IT IS VERY EXPENSIVE TO RUN OUT OF FUEL!

The Coast Guard won't help, they will refer you to a towing service. The towing service will come to your location to deliver fuel and you will pay for this service. It will be the most expensive fuel you ever purchased.

USPS encourages boaters to observe the "One Third Rule".
One third to get there,
One third to get back
and
One third for reserve.

Of course we realize that you may need more than one third to get there, but in that case never let your gas tank get below one third full before refueling, keeping intact the one third for emergencies. Always find out where you can get fuel before you leave on a long trip. When sea conditions and traffic permit try to cruise at the speed that yields maximum range.

The graph to the right illustrates the principle of finding the RPM that will yield maximum cruising range. Fuel consumption is a function of the throttle setting and so is speed.

The ratio of throttle setting and fuel consumption is the Range Factor.  As the throttle is advanced, MPH, GPH and MPG increase. As you continue to open the throttle you will reach a setting where MPG achieves a maximum value. This is the setting that will achieve maximum cruising range.

Continuing to advance the throttle increases the speed but at the cost of an exponential rise in fuel consumption; MPG decreases rapidly in this speed range, with consequent loss of range.
RPM 1000 1500 2000 2500 3000 3500 4000
GPH 2.2 2.5 2.9 3.3 4.0 5.8 8.9
RF 455 600 690 760 750 600 450

Of course, all boats are different and it is up to you to determine this valuable information for your vessel. To do this you need a fuel flow meter. This can be a temporary installation to get calibrated, maybe you could borrow one. In calm sea conditions, make a table, similar to that above, of engine RPM vs. fuel flow using values suitable for your boat. In the third row calculate the Range Factor (RF) by dividing RPM by GPH.

The highest RF number will give you the engine RPM for maximum range. Your actual range will depend on how much fuel you have and the boat's speed at best RPM. Remember that the best cruising RPM will vary with load and sea conditions. Also, the condition of your motor can affect it's fuel efficiency so keep it properly maintained. When making the table, you could use speed instead of RPM, but RPM is more reliable.