Boat Fuel Tank: Full or Empty Before Storage?

Is it better to fill your boat’s fuel tank before storing it, or should it be left empty? 

The debate over whether or not to fill the fuel tank on a boat’s engine before an extended storage period is common among boaters. When it comes time to store or winterize your boat, here are some things to keep in mind when deciding whether to fill up or empty out the tank.

Modern Boat Fuel Has Changed

The argument for an empty fuel tank is largely based on defunct fuel formulas and therefore outdated. 

Fuel containing sulphur and lead is long gone, and today’s marine fuel has a complex chemical makeup that includes renewables such as ethanol. This is great for the environment and our health, but not so great for your boat’s fuel tank. The trouble is that ethanol is hygroscopic, which means it attracts water molecules, and it’s also hydrophilic, which means it mixes with and absorbs water. This isn’t a problem during boating season since the fuel is regularly used and replaced. When a boat’s fuel tank is active, the ethanol within the fuel doesn’t have time to absorb much water. And the tiny amount it does absorb is burned along with the fuel in the engine’s combustion chambers.

However, when a boat's engine is left idle during storage, the ethanol keeps absorbing water until it reaches saturation, at which point the fuel undergoes what’s called “phase separation.” When phase separation occurs, the water/ethanol mix within the fuel settles on the bottom of the fuel tank. This layer of water/ethanol will corrode a metal fuel tank, and can get sucked into the fuel system when the motor is next run, which then requires injectors or carburetors to be drained and cleaned.

Preventing Phase Separation

Minimizing the amount of water that ethanol fuel comes into contact with reduces the amount of water it can absorb, and therefore the possibility of phase separation. 

The most abundant source of water for ethanol fuel is condensation, which will form on any flat surface, particularly in cold weather. This includes the inner walls of your boat’s fuel tank. The exposed surfaces of a partially filled fuel tank will condensate, which enables the ethanol fuel to absorb water and results in phase separation. Filling the fuel tank when winterizing or storing your boat eliminates exposed surfaces for condensation to form on, and reduces the likelihood of phase separation.

Knowing all of this, you could say the debate is resolved and we can all agree that it’s better to fill your boat’s fuel tank before putting it into storage. Just add a marine-specific fuel stabilizer to the fuel after filling it up to keep it fresh for the months ahead.