• The best practical recommendation is to continue to top off a boat’s fuel tanks to about 95% full, leaving room for expansion. A tank that is almost full limits the flow of air into and out of the vent, which reduces the chance of condensation adding water to the fuel. Anglers who fish over the winter should also top off their boat’s gasoline tanks between outings to prevent condensation.

Note that some mechanics mistakenly advise that leaving a tank partially filled allows you to “freshen” the old fuel by topping off the tank in the spring. Leaving a tank partially filled with E-10 invites phase separation, which cannot be remedied by adding fresh gasoline. Once E-10 phase separates, the water will remain at the bottom of the tank. Midwest marina owners report that phase separation typically occurs when boats were stored with tanks only one-quarter to one-half full.

• Draining fuel tanks of E-10 gas, while completely eliminating any chances of phase separation, is potentially dangerous and not recommended.
• Once phase separation occurs in E-10 gasoline, additives and water separators can’t help. The only remedy is to have the gas and ethanol/water professionally removed from the tank.
• Ethanol is known to chemically react with fiberglass fuel tanks, which can cause them to deteriorate and potentially fail. This is most common with tanks built before the mid-1980s. Unless your boat’s manufacturer can confirm that your tank was built to withstand ethanol, the only remedy is to not use E-10 gas (which may not be possible) or to replace the tank with a non-reactive material such as aluminum.
• While ethanol does attract moisture, never try to plug up a fuel tank vent to prevent moist air from entering a tank. Without room to expand, the additional pressure could rupture fuel system components.
• With any fuel that sits in a tank for a long time, it’s important to add a stabilizer. But understand that stabilizers do not prevent phase separation.