Estimating Fuel Consumption
We are often asked how we calculate our boats fuel consumption on an older boat. Older boats may not have engine monitors that do the calculation automatically and retro fitting an older engine with fuel flow monitors can be expensive, even more so for a diesel engine where you also have to account for return fuel flow. Knowing the total fuel a vessel carries there are few calculations to use, gallon per mile (GpNm), gallon per hour (GpH) and Speed.
In order to get these numbers the use of a trip log needs to become a habit. Things that need to be logged are, engine hours for each engine, generator hours for each generator, nautical miles (Nm) per trip and amount of fuel pumped during fill-up.
Start the log by filling the fuel tanks completely and before starting the engines or generator, log the hours for each. Starting from the point of fill up, track the nautical miles for each of your trips until you fill up again. First let's get the GpNm.
Fuel Filled / Nm = GpNm
If we pumped 250 gallons of fuel and traveled 100 miles, then we burned 2.5 gallons per mile. After completing three trips, we can get a basic estimation of your range by dividing the overall fuel capacity by the average of the miles per gallon calculations.
Trip 1 - 250 gallons pumped - 100 Nm = 2.5 GpNm
Trip 2 - 100 gallons pumped - 120 Nm = .83 GpNm
Trip 3 - 200 gallons pumped - 90 Nm = 2.2 GpNm
2.5 + .83 + 2.2= 5.5 divide by 3 and you get 1.84 gallons per mile. If you hold 400 gallons of fuel you should be able to get close to 217 miles out of full tanks. This is a basic calculation and the more data points you have to calculate the better. While speed is not calculated here, it is a factor as your fuel consumption will increase the faster you go. This is reflected in the amount of gallons used per mile, the lower the number the slower the speed, generally.
Figuring out gallon per hour is a little more complicated for multi-engine boats but not impossible. Again, speed is not used in the calculation as we will base this on the number of hours the engine is running, whether in idle or underway. To do this we need to log the engine hours of all sources of fuel consumption, port, starboard and generator. On your initial top off from the GpM example, log the engine and generator hours before starting the engines. This is your starting hours at the next fill up, log the hours again before starting the engines. It is not uncommon for port and starboard engines to have different hours and it will not affect the calculations. The difference in hours is usually from how you run the boat, like letting the port engine run longer because it take longer to get to operating temperature or longer run times during maintenance, etcetera.
Port engine starting hours - 1230
Starboard engine starting hours - 1330
Generator starting hours - 500
Trip 1 - 250 gallons pumped : port 1232 - Stbd - 1333 - Gen 502
Port hours 2 + starboard hours 3 + generator hours 2 = 7 hours
From the calculations we have 7 hours of total engine(s) runtime on 250 gallons of fuel. We average the engine hours by dividing by 3 for 2.3 hours total and then divide the gallons pumped by that to show we burned 108.7 gallons and hour. We will use a this method to get the numbers for the other trips to continue.
Trip 1 - 250g - 100Nm - 2.5 GpNm - 108.7 GpH (2.3hrs)
Trip 2 - 100g - 120Nm - .83GpNm - 12.5 GpH(8hrs)
Trip 3 - 200g - 90Nm - 2.2GpNm - 133.3 GpH (1.5hrs)
With this basic data we can now calculate the speed on each trip. This will not be hyper-accurate as we are calculating the generator hours as well but it does get us very close. To determine speed, use the formula of speed=Distance/Time. On Trip 1 on we traveled 100 miles in 2.3 hours for a speed of 43.5 miles an hour. Since we are using Nautical miles we need to divide the mph by 1.51 to get the knots. 43.5 mph/1.51= 28.8 Kts (knots).
Trip 1 - 250g - 100m - 2.5GpNm - 108.7 GpH (2.3hrs) @ 28.8 Kts
Trip 2 - 100g - 120m - .83GpNm - 12.5 GpH (8hrs) @ 9.9 Kts
Trip 3 - 200g - 90m - 2.2GpNm - 133.3 GpH (1.5hrs) @ 39.7Kts
You can use this data to help find the more efficient cruising speeds and distances. This should help you plan and extend your time on the water. In our trip and fuel log, these calculations are done for you by filling in the spread sheet. Use the button below to go to our files section where you will be able to download the excel file and access the tutorial for the sheet.