Mercury 25hp 2-stroke MPG

[willy13]

I recently bought a 12 ft Sorensen Catamaran with a 1996 Mercury 25hp 2-stroke(OG390451). Weight of boat, motor, gear and 2 people is about 850 lbs. Any guess on how many miles per gallon with this setup at crusing speed? Top speed is 25 mph, and crusing speed would be about 18 mph. I ask because we are bringing this boat to Marco Island, Florida and visiting near by islands.
Thanks!
William

[bassfisher444]

Figure roughly 2.5 gallons per hour wide open, so 10 MPG running 25 MPH. Better running at part throttle.

[willy13]

Thank you, thats very helpful

[EuropeanAM]

Keep in mind that simply a change in loading can account for a substantial difference in fuel consumption. Recommend carrying a fuel load of at least 50% more than calculated until you have some confidence in what will remain after each trip.