Hi Pete,
The rudder doesn't have to sit that deep below the prop Ø, in fact it's a sure way to damage it it you run over some debris.
It's better to shatter the prop than ripping the rudder from the hull.
The position of the prop puzzles me; it's a strange hybrid between a Crackerbox (but they have the propshaft parallel to the keel) a submerged prop (but that would sit under the hull) and a surface piercing drive (which would have the prop at 10% of the hull length from the transom, with the shaftangle as shallow as possible).
How did you come up with this construction?
I seriously doubt if this is going to work properly, let us know how the testrun goes.
Regards, Jan.