A long time ago, I stood on a Kaplan turbine blade that was being hardfaced. I think an axial section would show the blades overlapping each other quite a bit. I suspect that you'd have to make the root chord a lot shorter in order to get the blade to swing past vertical, and you'd lose some efficiency to that.
At which point the blade twist would be all wrong, and you'd be fighting the curvature of the volute, too.
Mike Halloran
Pembroke Pines, FL, USA