If your boat did not come with a fuel gauge I would not bother installing one as they usually inaccurate and unreliable. Invest in a cheaper "Hour Meter" that when connected to the Key Switch will measure engine run time. It is always recommended that you calculate The Average Burn Rate for your engine in Gallons per Hour. You fill up the tank and after running the boat for a few times you go to top off the tank again. The meter at the fuel pump will tell you how many gallons you had used and the hour meter will tell the time it took to use those gallons so you will have that your burn rate = Gallons/Hours. A Yanmar 2GM will burn a little less than 1/2 Gallon per Hour and this would be a good starting point until you develop your own average burn rate. Prepare a log and include the date the tank was topped off, the reading on the Hour meter, the number of gallons taken in and the calculations for the burn rate. Make an entry every time you top off and pretty soon you will have narrowed down a pretty consistent number for the burn rate. Now it becomes real easy and very accurately to tell how much fuel you have left in the tank at any given time. One more piece of information that you need is the size of the fuel tank in gallons. Be aware that not all the fuel is usable as the engine will start sucking air and die before the tanks is completely empty. It is always prudent to create a reserve of 20% of the total tank's capacity for emergencies and to cover the unusable fuel capacity. For example if you have a 30 gallon tank consider you have a maximum of 24 gallons available for travel use and forget about the other 6. Ok, The way you can tell how much fuel you have left is to glance at the Hour Meter and the log and calculate how many hours you have ran the engine since last fill. Then multiply your burn rate by the number of hours, (example if your BR is 1/2 Gallon per hour and you have ran 6 hours you have used 3 gallons) Then you substract the number of gallons used from the total usable capacity of the tank or in this case 24 gallons minus 3 will tell you have 21 gallons left. This gets pretty accurate and I can tell you most can predict how much fuel they will take on at the filling to perhaps a 1/4 to 1/2 gallon accuracy. It is always best to simplify calculations so that you can conduct them mentally. Round off your burn rate to the nearest 1/4 GPH and you Hour readings to the nearest full hour. An d then it become a simple Gallons divide by 4 or 2 whichever is the case. Once you know quite accurately how much fuel you have on board you are ready to set Range. Range is calculate on the water as it is dependent on prevalent weather conditions. If you are averaging 4.5 knots and you know you have 20 gallons left and you know you burn 1/2 GPH then you know you have 40 hours of fuel and at 4.5 Knots you could cover a distance of 180 Nautical Miles. Since conditions change the Range should be recalculate once every hour. Once you start your log and practice the calculations you will find they become second nature and will give you the confidence to know you should never run out of fuel or run to the fuel dock unnecessarily. Before I forget the Average Burn Rate is independent of engine RPM, condition of the hull, currents or wind. The only factors are Gallons and Hours. Condition of the hull, currents and winds will influence boat speed which is the dominant factor to Calculate range and that is why Range should be calculated on the water and recalculated at least once every hour. Also always know the distance to your closest refueling point. Know how far can you go and know when you need to alter course to your refueling point.
