From Dave Gerr's "The Nature of Boats:" Ten percent (of the prop diameter)
is the bare minimum (tip clearance). 14 to 18 percent is the standard ideal
distance though bigger is better. High speed planing hulls should have 20
percent. Fair the edges of the prop aperture.

He also discusses propeller sizing and other factors.

Roger

http://home.insightbb.com/~derbyrm
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There is a rule of thumb about prop clearance but I forget it but you
can probably google it. I think that with a 2 blade that can be
aligned with the keel, you should use the largest prop that will allow
this clearance.