Under average load a prop pitched to run engine at its peak torque RPM would have what effect? My goal is gas mileage at slowest plaining speed.
fuel economy and boating are polar opposites. if you want fuel economy get a sail boat, kayak or row boat.
that being stated, you prop your boat so your motor hits the middle to upper end of the RPM range recommended by Mercruiser.
in I/O will burn 0.4# of fuel per HP commanded per hour.
your commanded HP will depend on the location of your throttle, the direction and force of the wind, the direction and force of any currents, the load in the boat, the position of your trim tabs, length of the boat, fresh water vs salt water, the cleanliness of the bottom, the condition of your prop and the angle of your drive.
since the HP will be a variable that changes by the moment, the only thing you can do is prop the boat properly, install a fuel flow meter, and play with trim tabs and drive trim until you get the best economy at that particular moment.