I lost the original thread from somebody asking about prop size for M-18 on a C27. The standard prop is a 2 blade 13x10 RH. I have same boat/same engine. Last haulout, I had the prop repitched to 13x12 (2 inches from original is about all they can do). The result is an increase from 5.2kt @ 2600rpm to 5.6kt. At 3000rpm, I can get 6kt. The increase does not load the engine down, in fact, I can still get to red line. Hope this helps. Don #6293