[Fr. Frank]

Let me begin by making a statement: SeaTow and TowBoatUS make a great deal of money off people who don't monitor their fuel flow and instead depend upon their (often faulty) fuel gauges. Similarly, as a Civil Air Patrol/US Air Force Auxiliary captain I can tell you that fuel exhaustion causes a great many aircraft to stop flying unexpectedly, often with fatal results. Far too often the cause is pilot error, i.e.; the pilot just looked at his gauges and payed no attention to fuel flow in comparison with flight duration.

You need to know your actual useable fuel capacity and your fuel burn rates at a variety of speed/power settings.

My 1972 20' SeaCraft Seafari came equipped with what the SeaCraft sales brochure said was a 30 gallon fuel tank. Even after running the tank "dry", I was never able to get more than 27.4 gallons of gas and 1/2 gallon of oil into the tank, for a net 27.9 gallons usable fuel.

When I replaced the tank with a new one made using the old tank as the template, it held 27.2 gallons of pre-mixed fuel when I filled it up for the second time. My new tank was labeled by the manufacturer, RDS, as having 27 gallons capacity, even though it held 30.3 gallons on its initial fill up. When I asked about the difference, they indicated that they are in the process of beginning to label their marine tanks with the usable capacity to avoid confusion.

At any rate, I know my best fuel burn and my average fuel burn, not from a Navman, but from doing the following:

Buy a 6 gallon portable tank or two. Run your boat in normal safe operation, (including normal repeated acceleration) to see how long it takes to exhaust the 6 gallons. Refill the tank and do it twice more. Then refill and run at cruise speed and WOT (or 95% power minimum) and calculate fuel burn at those settings.

Know your fuel burn. It might save your life, and the lives of people you care for.