Ethanol blended fuels, such as E-10, are getting more and more common, and as their popularity has increased, so has the talk concerning the potential damage that these newer fuels can inflict upon outboard motors. Corrosion, rotted fuel lines, water contamination, and a vast array of other problem have been blamed on ethanol. So is ethanol blended fuel going to destroy your boat motor? Probably not, say the companies that make those motors.
Many of the problems associated with ethanol blended gasoline are either inaccurate or easily prevented. Ethanol’s supposed tendency to absorb water out of the air is just not true, according to Mercury Marine’s “The Myths of Ethanol and Fuel Care”, “The primary cause of water collecting in fuel tanks is condensation from humid air.” This condensation can occur whether the tank contains E-10 or regular gasoline. Ethanol can absorb a small amount of water after it has condensed, but this can pass safely pass through the engine. The best way to prevent excess water from entering a fuel tank is to keep it full of fuel, thereby reducing the amount of room for air.
Ethanol is potent solvent, which can cause some issues when using blended gas for the first time in a tank previously used only with regular gas. The ethanol in the fuel can dissolve any residues stuck to the sides, causing a gummy material to form in the tank. This should only be a concern when first making the transition to E-10, and can be prevented by thoroughly cleaning the fuel tank beforehand or, as Honda and Yamaha advise, placing a 10 micron fuel filter between the fuel tank and the motor.
Honda, Mercury, and Yamaha, Three of the biggest manufacturers of boat motors, specifically state that their products are warrantied for use with ethanol blended E-10 gasoline. They do suggest a few precautions, such installing a fuel filter and adding a fuel stabilizer to the tank if it will be sitting unused for months on end. An critical detail in all three of the manufacturer's recommendations is to NOT use any fuel consisting of more than 10% ethanol, such as E-15 or E85, as these fuels can void the warranty and damage the motor.