Skip to main content

Your Boat's Fuel in Winter

  • Author:
  • Updated:

Fill ’Er Up?

With the ethanol debate continuing to rage, should you gas up your boat before winterizing or leave the fuel tank empty?

If you live in a part of the country that gets cold this time of year and you own a gasoline-powered boat, your mechanic (or whoever it is that does your annual winterization) is probably going to ask you at some point to fill the gas tank to at least 90 percent capacity. With gas prices at close to $5 per gallon at most marinas, that’s a major chunk of change for most boat owners.

Click to enlarge

So the question may reasonably arise: “Should you?”

I live in Maine and when I get ready to winterize my boat, which is powered by a MerCruiser 350 Mag MPI, I run the fuel to below one-quarter of a tank and add a little more than the recommended amount of Sta-Bil fuel stabilizer and Star Tron ethanol treatment to the remaining fuel. I also remove the fuel-water separator and pour out some of the gas in the filter, replacing it with some Star Tron and two-stroke engine oil to keep the fuel injectors in the engine lubricated. Then I complete the winterization process. I’ve had the boat for six years and have had no fuel-related problems.

My boat is a model-year 2000 and, according to my MerCruiser owner’s manual, I’m following the engine manufacturer’s recommended approach by running down the fuel to nearly empty. In a September 2011 webinar, however, MerCruiser reversed its position and now recommends that boats be stored with fuel tanks at 95-percent capacity. The American Boat & Yacht Council (ABYC) and many state fire marshalls’ offices also prefer that boats be stored with tanks at 95 percent full so fumes don’t accumulate in an empty tank, creating a chance for an explosion if there’s a spark.

At service facilities in New England, the answer to the full or empty question varies. Tim Dorr of Dorr’s Marine Engines in Windham, Maine, is certified on both MerCruiser and Volvo Penta products. He likes the fuel level at between one quarter and one half and he treats the remaining gas. Port Harbor Marine, which has multiple locations in Maine, requests tanks be as close to empty as possible. And at the Brewer group of marinas, which stretches from New York State to Maine, they take yet another tack. The service departments recommend filling the tanks to 95-percent capacity with ValvTect Marine fuel. Brewer purchases treated fuel from ValvTect Petroleum Products ( in Northwood, Illinois, in bulk. ValvTect president Jerry Nessenson says his company’s Marine Gasoline Additive stabilizes fuel for a full year. 

Of course, the main reason for concern over gasoline stored over long periods can be synopsized in a word: ethanol. The label E10 means that a particular gasoline has 10-percent ethanol and E10 is approved by the National Marine Manufacturers Association. The more highly concentrated E15 is not approved, though. 

In any case, the problem with fuel that contains ethanol stems from a process called phase separation. When gasoline that contains ethanol sits unused for long periods the ethanol absorbs water and the alcohol content of the fuel increases. Gasoline bought at land-based stations remains stable for no more than 90 days before it breaks down. Because water is heavier than gasoline, it sinks to the bottom of a boat’s gas tank. Come spring, when you start your engine and the fuel pump starts drawing what is supposed to be gasoline, it’s sucking in the water.

Even when fresh, E10 gasoline tends to contain a small amount of water (an ounce or two per gallon) but most engines can still burn the stuff consistently. Phase-separated gas, on the other hand, can contain up to 90 percent alcohol and an engine designed to run on gasoline doesn’t work so well when trying to burn fuel with a high alcohol concentration.

“Of all the people who have lost expensive engines, I’ve never run into one who was using 10 percent (ethanol) or less—they were all constantly running phase-separated gas, which is 60 to 90 percent alcohol,” says Gail Macri, president of MLR Solutions of Hampton Bays, New  York, one of the earliest companies to offer ethanol test kits. Macri originally planned to make ethanol test kits available on a temporary basis, but has since sold thousands to hosts of businesses, from gas stations to dealers for boats and snowmobiles. You can order the company’s most popular kit (AFTKD15-01 for $31.95 plus shipping) at And for about $8 you can get the Quicksilver 91-879172T Ethanol Tester through your local Mercury dealer. Quicksilver is the accessories division of Mercury Marine.

This article originally appeared in the December 2012 issue of Power & Motoryacht magazine.