Boat Fuel Additives Explained: What Stabilizers, Ethanol Treatments and Boosters Really Do

A stabilizer slows the chemical reactions that turn fuel into gum and varnish, which clog carburetor jets and injectors. Mercury and Yamaha both say their stabilizers keep fuel usable for up to about a year in storage when added to fresh fuel. Both makers recommend treating fuel regularly, not only at layup; Yamaha suggests treating every tank.

How to use one well:

• Add it to fresh fuel, ideally as you fill so it mixes. Treating old, stale fuel does not make it fresh again.
• For storage, run the engine about 10 minutes after treating, as Mercury describes, so treated fuel reaches the carburetor or injectors.
• If you fill portable cans, add the stabilizer before you fill the can.
• Don't count on a stabilizer forever. MasterCraft's storage instructions say even properly treated gasoline shouldn't be stored more than a year, and that adding stabilizer in spring won't undo gum or water that formed over winter. Its gasoline storage routine also adds a biocide alongside the stabilizer; follow your own engine maker's list.
• How full to store it depends on the tank. Mercury's fuel page says an older vented tank should be filled to about 95% (room for expansion) to cut condensation, while a newer sealed, EPA-style tank can be stored with a low level as long as the fuel in it is treated. Ask your dealer if you're not sure which you have.

Products sold as "ethanol treatments" are mostly stabilizers with extra corrosion inhibitors for E10. Yamaha also markets one of its products for protecting fuel system metals against ethanol-related corrosion. What they cannot do, according to both Mercury and Practical Sailor testing, is stop phase separation. In Practical Sailor's test of five well-known products, none prevented cooling-induced phase separation.

Your real protection against ethanol problems is: buy fresh fuel from a busy station, use ethanol-free gas when you can, keep water out of the tank, and use a water-separating filter. The ethanol fuel care guide covers all of this.

No additive makes E15 safe. Many pumps now sell E15 (15% ethanol), often cheaper and sometimes labeled "Unleaded 88." The EPA's E15 rules say it cannot be used in nonroad engines such as boats, and Mercury says its engines are built for no more than E10 because higher blends attack metal, rubber and plastic fuel parts. Read the pump label before you fill a boat or a portable can. MasterCraft warns its owners that E15 is now sold year-round, including through the summer boating season, so the risk of grabbing the wrong nozzle no longer goes away in June.

If your fuel sample shows a cloudy layer or water at the bottom of the jar, Mercury's answer is clear: remove the water, dispose of the bad fuel, clean the tank and start over with fresh fuel. Some products claim to "restore" separated fuel by absorbing the water back into it. Practical Sailor did not recommend routinely using high doses for that, especially in two-stroke engines, and engine and fuel makers say phase-separated fuel cannot be safely treated. Have a pro pump out the tank.

Many car-store gas dryers are alcohol based. Your E10 gasoline already contains ethanol, and marine stabilizer makers such as Yamaha advertise alcohol-free formulas for boats. Do not add car gas-line antifreeze to a boat tank unless your engine maker approves it. Small amounts of water are the water-separating filter's job; large amounts mean the tank needs pumping out and the source of the water found (see the fuel tank inspection guide).

Cleaners contain strong detergents that remove carbon and varnish from injectors, valves and combustion chambers. Mercury describes using its deep-cleaning treatment periodically during the season, and Yamaha sells one meant for every fill-up. Use the product your engine maker recommends, at the label dose and frequency. A heavily varnished carburetor still needs to be cleaned by hand (see the carburetor cleaning guide), and a badly carboned engine is a dealer job.

Use the octane your owner's manual specifies, measured at the pump. Practical Sailor points out that higher octane only helps an engine that is knocking because of its design; it does not add power to an engine built for regular. If your engine knocks on the correct fuel, have it checked instead of masking the problem with a bottle. (Westerbeke's FAQ does list octane booster as an option for its gasoline generators; as always, your own engine manual decides.)

Diesel fuel can grow bacteria and fungi wherever a little water is present, especially in warm weather when the boat sits. The signs are brown, gray or black slime in the filter bowl or on the filter element. Practical Sailor explains:

• Keeping water out (a good water-separating filter and a dry tank) is the main defense.
• Real biocides must be registered with the EPA as pesticides. Magnetic "bug killers" showed no effect in Practical Sailor's tests.
• Killing a heavy infection releases a surge of dead slime that clogs filters. Do not shock a badly infected tank right before a rough trip; plan for tank cleaning or fuel polishing (see the diesel fuel polishing guide).

Use a biocide at the maintenance dose on the label if your boat sits for long periods or has had water problems, and at the shock dose only as part of a cleanup plan.

Cetane measures how quickly diesel ignites; higher cetane mainly helps cold starting. Practical Sailor ranks cetane improvers as "nice to have, not essential" for most boats, and found that some conditioners did help against corrosion and lubricity. Check your diesel engine manual for approved products. Diesel exhaust fluid (DEF) is not a fuel additive: never put it in the fuel tank.

Westerbeke's FAQ recommends a stabilizer for diesel, mentions cetane boosters for cleaner burning (less soot on the transom) and an anti-algae treatment for diesel tanks, and stresses following each maker's directions because more is not better. Not every diesel maker agrees: Yanmar's FAQ says it does not advise any fuel additives, and recommends keeping the tank full to limit condensation and draining water instead. If you own a Yanmar, or your manual says the same, ask the dealer before treating the tank (a biocide to clean up an actual microbe problem is a conversation to have with them, not a routine dose). Westerbeke's lay-up advice also warns that any additive must be compatible with your primary filter/water separator, so check the label before you buy; some additives are designed to disperse water into the fuel instead of letting the separator catch it.

Practical Sailor's mixing tip for diesel: do not pour concentrated additive into an already full tank, where it can settle out. Add it while fueling, or premix it in a gallon of fuel first.

• Read the label dose and measure it for the gallons you are adding, not the tank size.
• Know your tank size (on the tank label or in the boat's manual) so you can calculate.
• More is not better. Overdosing can leave deposits or, with biocides, attack metal parts.
• Two-stroke owners: use only products the engine maker approves; some additives can affect oil mixing.

Pick one stabilizer or ethanol treatment that your engine maker approves, plus a biocide on a diesel boat if needed, and stop there. Pouring three or four different bottles into the same tank mixes chemistries nobody has tested together, and owners on forums regularly report engines that ran worse after a cocktail of treatments.

Write the date, product and amount in your maintenance log every time you treat fuel. When the engine starts hard next spring, you and your mechanic will know what went into the tank, and you can see whether your routine works.