Why Should a Boat’s Gas Tank Never Be Completely Filled?

Depending on the type of boat you have, you will either use ethanol-free gas, ethanol gas, or diesel fuel. The majority of boat owners with average-sized or small boats use ethanol gas because this is the most widely accessible form of gasoline. If you are using ethanol gas, it is important to know how high to fill the gas tank because ethanol gas can undergo phase separation or experience water contamination.

Never fill up your boat’s gas tank more than 90% full to give the gas room to expand and to prevent phase separation. The empty space limits the amount of air that flows through the vents which reduces the likelihood that warming temperatures cause condensation, and ultimately phase separation.

Read on to find out why you shouldn’t fill up your boat’s gas tank completely, what phase separation is, and how to maintain the gas inside for the best performance possible.

Why Shouldn’t I Fill My Boat Gas Tank Up Completely?

There are two main reasons why you shouldn’t fill up your boat’s gas tank completely. The first is to allow room for the gas molecules to expand. Eventually, the temperature will fluctuate, and when the air grows warmer, the gas expands in the fuel tank. This can cause overflow out of the vents and dangerous gas vapors to escape. You can also get a hefty fine for gas leaks in water.

Second, you don’t want to fill up the tank completely because the fill hose is not designed to store gas, which it would have to do if the tank was filled up all the way as the fuel will slosh around when you are driving. Leaving the tank 90% full will keep your fill hose clear and prevent gas overflow.

How High Should I Fill My Boat’s Gas Tank?

Before storing your boat, you should leave the gas tank 90% full with a fuel additive. Leaving the tank nearly full gives the gasoline room to expand and prevents phase separation by regulating the amount of air traveling into the fuel tank and ultimately decreasing the chance of condensation.

What is Phase Separation in Boat Gasoline?

Ethanol is known for its ability to absorb water ten times more than regular gasoline while still being able to run successfully through the engine. If too much water gets absorbed, however, then a process called phase separation occurs.

Phase separation, specifically, is when the ethanol-gasoline becomes contaminated by too much water. The ethanol will then attach to the water molecules and separate from the other type of gasoline. This leaves two layers of liquid, gasoline at the top and an ethanol-water mixture below. The gas will then be useless.

Can I Reverse Phase Separation?

No, you cannot reverse phase separation once it has happened. There are a lot of products out there claiming to reverse the separation, but it isn’t possible. If it occurs, you should properly drain the fuel tank completely before replacing it with new gasoline.

What Type of Gas Should I Use for My Boat?

While experts recommend using ethanol-free gasoline for your boats, this is extremely difficult in most cases because 98% of gasoline in the U.S. contains ethanol in order to reduce air pollution.

Gasoline will usually contain 10% or 15% ethanol (E-10 or E-15). Never use E-15 gas because it will damage your boat engine and most likely destroy it.

There are many ways that E-15 gas can damage your boat engine. It can damage the fuel hose, rubber parts, and metal components. It also increases the amount of oxygen in the fuel, which makes the engine run hot and causes carbon buildup, overheating of the valves, and eventual engine failure.

Any risk that comes with using incompatible gas with your boat engine will occur using E-15 and will most likely void your boat warranty. When you are pumping gas, be extremely careful to put E-10 in instead of E-15.

All boats that have been built in the past approximately 10 years can safely and successfully use E-10 gas. Refer to your owner’s manual for fuel requirements to be sure.

You should always consult your owner’s manual and your boat dealer or expert for recommendations on filling your gas tank, but the general rule is to only fill the tank to 90% full.


Should I use a fuel additive when storing my boat?

It is recommended that you use a fuel additive before storing your boat or if you don’t use it very often. You can find the types of additives suggested for your boat in the owner’s manual. There are engine cleaners, ethanol neutralizers, and fuel stabilizers you can add to the tank according to the different needs and frequency of use of your boat.

How long does E-10 gas last in my boat’s fuel tank?

If you don’t treat the fuel, E-10 gas will only last 3 months in the tank before it separates and becomes useless. If you properly treat it with recommended additives, the fuel can last 1-2 years.

Is it okay to leave my gas tank nearly empty?

Some boaters decide to leave their gas tanks nearly empty so they can “refresh” the tank with new gas later on to replenish octane levels. However, if there is less gas in the tank, then there is more air flowing in that can bring moisture into the tank with it. This speeds up the process of phase separation that ultimately renders the little fuel in your tank useless. Too little gas means there won’t be enough ethanol in the tank to absorb all of the moisture carried in with the open air.