I'm converting an ancient board to experiment with ground effect. I've cut the back off, so it's nice and wide, made it a single concave, with flats near the rails. Mounted 2 long US finboxes on the edge of the flats, about 70mm in from the rails.
After I'd mounted the boxes parallel, I had a thought they probably should be toed in. The displaced water, is going towards the rails, so the flow at the leading edge of the fin, is coming at whatever this angle is, +/- the boards angle of attack.

So my question is, does anybody have a clue how to work out what this flow angle is?

It's a bit late now to change the box angle, but as I haven't made the fins yet, I can make them assy, that should solve the problem.

And don't worry, my first experiments will be on light wind days only!
After Elmo's dire warnings, I'd hate to end up with a broken neck.