Is there a rule of thumb describing how close the tip of a prop blade can be to the hull?

Not really. It depends on the boat. With surface drives some will have struts 3" or more aft.

Thanks for your answer. I didn't makes myself clear. I'm interested in the distance between the prop tip and the bottom of the hull.

I think a little more info on the boat would be helpful. Ron's answer would be accurate for a sport or scale hydro and some riggers. Knowing what kind of boat you have would help in giving an answer that would be boat specific