Ethanol in Marine Gas Engines

Racor filter

By Andrew Spaulding

If you’ve followed any of the industry rags recently, E15 and what it might do to your marine engine should be on your mind. E15 is the common term to designate gasoline that has 15% ethanol in it. We are already using E10 gasoline and the federal government, through the EPA, would like us to be using E15. While the benefits of using ethanol to reduce harmful exhaust emissions are certain, since ethanol can be harmful to marine engines and fuel systems, and E15 is in the news, I thought I would go over ethanol and its effects on marine systems.

So, what problems arise from ethanol? There are two main issues: One, ethanol is a stronger solvent than gasoline; and two, ethanol has a strong chemical affinity for water. One might argue that as a stronger solvent ethanol would do a good job cleaning out a gasoline fuel system. The problem is that it will remove gasoline varnish deposits and surface corrosion from metal tanks and other system components, potentially causing clogs in small passages in the fuel system such as carburetor jets.  Ethanol also has the ability to dissolve some plastic resins which can cause serious problems in some fiberglass fuel tanks.

Ethanol’s strong affinity for water means that it would rather chemically bond with water than with gasoline. When the percentage of water in E10 gasoline reaches about 0.5% the bond between ethanol and gasoline breaks down, and the water molecules bond with the ethanol causing what is called phase separation. Gasoline will float on top of the ethanol potentially leaving the fuel pick up at the bottom of the tank in a water-ethanol mix which can severely damage an engine. Even if there isn’t enough ethanol and water at the bottom of the tank to cause this, the gasoline left over after phase separation has a lower octane level (approximately 2-3 points lower) which can be below the recommended level for the engine. 

If all of this is true, why don’t we have more ethanol related problems in our cars? Well, car tanks are typically smaller than boat tanks and filled more regularly so phase separation has less of a chance to occur. Also, automotive tanks have closed vent loops or are unvented which allows less moisture to enter the tank. Boats are used in wet environments (no news here) so the overall chance that water will enter the tank one way or another is higher.

As a boat owner what can I do to help minimize the impact of using E10 gasoline? Since as little as 16 ounces of water in 25 gallons of gasoline can induce phase separation (less at lower temperatures), anything that you can do to lessen the amount of water getting into your tank will help. Keeping your tanks full will minimize the amount of condensation that forms in the tank. Also, run through your tanks regularly so that you can fill them with fresh gasoline. Of course, for the casual boater these two things can be at odds with each other, but do the best that you can. Even running the tank out by then end of the season so that you can put in fresh gasoline for winter storage will help.

Installing a Racor gasoline/water separator with a drainable bowl is a practical fuel system upgrade that will allow you to determine if there is any water being pulled out of the tank. Also, MerCruiser and Volvo gasoline primary spin-on filters will trap water. The bottoms of these filters are not clear due to USCG regulations, but they can be periodically drained to check for water. If you find evidence of water in the tank from a Racor or primary filter, you should have the tank cleaned, or at least pump out the bottom of the tank to remove as much water as possible.

All of the problems mentioned above get worse if we switch from E10 to E15 gasoline. Even if everything is okay on your boat now with E10 at the pumps don’t be complacent if E15 gasoline gets to marine engines. If you trailer your boat and fill up at an automotive filling station, be very cautious as I’ve read reports of E15 at automotive pumps not being properly labeled as such. Let us know if you have any questions regarding ethanol in gasoline as these issues can end an already short boating season.