Probably below a few inches, in part because of the shape of the hull.
Although if I put them at the widest part, I think they'd end up about
midway. So rub rails that stick out a bit would seem to the the answer?
I am considering them because I single-hand rather a lot and so seem
to join up with things more than I would prefer. So the prospect of
doing it with the rub rail rather than the hull seems a happier
outcome. Pier poles seem to be my worst enemy.