The rule is to calculate 90% of the total fuel carried and only count on that to be available. In your case you carry 160 gallons but the usable fuel is only 144 gallons(90% of 160) The main reason for using the 90% rule is that your tank pickup tubes don't go all the way to the bottom of the fuel tanks to avoid picking up sludge and water that accumulates there. If I were you I would install a fuel flow meter. Depending on the year model of your motors and the type of chartplotter you have it's an easy thing to do. Barring the above you can install a "flow Scan" for both motors or the old tried and true method of filling the tanks and taking the boat out for a normal day of fishing and then go back to the fuel dock and refill the tanks to get a real world idea of fuel burn.
|