurfing. But among advanced wingers, the new Armstrong HA range beats the older Armstrong foils hands-down. Compared to the old Armstrong HA range which was not universally liked, that is certainly true with respect to glide, stall speed, and pumping, without any loss in carving ability.

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colas said..
You mean somebody just discovered that the profile used by all airplane wings for more than a century is better for providing lift?

. If you only look at lift, you want the fattest foil you can find. As slowboat has pointed out for windsurf fins, the "thickness penalty" is less than you'd think. But while just making the same shape thicker increases lift, it also increases drag, so you don't increase glide ... unless you also shave off the bottom profile. Well known, of course, but looking through the airfoil database, I was surprised how many airplane foils have a lot of profile on the bottom - here is one example:



Design criteria for air planes are quite different, though. Stalling a foil means for winging means dropping down and maybe getting wet. Stalling an airplane wing has much more dramatic consequences.

Yep, Armstrong catching up :-). Their 'innovation' is to design in low-speed lift so you can wing with a smaller foil and thereby limit the degradation in top-end speed the design brings.