Fill Her Up?

It used to be common to fill fuel tanks before storing your boat, but that was before gas contained ethanol. What’s the best practice today?
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Bill Gardella

Rex Marine, Norwalk, Connecticut

Some folks think less gas is better, because ethanol in E10 gas absorbs water. But filling the tank leaves less space for moist air and less space for explosive vapors to accumulate, so it’s safer. You don’t want to fill the tanks all the way: If you fill them to 95 or 100 percent on a cool day in the fall, when it warms up in the spring, the gas can expand, run out through the vent and form a puddle on the ground under the boat. Our standard is to fill to 80 percent capacity. Same with diesel, but less for safety than to minimize condensation; all kinds of stuff can grow in water sitting at the bottom of a diesel fuel tank. If the fuel requirement for storing an engine isn’t explained in the owner’s manual, invest in a factory service manual, or have the yard do it. 

JB Turner

Front Street Shipyard, Belfast, Maine

Most boats we store for the winter are diesel. My recommendation is to always leave the tank as full as possible. If there’s a lot of moist air in the tank, water can condense and contaminate the fuel, which can lead to algae and bacteria growing. The less air over the fuel, the less condensation issues there can be. If the tank is full, it can’t add water to itself. That is really the issue over a long, cold Maine winter, assuming outside or cold storage. In general, we do not add anything to the fuel. There are certainly plenty of additives available, but over the years I worry that it just ends up affecting the injectors and other components of the fuel system. When a boat’s in the yard for longer-term storage, for renovation or a rebuild, we offload the fuel from the tanks into approved Intermediate Bulk Containers.

David Grochocki

ValvTect, Buffalo Grove, Illinois

Fill the tank to three-quarters so you don’t get condensation, and add a fuel treatment at the correct concentration. Some treatments use one dose for everyday use, but twice as much for long-term storage. ValvTect XP+ for gasoline, or BioGuard Plus 6 for diesel, are formulated to be effective for at least a year in all kinds of climates. XP+ prevents phase separation in ethanol gas, disperses moisture, guards against corrosion, and removes carbon deposits. BioGuard Plus 6 contains a biocide, moisture dispersant, corrosion inhibitor, cetane booster and lubricity improver to offset the effects of low-sulfur diesel. You can add these products to any fuel, or buy ValvTect marine gas and diesel, which have the additives already mixed in. When it’s time to store the boat for the winter, you’re all set.

This article originally appeared in the November 2019 issue.