The next time you stop to fill up on the way to the lake, be careful what you put in your boat.  Saving a few pennies on fuel now could cost you dearly in the long run.

Ethanol in Gasoline

Gasoline with modest amounts of ethanol (up to 10%) has been widely used year-round throughout the US for many years.  In fact, according to the US Department of Energy, 98% of gasoline sold in the US contains at least some ethanol.  While E10 gasoline is fine for automobiles, it can be somewhat problematic for boats depending on how they are used.  Specifically, boats that sit for long periods of time are especially prone to “phase separation”.  This is what happens when Ethanol molecules combine with water and sink to the bottom of the tank – separated from the gasoline.  When this is pulled into your engine, starting and running problems will likely ensue and major damage could occur.

However, because E10 gasoline is so prevalent and considerably cheaper than 100% gasoline, many boaters still use it – myself included.  Because we live on a lake in a rural area, my wife typically fills our portable gas tanks when she is fueling her car in town.  I get whatever she uses.  And this is generally fine provided that you take certain precautions.  One precaution is to add an ethanol treatment like this one every time I put gas in my boat.  Another precaution is to completely fill or empty my gas tank at the end of the season or whenever the boat is not used for an extended period of time.  Doing these things will minimize the chances of phase separation from occurring.  However, one word of caution – gasoline with ethanol has a lower boiling point and therefore is more prone to vapor-lock, especially on very warm days after stopping your motor and then trying to restart it later.  This has happened to me twice over the past 10 years (including the most recent 4th of July holiday) and is a bit of an inconvenience because the engine will not start for a considerable period of time (until the fuel cools off).

But E15 gasoline is a different matter entirely.

The E15 Issue

Historically, gasoline containing 15% ethanol (E15) was restricted from use during summer months because it contributed to smog more than gasoline with no or lower levels of ethanol.  However, starting in 2022 and again in 2023, a waiver was signed into law to permit the use of gasoline with up to 15% ethanol during the summer months.  So now E15 gasoline is readily available during the peak boating season.  Problem is, it is not approved for marine and many other non-automotive applications.

According to the Mercury Marine website, “while it might be a worthwhile alternative for some cars and trucks, E15 gasoline is not designed for marine engines. Ethanol attracts water from condensation and the air, which in turn can wreak havoc on an internal-combustion marine engine.”  The company goes further and states that “warranty coverage on a broad array of repairs and services will become void on any Mercury engine that has been operated using fuel with an ethanol content of more than 10%.”  And they are not alone.  Yamaha likewise cautions that E15 should never be used in any Yamaha Outboard, and the US Coast Guard states that gasoline with15% ethanol is federally prohibited for recreational vessel use.

What to Look Out For

Gas stations are required to provide a warning label like the one you see here to prevent misfuelling.  If you see this, or E85 (85% Ethanol which is even worse), definitely do not use it for your boat.  However, with all the flashing lights, music and other distractions at some modern fueling locations, it would be fairly easy to overlook this warning.

The other thing to watch for is fuel labeled as Unleaded 88, Regular 88 or Clean88.  These are all different names for E15 gasoline.  Because ethanol has a higher octane level than regular gas, it can be blended with regular unleaded (87 Octane) gasoline to boost the octane level – hence the “88” name.

While 100% gasoline with no ethanol is clearly the best for your boat, fueling with E10 is acceptable and most modern marine engines are designed to handle this.  However, under no circumstances should you use E15 or higher levels of ethanol as this might result in a major engine failure and leave you boatless for an extended period of time.