Since most of the time people on cats motor sail I would plan on 1 gallon an hour for both engines( 1/2 gallon each) the generator probably burns 1 quart an hour. You should not have to motor more than 25 to 30 hours to go from e end of the BVI's to the other. If you sail when able or at least motor sail at a lower RPM you could cut the consumption dramatically. What ever it cost the vacation is worth it. You can more than make up for the high cost of fuel with the cheap cost of rum. If you use water wisely it should not be a problem, plus top off at Saba Rock or Leverick bay for free. The moorings at Saba are right in front and they are labeled.
Fair Winds