E10 petrol in outboard engines: fuel, approvals and winter storage

What E10 is

E10 is petrol containing up to ten percent ethanol; E5 contains up to five percent. At European pumps the grade is shown in a circle marked E5 or E10. Diesel engines are not affected.

Ethanol attracts water and can attack some rubber and plastic parts. In a car that is refuelled every week this rarely shows. A boat often sits for weeks, and the tank breathes through a vent that lets damp air in.

Long periods without use: water and phase separation

When ethanol blended fuel takes up too much water, it separates: a layer of water and ethanol collects at the bottom of the tank, with petrol above it that has less ethanol and a lower octane rating. Shaking the tank will not reverse this phase separation. If the engine draws from the bottom layer it runs badly or stops.

Suzuki Marine USA lists corrosion of metal parts, damaged rubber and plastic parts and starting problems among the effects of too much ethanol or long storage, and says phase separation is more common when boats are not refuelled regularly. Honda Marine notes that, depending on the region, fuel can start to deteriorate in as little as 15 days. In practice:

• Buy fuel fresh and only as much as you will use soon.
• Have a water separating fuel filter fitted ahead of the engine and check it regularly.
• Keep spare cans tightly closed, cool and out of the sun.
• Do not run old or cloudy fuel through the engine; have it disposed of properly.

What the manufacturers approve

Your engine's owner's manual says which fuel it may use, usually in the fuel chapter, together with the octane rating it needs. We found published manufacturer statements for two brands:

For every other brand, the owner's manual of your engine is the reference. If you no longer have it, ask the brand's dealer, quoting model and serial number. An approval for the engine says nothing about the tank, hoses and filters on board, so on an older boat also ask the boatbuilder or your yard.

Fuel and tank in winter storage

Over winter the fuel sits for five months or more. The owner's manual sets out how to lay up the tank, and makers differ. Honda Marine recommends using a fuel stabiliser with fresh fuel when the boat is stored, and refers to the manual for adding stabiliser and draining the tank. Typical steps are:

• Add stabiliser as the maker specifies and run the engine briefly so it reaches the carburettor or injection system.
• Drain the carburettor or float bowl if the manual says so.
• Store portable tanks and cans as the manual specifies.
• Check the water separator and filter and drain off any water.
• In spring, check for water at the bottom of the tank before the first start.

The rest of the lay up work is in our winter storage checklist.

Wrong fuel in the tank

If there is petrol in the diesel tank, diesel in the petrol tank or water in the fuel, do not start the engine; have a workshop drain the tank.

Many hull policies exclude mechanical breakdown of the engine. Some offer an engine or machinery extension that names misfuelling or contaminated fuel. Look in your own policy for how it treats misfuelling, contaminated fuel and diesel bug, whether there is an age limit for the engine, and whether it asks for proof of servicing.