As winter approaches, a common question from boaters is whether it is better to store a boat with a full or empty tank of gas.  The short answer is “yes” – just be sure to not do in between.  The issue stems from the usage of ethanol in gasoline and something called “phase separation” that can cause serious damage to your marine engine.

What is Phase Separation

Source: BoatUS

The vast majority of gasoline used today, even in boats, contains ethanol.  Under normal usage, this is not a problem and the addition of ethanol to gasoline actually boosts the octane level.  The problem occurs when the fuel is stored for a considerable period of time.  This is because ethanol absorbs water.  When the amount of water absorbed becomes too great, then the gasoline + ethanol blend separates into two separate “phases” or components – the gasoline phase and a water/ethanol phase.  The heavier water/ethanol component sinks to the bottom of your tank where it can cause a host of issues.

You might be wondering where the water comes from.  It naturally forms from the environment.  Marine fuel tanks are vented and so they breath in outside air.  When the air inside your tank cools to the dew point, condensation forms.  Or when your cold fuel tank hits warm and humid air, the same thing can occur – like taking a cold beverage out of the refrigerator and setting it on the counter.

The problem is especially acute in humid environments because of the high moisture content.  But cooler, drier locations are not off the hook either because cold air is less capable of holding moisture and so is more likely to condensate.

What Harm Can It Do

When the ethanol/water combination drops out, it takes some of the octane with it reducing the overall octane level in the remaining gasoline portion.  Your engine might run rough and not be able to achieve the proper power when this occurs.

Even worse, the ethanol/water mixture might be drawn into your engine through the fuel intake near the bottom of your fuel tank which could cause severe engine failure or simply prevent your engine from starting.  This mix is also highly corrosive which could deteriorate parts of your fuel system.

How to Prevent Phase Separation

If you are using your boat and consuming fuel regularly, phase separation should not be a problem.  However, if you are winterizing your boat or don’t plan on using it for an extended period (say 2-3 months or more), then you need to take precautions.  The following are alternative ways to reduce the chances of phase separation from occurring.

Only Use Ethanol-Free Gas

The problem occurs because ethanol molecules absorb water.  If you eliminate the ethanol, the issue is largely solved.  However, you should still treat your fuel with a stabilizer to keep it from going “sour” since the octane level will degrade over time.

This might not a practical option for many boaters because ethanol-blended gasoline is much more plentiful and more economical than pure gasoline.

Completely Empty the Tank

And by empty, I mean completely empty – not just low. This way, you eliminate the possibility of ethanol combing with water from the atmosphere.  Problem is, unless your boat uses small, portable gas tanks, this is difficult to do.  Even if you run the engine until it quits, there will likely still be fuel in the tank depending on the location of the fuel pick-up line.

Fill Your Tank to 95% Full

Perhaps the most practical solution, and the one recommended by Mercury and many others, is to fill your gas tank to 95% of capacity (to allow for fuel expansion).  By doing so, you minimize the area inside your fuel tank for condensation to form.

Mercury also recommends adding an ethanol treatment to your fuel and running the engine for 10 minutes so that it has a chance to work through the system.  There are many ethanol treatments available.  One product that I have had good success with for years with my two boats is Sta-Bil Marine Treatment.  I simply add 1 oz per 10 gallons every time I fill up to minimize the chances of phase-separation from forming (especially since, once this occurs, there is no way to reverse the problem without completely pumping out your tank).

But under no circumstances should you store your boat with a partially filled gas tank for an extended period of time (even with fuel treatment).  If you do, there is a good chance your engine will not be happy with you next Spring.  Nor will your spouse when he/she sees the repair bill.

Please note that I provided a link to a product that I personally use (Sta-Bil Ethanol Treatment) for your convenience.  If you click on the link and buy the listed product or any other, I will receive a small commission that will help support BoaterInput.


Frequently Asked Questions: Fuel & Fuel Systems.” Web Blog Post.  Mercury Marine.

Riechers, Mark.  “Ethanol, E10, E15.  The Facts, The Myths, and The Politics.” PowerPoint Presentation.  Brunswick Corporation.  November 2016.



Leung, Anita.  “Boater’s Guide: Choosing the Right Fuel Treatment to Combat Water in the Fuel.” Web Blog Post.  Chevron Lubricants.  8 November 2021.

Jerry Mona - BoaterInput

About the author

Jerry Mona is an avid boater and angler and long-time boating industry insider. With over three decades of experience, he is often considered to be the leading research expert with boaters and has helped numerous manufacturers and trade associations to understand the needs, wants, attitudes and behaviors of boaters. He now shares many of his insights about boats and boaters for free on his website.

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