Keep your marine fuel tank full

We’ve all heard it’s a good idea to keep our marine fuel tanks full, but do you know the reason why?

A large quantity of fuel acts as a heat sink, which means the fuel takes a long time to heat or cool enough to match the ambient temperature. While the outside temperature might vary 20 or more degrees daily, the fuel temperature might only vary three to five degrees.

Air taking up unused space in your tank expands and contracts with the temperature change, pushing out through the vent as the day warms and pulling outside air back in overnight as the temperature cools. When this happens, the ambient air entering the tank touches the walls; any moisture in this air condenses onto tank walls and drips into the fuel.

Warm air holds more moisture than cold air. Your fuel is almost always colder than the outside temperature, so any air drawn into the tank contains more moisture than it can hold inside the tank. That extra moisture ends up in your fuel. Boat engines don’t run well on water, and removing it can be a problem.

The easy solution is to keep the fuel tank full, leaving little space for air to go in or out and little tank surface for condensation to accumulate. If your boat uses ethanol-blended fuel, moisture combines with the ethanol and becomes a mess in the bottom of your tank. The current wisdom is to drain the tank for the winter when using ethanol-blended fuel. –Ron Micjan