Most folks run a 15 x 17 or 14 1/2 x 19 prop. MJ used to run a 15 x 15 to get added torque for carrying heavy loads. 
You should be getting more RPM's with the 15 x 15. Are you sure she's firing on all cylinders? Makes me think a cylinder or two are not with the low rpms and the low speed vibration.
__________________
1985 Wellcraft V-20, Evinrude ETEC 150: SOLD
1979 Marine Trader 44, twin Ford Lehman 120s
2006 Panga 14, Tohatsu 20
|