DWF said..
You've discovered an industry wide problem. Huge wings mated to masts not stiff enough. It's something that shows up big time with SUP, but not prone, so many are not aware of how bad the issue is.
Is it only an issue with people foiling strapless on a SUP with feet close to the rail?
With straps, I did not experience this problem, but looking at videos, it seemed that some strapless SUPers have their feet very far from the center line, and thus are exerting a lot of torque on the board, and I seemed to notice the mast flexing, which does not seem to happen with people with their feet on the center line (or close to it).
Surf foilers have narrower boards and so have not this problem.
I recall also asking the Gong shaper to put the front strap inserts more towards the rails, as it is definitively easier to paddle a SUP board this way, and he relied me that he did not want it as it made the mast flex, so it tends to confirm this hypothesis.
So, this actually may not be a problem if you have your feet close to the center line.