Inboard boats consume fuel in the same manner as a car but the consumption rate is calculated differently. A car's consumption is rated in miles per gallon (MPG), based on the distance travelled which is recorded on the odometer. The fuel consumption rate of a boat is calculated not on distance travelled, but on the hours of operation. The calculation method will obtain the exact fuel consumption on the normal operation of your boat. The estimation method will determine the fuel consumption according to the size of your inboard motor.
Fill your boat with fuel in all tanks.
Drive the boat for four hours at your normal pace of stopping to fish or float, top speeds and cruising. End your four-hour journey back at the fuel pumps.
Refill the boat fuel tank(s). Write down the exact number of gallons that it takes to top off the tank(s).
Divide the number of gallons of fuel when you refilled by the number of hours you operated the boat. If it's exactly four hours, then divide by 4 to obtain the fuel consumption rate in hours for your normal driving habits. If it's a bit different, divide by the actual number, e.g., 4.25 if you were out for four hours and 15 minutes.
Write down the horsepower (HP) of your inboard boat motor. The HP is printed on some motors; consult your owner's manual if this is not the case.
Multiply the HP by 0.055 for a diesel engine to estimate the fuel consumption. This is based on the calculations of Boat Safe that, "Diesel engines consume about 1 gallon per hour for every 18hp used." Alternatively, you could divide by 18.
Multiply the HP by 0.10 for a gasoline boat engine to estimate the fuel consumption. According to Boat Safe, "Gasoline four stroke inboard engines need about 1 gallon per hour for every 10hp used." Divide by 10 instead, if you like.
Multiply the fuel consumption per hour by the number of hours you plan to be on the water to determine how much fuel is needed for an outing. The estimation method takes into account that the boat will be running at top speed 100% of the time. Lower your estimate number by 50% if you plan to float half of the time of your outing or travel at lower speeds to determine how much fuel you need for a trip.