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):
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.
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.