Since the stabilizer is creating an opposite moment to the front wing (to counter-balance the forward pitching from the tow point being so high up on the rider's waist) then the dihedral angle will be opposite but complementary to the front wing. I do not think this is intended to replace vertical control surfaces previously built into older fuselage designs. I think the turned wingtips we see on a lot of stabilizers (Mike's Lab, Delta, Enata) are there for wingtip efficiency, not providing a control surface.
gofoil -stab vee, tips down, works great, super stable.
most everything else -stab tips up, works, varying stability by design/use
Inconclusive.
I'd want to absolutely eliminate the variable of "rapidly designing prototypes to see what works using easily CNC'ed materials" before immediately deciding that those brands went with flat wings based on hydrodynamic considerations. I think it's much more likely that their R&D process is more conducive to machining a bunch of easy and cheap to make wings and then going to the beach and finding the setup that works. If they had the ability to deliver a product to market that was a complex polyhedral wing shape, it's likely that's what we'd see. Modern manufacturing restrictions and the price point those brands target prohibit such extensive development so I think we're seeing a lot more flat shapes as they're faster and easier to rapidly iterate. Plus as soon as your customers think flat = waves then you've automatically created a marketing niche to exploit with your messaging.
Granted shaping G10 protos and producing CNC copies is an efficient method, keep in mind: Zeeko and Ketos both have been around a long time and have expertise with molded carbon, as well as offering flat g10 wings for some purposes. Zeeko prior to 2015 only made molded, anhedrally curved carbon foils to my knowledge. These are still made, and updated.
I think the flat carver wing, 710cm^2 came out in '15 for wave/light wind/higher lift use, not just to be cheap.
Same year, they offered a molded anhedral glass wing to update the original flat G10 wing on their aluminum/glass foil. They still offer the carver wing for waves/carving, it is still flat. So the rationale of g10 to be cheap is out imo. Being able to slam G10 into a sandbar many times with minimal damage matters, though.
Ketos is a subsidiary of a decades old composite company. Except the new flat g10 stab ("G10, program, maneuverability, reserved for confirmed or expert rider" on website), everything they make is curvy carbon. Their foils aren't cheap and they seem to be always putting in development effort. Nothing looks banged out in a hurry.
here's the wave one I referenced, head on:
Clearly, it is designed with anhedral, but not a lot. Looks like it was made flatter for a reason.
I think nobody uses a dihedral because you can't easily roll a dihedral wing into a turn if you're also pulling against the kite because they're naturally stable.
I doubt they are stable anymore when you have a CG 2-3 wingspans vertically displaced above the wing, and the CG moves farther outboard than the effective CL. The dynamics are not simple, but we have both a mobile and very high CG. Seems like a dihedral kitefoil could be prone to induce hinging at the feet and nasty crashes. But I've never tried one, naturally.
I don't think riders are inducing a yawing moment on a hydrofoil, there's not a control surface against which to push or pull that would make this possible.
With a kitefoil, no moving control surfaces, just a moving CG. Moment is applied the same way as with this:
If you just picture a perfectly vertical foil riding through the water, what we perceive as "yaw" or rather the rate of change of the nose angle of the board is really a product of rolling the wings. Yes, I guess from an external observer's standpoint you could see a guy riding along on a hydrofoil wagging his nose back and forth in the "yaw" plane but it's not because he's applying yawing forces to the hydrofoil to do so, it's because it's basically just a rolling turn in that direction with a commensurate change in kite power/angle or pitch to maintain ride height. There's no rudder on a hydrofoil.
I most often ride the large carver wing with low to moderate kite power and find yaw readily perceptible.
Visually, with the board vertical, kite high, yaw is the nose changing heading relative to the direction I'm moving. This easily and often happens flying high but at moderate speed without a lot of power, riding over top of small swells without trying to carve any, for example. There is some tolerance of yaw, if greater than this tolerance, you have to respond to it with a little matching roll to avoid falling, and voila, you have started carving S turns downwind on little waves. My favorite thing to do foiling, so far.