Ethanol and your marine engine

With so many options and opinions out there, it's hard to know which type of gas is best for your boat's engine. Let us help you sort it all out.
When it comes to ethanol-blended fuel and your marine engine, it’s not a marriage made in heaven. This fuel, otherwise known as E-10, contains 10% ethanol and 90% gasoline. In the US, ethanol for fuel is essentially produced by removing the starch portion from plant material (such as corn, sugar cane or grasses) and fermenting it, then distilling the fermented starch and removing the excess water, resulting in a highly refined, approximately 200 proof ethyl alcohol. The EPA, the government agency responsible for setting national guidelines that regulate the content of engine fuel, has allowed the use of ethanol in gasoline up to 10%. But here’s the issue: ethanol has an attraction for water and will more readily mix with water than gasoline. And that’s a problem when it comes to your boat’s engine. 

It has been reported that the fuel system components of Mercury, Evinrude and Yamaha engines will withstand up to 10% alcohol content in gasoline (E10). While E15 (15% ethanol, 85% gasoline) is a higher octane fuel that has been approved by the EPA for light duty vehicles, model year 2001 and newer; boats were not approved for E15. In fact, Mercury and other boat-motor manufacturers are emphasizing that marine engines should not be powered by E15. The National Marine Manufacturers Association (NMMA) and the Outdoor Power Equipment Institute (OPEI) have also gone on record to warn consumers to pay special attention when purchasing gasoline greater than 10% ethanol, as it may harm marine engines and can void warranties.

Since E10 became prevalent, there have been two opposing camps when it comes to winterizing your boat. One camp says to keep the tank full, while the other says to empty it completely. The debate circles around the fact that ethanol holds water and can only absorb so much water before it becomes completely saturated. When this “phase separation” occurs, the corrosive ethanol-water mixture sinks to the bottom of the fuel tank and can damage seals, O-rings, injectors, and other parts when the engine is run. If the tank is left partially full during the winter season there is more room in the tank, which increases the likelihood of condensation forming and allows less ethanol to absorb the remaining water. This is why the opposing camp’s opinion is to completely empty the tank and even the fuel lines and filters, if possible. But this is often easier said than done. So what are the engine manufacturers recommending?


Mercury Marine’s position is to follow the instructions for normal boat storage found in the operation, maintenance and warranty manual. They believe that if you are planning to store your boat for more than two months, it is best to completely remove all fuel from the tank. But, if it is difficult or completely impossible to remove the fuel, then maintaining a full tank of fuel with fuel stabilizer added to provide fuel stability and corrosion protection is recommended. In this case, Mercury Marine’s website suggests adding the stabilizer and fuel treatment to the tank at the recommended dosage, running the engine for 10 minutes to allow the system to be cleaned, and then shutting off the fuel valve to interrupt the amount of exchange with the air that might bring in condensation. They advise not to cap the tank vent and not fill the tank with fuel to the point of overflowing, as some extra space should be maintained to allow for expansion and contraction of the fuel as temperatures change. And finally, a partially full tank is not recommended because the void above the fuel allows for air movement that can bring in water condensation as the air temperature goes up and down.

As for our 23’ Chaparral with a MerCruiser engine, we have elected to follow Mercury’s recommendation to winterize with an empty tank. It just makes more sense, especially since our winter storage facility is located in southeastern Wisconsin, where the term “polar vortex” may have originated. But no matter what you decide, please check your engine’s owner’s manual for all engine and fuel information. If you do not have access to the owner’s manual for your equipment, contact the engine manufacturer or an authorized dealer. And remember, the EPA’s E15 approval is for vehicles 2001 and newer. If this fuel is used in boats, potential problems can include engine overheating, fuel line ruptures, a breakdown of rubber pieces in engines, expensive repairs and engine failure not covered by warranty!

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About the author: Karen has been boating for 25 years and is a Markel boat insurance customer. Her 23’ Chaparral is slipped in the Chain O’Lakes, in northern Illinois, where she and her husband spend as much time as they can during the summer months.

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