The best way to find out how much fuel you have is to fill it up to the top and then you may surmise you have around 9 gallons. Like py26129 suggests the best way of keeping track of fuel usage is to track fuel consumption over measured time. What diesel engine do you have? A Yanmar 1GM burns about 1/4 of a gallon per hour (GPH), a 2GM about 1/2 GPH and a 3GM about 3/4 GPH. The best instrument to measure running time is an "hour meter" that is connected to the key switch and it starts the meter when the key is turned on to start the engine and switched off after engine shutdown. The other component is the engine burn rate; I have provided starting figures for 1,2 and 3 cylinder Yanmar engines but everyone should calculate the burn rate of their individual engine. To keep it simple we calculate the average burn rate according to our usage so the figure is an average which includes how long we idle, how long we run at slow speed and mostly the consumption at maximum cruising RPM. This average gives good enough results for our needs. After you top off the tank run about 5-6 hours and then top the tank again; the meter at the pump will tell you how many gallons were consumed and dividing by the hours operated you will get your first GPH calculation. Repeat the process every time you fill up and pretty soon you will find a concentration of results which will define your average burn rate to be used for calculations. It is easy to round off results to 1/4, 1/2 ,3/4,1 gallon to facilitate mental calculations. I keep a log of the date I top off the tank and the number of hours in the hour meter. I make a habit of when I get on board to read the hour meter and then I go to the log and subtract the difference to know how many hours I have ran since my last fill up. Lets say I have ran 6 hours and my engine burn rate is 1/2 GPH I can deduct I have burned 3 gallons. If I know my tank holds 25 gallons I know I have 22 gallons in the tank. Another fact is that not all the fuel in the tank is usable; at some point depending on the shape of the tank and the condition of the seas the fuel pump will start sucking air before the tank is empty. In disturbed seas low fuel will start sloshing around the tank interrupting the pumping action. It is always good seamanship to maintain a fuel reserve to include the non usable amount. Many reserve between 15% and 20% for emergencies. So out of your 9 gallon tank you may consider to have 7 usable gallons. This information can be used to predict and calculate range on a trip but it has to be done on the water during the trip to account for the prevalent conditions. If conditions change and you recalculate range and see that you do not have enough fuel to reach destination you may divert to a refueling destination or await until conditions change. Depending on wind and currents a boat can travel 1 mile in one hour or travel 8 miles in one hour but it will burn the same amount of fuel. Knowing the usable capacity of your tank, the burn rate of your engine,and the time since the last fill up will allow you to calculate how much fuel you have left at any time. How far you can go in the fuel requires on the water measurements of speed (knots) nautical miles per hour. If you are averaging a steady 4.5 knots then you can calculate that you will have covered 4.5 nm in one hour after a fuel consumption equal to your average burn rate. These are not exact figures but they have been proven to be "good enough" for a sailor's needs. I can tell you I run average figures and have never ran out of fuel and every time I top off I'm not off by more than 1/4 to 1/2 gallon and usually to my good. You could keep time with a watch but it is easy to loose track and hour meters are relatively inexpensive and easy to install. I prefer the analog type vs the digital. Had a fuel gauge and it only could be reliable to tell me if there was some fuel or none. Hope this helps.
