The short answer is Mercury says buy a 14 diameter by 11 pitch. They are right.
Their prop has a softer hub. If you don't use it you will hear gear rattle at
low RPMs (basically running pure idle speed in gear)
My 75 2 stroke with the same foot made the same noise. It was just that the
coughing and belching smoke disguised it :-)
These are great motors on a toon