[Fr. Frank]

Denny's pretty close. Something on the order of a 13x15 cupped SS should be close. SS is much better for offshore. Don't use the Quicksilver Laser II, though, as it's designed as a bow-lifting prop and you need more sternlift. The Vengeance series is okay, but just okay. The old SS Black Max props (no longer made in SS) are better, or best, IMO, the PowerTech M135 series in a 15P.

Denny's right about the mounting position, too. Bottom of the engine cavitation plate about 1" above the keel line with the motor at neutral trim (parallel cavitation plate and keel line)

As an "old hand", I'm gonna throw some useful info at you, but it'll be a bit wordy.

The inline 6 "Tower of Power" Mercury motors are fantastic motors and nearly bulletproof when cared for, but there are some idiosyncrasies to worry about. Never, Never run these motors between 6200 and 6900 rpms - they'll self-destruct quickly from engine harmonics. (Believe it or not, running higher rpms than that is okay up to about 9000 rpms, as long as you run the right fuel-oil mix. Mercury Marine used to race these motors turning 8000 to 9000 rpms, making over 225 hp out of the 115/125 hp motors, and over 250 out of the inline 150 motors.) But these motors love to run at 5400 to 6000 rpms. I have owned no less than 9 inline 6 Mercury motors in my life, and still have an '86 115 in the shed.

I suggest you be very careful with the fuel/oil mixture, based upon your average rpms. If you run mostly over 4800, I suggest a 40:1 mix rather than 50:1 mix, that'll keep your top cylinder better lubed. However, if you run mostly below 4800, stay with the 50:1, or your spark plugs will need periodic cleaning.

Very important - make sure you have a good thermostat. With water temps below 70* and no thermostat, you'll burn fuel very inefficiently, and foul your spark plugs quickly because the motor won't ever get up to a good operating temperature. You can run without a thermostat in water above 80* in Florida in the summer if you have a brand-new impeller, but it's not a good idea, generally.

Speaking of that, make sure you change your water pump impeller every year, or every 100 hours, whichever comes first. This is another Very Important Procedure. Even if you only put 20 hours on the motor one year, change the impeller anyway. More inline 6 motors are killed from overheating than any other cause, and old pumps sometimes fail to adequately cool the top cylinder, especially if the thermostat is stuck open or missing.

Every time you run it in salt, rinse and flush thoroughly. Use Boeshield or Fluid Film on the motor liberally.
Run PowerTune through all three carbs while running in gear a couple of times a year, or run RingFree a couple of times a year.

It's a very good motor. It's also the genesis for the current Verados, because it's a naturally balanced motor.

Don't expect great fuel economy, but they're still better than the OMC motors of the '70 to '87 time frame, just nothing like today. Your cruising rpms for best economy with this motor will likely be near 4800 rpms.