I am only achieving 5400 rpms @ W.O.T . with michigan 13 1/4 17 prop and that is without full tank gas,or onboard water. My owners manual say 5750 rpms is the sweet spot. The next size down is 13 3/4 15 in michigan wheel. Evinrude has a 13 1/2 15. Can the 13 3/4 15 be o.k. or will it be to big of A diameter is my question ? Will the extra 1/2 inch cause any issues with cavitation/or slippage ? Any Ideas ? What the largest recommended dia. prop for the 115 ? Thanx