With 13 persons in board you will use a lot of water. Saba water tankage is only 185 gallons, which we go thru in <2.5 days with 4 couples on our Saba 50 Cuvée though we have freshwater flush electric heads and watermaker. I'm surprised there isn't a watermaker onboard your 6-cabin boat.

The genset burns 3/4 gph with all A/C units on, and 1/4 gph just charging batteries but you want to load up the genset with at least a couple A/C units.

One engine at 1800 rpm burns 1 gph and will drive you at 6 kts; 2300 rpm burns 2 gph and will get you to 7.4 kts. Running both engines doubles the fuel burn and adds 0.8 kts to the above speeds. Other than while manuevering or heading into heavy seas, I'll motor with one engine at 1800-2300 rpm to reduce engine hours, fuel consumption, and cockpit noise.

Fuel tankage is 124 gallons each in stb and port tanks. Genset is fed from stb tank. I just did a 2 week, 425 nautical mile trip with light winds, humid evenings and visits to Anguilla and Culebra - used 133 gallons (40 gal per engine, 53 for genset).

I always refueled at Delta Petroleum on east side of Roadtown harbor the last morning before returning to TMM.

I'm guessing the cheapest place to refuel would be on St Thomas, if your travels took you outside the BVI.