Winterizing Your Boat's Fuel: Stabilizer, Full Tanks and Keeping Water Out

Some Northwest boats cruise all winter. Others sit at the dock or on a rack until spring. Either way, a boat that sits for a while needs its fuel looked after, and the advice you hear on the dock doesn't always agree. "Top it off." "Run it dry." "Just add stabilizer."

Here's what the engine makers say, where they disagree, and the few things everyone agrees on.

Gas boats: what ethanol does over a winter

Nearly all gas sold at US roadside stations is E10, 10% ethanol, and many fuel docks sell it too. Ethanol absorbs water. A little is fine: the EPA says E10 can hold up to about 0.5% water at 60°F, roughly 3.8 teaspoons per gallon, before the water and ethanol drop out of the gas and settle to the bottom of the tank. That's called phase separation, and the colder the fuel, the less water it can hold.

If that layer reaches your fuel pickup, Mercury warns it can cause "poor performance or even damage," and the gas left on top has a lower octane rating. Once fuel has separated, BoatUS says no additive or stabilizer can fix it. It has to be removed by a professional.

In one ethanol-industry-funded lab test using small lawn-mower tanks in a warm, humid chamber, the gas went stale before it separated. That's one small test, not a boat tank in a Northwest winter, but it's a reminder that old fuel is a problem in its own right.

One more gas rule: don't put E15 in your boat. The EPA has approved E15 only for 2001-and-newer cars and light trucks and flex-fuel vehicles, not boats. Watch for it if you fill a trailer boat at a roadside station.

Fill the tank, or leave it low?

This is where the advice splits, and it depends on your engine and your tank.

WhoWhat they say
Mercury (gas), older vented tanksFill to about 95% with fresh fuel. Leave room, because fuel expands and can push out the vent.
Mercury (gas), most newer boats with low-evaporative-emission fuel systemsThese tanks don't vent freely and "can be safely stored with a low fuel level, but any fuel remaining in the tank should be treated with stabilizer." Not sure which you have? Ask your dealer.
Yamaha (outboards)Store 7/8 full.
BoatUSLeave a built-in gas tank nearly full; empty portable tanks.
Yanmar (diesel)Drain the tank completely or fill it.
Volvo Penta (diesel; D-series operator's manuals)Drain water and contaminants, then fill completely.

What we'd do: follow your engine or boat maker's manual first. If you fill, leave room at the top. The BoatUS Foundation and Washington Ecology both say not to top off; the BoatUS Foundation suggests about 90% so fuel has room to expand. If your boat lives on a dry-stack rack, check your marina's fuel-storage policy.

Whatever you do, never plug or cap the fuel vent.

Stabilizer: add it to fresh fuel, then run the engine

A stabilizer slows the breakdown of gasoline. A few things make it work:

  • Treat fuel when you pump it, not after it's gone stale. Stabilizer is made for fresh fuel, and once gas has phase-separated, no additive can fix it. Mercury also suggests storing with fresh, preferably ethanol-free, fuel.
  • Dose it the way the bottle says. Doses differ by product. STA-BIL 360 Marine calls for 1 oz per 10 gallons. Mercury Quickstor and Star Tron Stabilizer+ call for 1 oz per 5 gallons. Star Tron's gas stabilizer isn't meant for diesel.
  • Run the engine 5 to 10 minutes afterward, in the water or with the cooling-water hookup your manual specifies, somewhere well ventilated, so treated fuel reaches the whole system, not only the tank.
  • Know the limit. STA-BIL itself says gas stored more than 12 months loses volatility, and "NO additive, STA-BIL® brand included, will prevent volatility loss". Stabilizer buys you a season, not forever.

Ethanol-free gas helps if you can get it. It's sold at some Northwest docks, including Tacoma Fuel Dock and Blakely Island in the San Juans, but not everywhere, and winter hours vary, so call ahead.

Diesel boats: it's all about water

Diesel doesn't have the ethanol problem. Its winter enemy is water, because water in the tank is where microbes ("diesel bug") grow. Chevron puts it plainly: "The most important preventative step is keeping the amount of water in a fuel storage tank as low as possible, preferably at zero".

Before layup:

  • Drain the water separator and filter bowls, and drain water and sediment from the tank if your boat has a sump drain (from Yanmar's storage and maintenance steps).
  • Change the fuel filters if they're due on your engine's maintenance schedule.
  • Biocide is mainly for a known problem. Chevron suggests it once microbes reach problem levels. If you use one, follow the label: Biobor JF, for example, goes into a full tank, or in while you fill it, never an empty one. A heavy growth may need the tank cleaned, and water drained from a treated tank is toxic, so dispose of it properly.

Do you need a diesel stabilizer for one winter? Chevron says storage stability "should not be a concern for the majority of diesel fuel users," and suggests a stabilizer for storage of a year or more.

Keep water out: check the easy ways in

Engine and boat makers recommend a nearly full tank mainly to limit condensation: an emptier tank "breathes" damp air through its vent as temperatures swing, and some of that moisture can condense or be absorbed by E10. How much water this adds is less clear. An empty, vented tank kept in a barn on the Maine coast stayed bone dry at every check over more than eight years, and an engineer's worst-case estimate came to a fraction of an ounce a day, adding that if you find more water than that, look for another way in. Both are single tests or estimates, so they don't overrule your manual.

Whatever you decide about fill level, these are worth a look in a wet Northwest winter:

  • the deck fill cap and its O-ring,
  • the vent fitting on the hull, where spray and washdown can reach it,
  • and rainwater around the fill when you open it.

Don't let fuel reach the water

Winter maintenance means bilges, filters and fuel cans, which means spills.

  • A sheen counts. Under federal rules, oil that leaves a film or sheen on the water counts as a discharge that "may be harmful," and it has to be reported.
  • Never pump oily bilge water overboard, and never use soap or detergent to break up a sheen. Both are illegal. Oregon's boating agency also says not to use bilge cleaners, which only spread the oil out.
  • When you fuel, use an absorbent pad or collar at the nozzle, and stop before fuel comes out the vent.
  • If fuel or oil gets into the water, report it right away to the National Response Center, 1-800-424-8802:
    • Washington: also call Washington Emergency Management, 1-800-258-5990. Ecology says, "There are no penalties for reporting a spill unnecessarily, but there may be significant penalties for not reporting one".
    • Oregon: any amount of oil on the water is reportable. Also call OERS, 1-800-452-0311.
  • Used absorbent pads: Oregon's boater guide says a gasoline-soaked pad can be reused once it dries. Seal a diesel- or oil-soaked pad in two plastic bags and leave it by your marina's oil collection container, or take it to a household hazardous waste collection event. Washington Ecology says to dispose of pads properly, so ask your marina or county what they accept.

The short version

  1. Check your manual first. Fill levels differ by engine and by tank.
  2. Gas: fresh fuel, the right stabilizer dose, run the engine 5 to 10 minutes. Fill to your maker's level, usually nearly full with room to expand, unless your maker says a low tank is fine.
  3. Diesel: drain the water separator, check the filters, keep water out.
  4. Everyone: never plug the vent, check the fill cap seal, and keep fuel out of the water.

When spring comes, Pacific Northwest Fuel Mate™ can help you plan the first run of the season, from how much fuel the trip needs to the fuel docks along the way. Plan your next cruise →

This post is general information. Your engine's and boat's manuals come first.

Sources: EIA: Almost all U.S. gasoline is blended with 10% ethanol · EPA: Water Phase Separation · NREL water-uptake study (RFA) · DOE AFDC: E15 · Mercury: Fuel basics · Mercury: Stabilizing fuel · Yamaha: Extended storage · Yanmar 6LPA manual · Volvo Penta D1-30 Operator's Manual, laying-up section · BoatUS winterizing quiz (NSBC) · BoatUS Foundation: Fueling tips · STA-BIL 360 Marine · STA-BIL Storage · Star Tron Stabilizer+ · Chevron Diesel Fuels Technical Review · Biobor JF label (EPA) · Discover Boating: Winterizing your boat's fuel tank · MarineHowTo: Does an empty fuel tank condensate? · Charles O'Neill: Water condensation in fuel tanks · 40 CFR 110.3 · 40 CFR 110.4 · 33 CFR 153.203 · WA Ecology: Report a spill · WA Ecology: Clean, green boating · Oregon Clean Boater Guide · Oregon Clean Marina Guidebook · Tacoma Fuel Dock · Blakely Island General Store

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