Ethanol Blends and Boating – Do they Mix? 

Did you know that running your outboard or sterndrive on ethanol blended petrol (E10) can cause performance problems and permanent damage to your motor, fuel tank and fuel lines. 

Why does ethanol pose a problem for marine engines? The answer lies in the properties of ethanol. 

Ethanol is hygroscopic, meaning it will absorb large amounts of water. Ethanol-blended fuels are also prone to phase separation. This means that over time the weight of the ethanol, along with any water it has absorbed, will separate and sink to the bottom of the fuel tank. Phase separation begins to occur in as little as 2 weeks. 

These attributes are less problematic for your car. It usually operates away from water and its fuel is replaced every week or two. But boat engines operate in a water environment and are often used infrequently. A water/ethanol mixture that has sunk to the bottom of your tank will get picked up by the motor’s fuel system. Even small amounts of water can harm the fuel system and cause premature rusting. Phase separation also leaves a lower octane fuel on top. This separated fuel causes performance issues such as increased stalling, misfire, hesitation and difficulty maintaining speed while trolling. 

Ethanol is also a highly effective cleansing agent, solvent and antifreeze. Excess ethanol can break down solid material including rubber seals, plastic, fibreglass and even aluminium and steel. It mobilizes deposits including corrosive matter, varnish and rust which will travel through the engine and clog fuel filters, carburetor jets and injectors. High concentrations will clean the oil right off the internal components of a 2 stroke, making extra lubrication necessary to avoid premature wear. It can also contaminate fuel through deterioration of the tank surface, especially if the tank is older and made of fiberglass. 

All reputable brands of marine engines now sold in Australia and New Zealand are designed to operate on fuel containing no more than 10% ethanol. This will not apply to older engines and in cases when illegal amounts of ethanol have been added at the petrol station pump. Prevention, therefore, is your best weapon against ethanol damage and it is our advice to avoid using ethanol fuel blends in your outboard or sterndrive.

It is our strongest advice that you choose premium unleaded (95 Octane or higher) if regular unleaded is unavailable, before an ethanol blended fuel is used.

If your engine is E10 suitable, according to the manufacturer’s fuel recommendations in your user manual, and it is not possible to avoid ethanol blended fuel in your area then here are some suggestions for minimizing harm to your engine. 

• Test the fuel with an alcohol fuel test kit to ensure ethanol presence is less than 10%. 
• Use fresh quality fuel from a high turnover service station. Ideally replace it every 2-4 weeks and keep it no more than 90 days. 
• Avoid running on the bottom of the tank and check for water in your fuel tank when you refill. 
• Never mix ethanol-blended fuels with other regular unleaded or premium fuels. They react chemically and create a gel-like contaminant. 
• Maintain your engine. Keep it well tuned and lubricated and replace your fuel filter every 50-100 hours. If possible equip your motor with a water-separating fuel filter. 
• Fuel stabilizers and additives may help but are no substitute for avoidance and the above measures.