IMHO
(OS70 4 stroke) + (13x6 prop) = (perfect balance of thrust / speed / engine RPM)

OS lists a huge range of props for all of their engines and between my brother and myself we have bought them all.
To bad we wasted so much money.

61 4 stroke = 12x6
70 4 stroke = 13x6
91 4 stroke = 14x6

I am not a speed freak so these are the only sizes I will ever for these engines in the future.