One of the most basic questions a boat operator must be able to answer before leaving the dock is how much fuel will be required to safely complete the planned voyage. For obvious reasons, accuracy in estimating a boat's range is critical.
Boat manufacturers publish performance data detailing the expected speed and fuel consumption for their fleet and engine configurations. In the example table, we can see the fuel consumption in Gallons per Hour (GPH) at a given speed for our vessel.
To calculate the amount of fuel needed for a planned voyage, we need to know:
Distance - The distance, measured in Miles, to be travelled to and from the destination.
Speed - The speed, measured in Miles Per Hour (MPH), the boat will be operated at for the majority of the voyage.
Fuel Consumption Rate - The rate fuel is consumed, measured in Gallons Per Hour (GPH), at the Speed the vessel will be operated.
Let's assume we have planned a round trip of 63.25 Miles, and we expect to cruise at the most efficient 31.4 MPH as shown in the table.
Step 1 - Use the Speed of 31.4 MPH and Distance of 63.25 to solve for Time using the following formula:
Distance / Speed = Time
63.25 / 31.4 = 2.01
Our trip will take approximately 2 hours to complete, cruising at 31.4 MPH.
Step 2 - We can see from the performance data table that that at 31.4 MPH, 20.6 GPH will be consumed. We can calculate the total fuel that would be consumed by multiplying the GPH by Time determined in Step 1.
GPH X Time = Fuel
20.6 X 2.01 = 41.2
Step 3 - To calculate a margin of safety, multiply 41.2 gallons by 1.5.
41.2 X 1.5 = 61.8
We should start our voyage with a minimum of 61.8 gallons of fuel.
How can we be certain we have enough fuel in the tank?
Assume our fuel gauge reads about 1/3 full and we know we have a 300 gallon tank. It would appear we have approximately 100 gallons of fuel remaining so we are ready to go. STOP!
Fuel gauges on boats are notoriously inaccurate and should not be relied upon for calculating range. The best method to determine fuel level, is to know the capacity of the fuel tank and fill it to the top. Then, use the engine gauges to reset the cumulative fuel consumption to zero after fueling. From that point forward, the fuel flow will be tracked cumulatively and will display how much fuel has actually been consumed from the known starting point of 300 gallons.
Assuming the method above was followed, we can check the trip information on our engine gauges. On the gauges shown, we can see 260.8 gallons have been USED from the last time the tank was filled to its capacity of 300 gallons.
300 - 260.8 = 39.2
300 minus 260.8 means only 39.2 gallons remain in the tank. We need to fuel up before our voyage.
Notes:
Actual fuel consumption may vary from the manufactures tables and can be adversely impacted by wind, load and current. It is important to confirm your actual fuel consumption rate, and adjust accordingly.