That's some sailboat with a pair of 8V-71s! 
The short answer is the faster you go, the more fuel you burn per mile. The optimum cruising speed is a slow drift.
For diesels you can figure 0.055 U.S. gallons/hp/hour. Wide open this boat will burn 880*0.055 = 48.4 US gallons/hour.
The boat is probably a semi-planing hull given its displacement/length ratio. You can estimate horsepower for a given speed in two steps (this is from Dave Gerr, naval architect):
You can work out power use at different speeds. I get a maximum of about 17 knots for your boat, giving about 1/3rd of a mile per gallon wide open.
If you're in full displacement mode, assuming the hull is efficient in displacement mode, the rule of thumb is 2 HP per ton, or 63 HP for your boat. This should get you to hull speed of 9.3 kts. Fuel burn would be about 3.5 gal/hr giving about 2.7 mpg. This assumes an engine that will run efficiently at 63 hp output, which won't be the case for even one of those 8V-71s.
If you're looking at something that'll cost four grand to fill up, it'll be worth spending a few hundred bucks on a naval architect to walk you through the details of speed, range and fuel economy on this boat.
Good luck,
Tim
The short answer is the faster you go, the more fuel you burn per mile. The optimum cruising speed is a slow drift.
For diesels you can figure 0.055 U.S. gallons/hp/hour. Wide open this boat will burn 880*0.055 = 48.4 US gallons/hour.
The boat is probably a semi-planing hull given its displacement/length ratio. You can estimate horsepower for a given speed in two steps (this is from Dave Gerr, naval architect):
SL = speed/length ratio = speed in kts/SQRT(waterline length)
HP = (displacement in pounds) * (SL/10.665)^3
where the ^3 means to the third power.HP = (displacement in pounds) * (SL/10.665)^3
You can work out power use at different speeds. I get a maximum of about 17 knots for your boat, giving about 1/3rd of a mile per gallon wide open.
If you're in full displacement mode, assuming the hull is efficient in displacement mode, the rule of thumb is 2 HP per ton, or 63 HP for your boat. This should get you to hull speed of 9.3 kts. Fuel burn would be about 3.5 gal/hr giving about 2.7 mpg. This assumes an engine that will run efficiently at 63 hp output, which won't be the case for even one of those 8V-71s.
If you're looking at something that'll cost four grand to fill up, it'll be worth spending a few hundred bucks on a naval architect to walk you through the details of speed, range and fuel economy on this boat.
Good luck,
Tim
