This is a link about a gov't study and report on a 18.5 ft. boat in Sri Lanka. It shows the benefits of the inboard as opposed to an outboard.

It was an actual test. First without a gearbox, then with a 2:1 reduction gearbox. Engine about 6 HP.
I plan to build the same set up so i was interested to know WHICH prop they used. on the report it says 11"X 7" ????

Anyway go to: ftp://ftp.fao.org/docrep/fao/007/ad944e/ad944e00.pdf