Wheel 15 x 15 Michagans on my 52 Monticello with twin 7.4 Mercs. Get about 1mpg @ 2000 rpm at about 9.7 mph. WOT it drops to 3 gallons to the mile @ 20 mph.

My 43 Nautaline wheeled the same props with 2- 318 Chryslers and delieverd 1.2 mpg @ 2200 rpm going approx 11moh and .6mpg @ 4000rpm and approx 24mph.

But I never bought a houseboat to get there quick, I can enjoy a cocktail at 10 mph as much as sitting at the dock at 0 mph