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Ethanol additives will destroy your boat, ruin your marriage, and cause California to calve off into the sea

A non-ethanol gas station in coastal NC. Price per gallon across the street is $3.45. People are willing to pay a premium for their ethanol fears. Photo by Andrew David Thaler

Ethanol. For many boat owners in coastal North Carolina, it’s a dirty word. Since the mid 2000’s, various federal and state regulations have mandated the addition of up to 10% ethanol in gasoline. The reaction has been a combination of legitimate concern and hyperbolic declarations of doom (ecoterrorist have taken over our government and it’s a vast conspiracy to force you to buy a new car every three years – yes, someone said that too me). The rationale for federal mandates come primarily from the Energy Policy Act (2005), the Renewable Fuel Standard Program (2006), and the Energy Independence and Security Act (2007), while state mandates tend to deal with air quality and the recent appearance of methyl tert-butyl ether (MTBE) in the drinking water (ethanol fulfills the same role as MTBE in gasoline). Renewable resources, energy independence, national security, and clean air and water, it would seem that ethanol has a little something for everyone.

Ethanol in fuels and as an additive is not new. Most of the off-the-shelf fuel treatments designed to clean your engine and boost octane are ethanol based and have been around for more than 50 years. Many gas companies that advertise that their fuel will keep your engine clean use ethanol (along with other, proprietary compounds) to accomplish that task. Most carborator cleaners rely on ethanol. So why the backlash?

Part of the backlash stems from the belief among a certain subset of the population that anything the government forces you to do must not be in your best interest. As an a priori irrational assumption, it is impossible to argue against that assertion. Beyond the fanatical fringe, I’ve talked to many boaters, mechanics, and otherwise trustworthy people who are also concerned with the use of ethanol in fuel, specifically with regard to outboard motors, so I decided to dig a little deeper, and find out if there was any truth to the rumor that ethanol will destroy your boat.

The answer, surprising at it seems, is that while most boaters will never notice the switch (and some may experience an increase in fuel efficiency), in a few, very specific, circumstances ethanol additives in fuel can damage your motor.

The Good News

Most outboard motors built in the mid-1990’s or later were designed to handle up to 10% ethanol in fuel (E10) and will have no problem with new gas. Ethanol is a solvent, and will actually keep your engine cleaner than older MTBE fuels, extending the life of your outboard.

The Bad News

The problems with ethanol can be lumped into three major categories based on its chemical properties.

  1. Ethanol is a solvent
  2. Ethanol absorbs water
  3. Ethanol reacts with MTBE

As stated above, ethanol is a solvent. This means that it will clean your engine and fuel lines as you use it. However, putting ethanol into an already dirty system, such as what you’d expect from an older motor (and especially a carborated motor), will dislodge carbon residue that may work its way into the engine, potentially blocking fuel flow or damaging internal parts. Old rubber gaskets that have been hardened may also wear out faster, causing engine failure. These problems can be avoided by overhauling your older motor before switching to E10 fuel. Ethanol will also cause fiberglass to degrade, so if you are unlucky enough to have a fiberglass fuel tank (popular in the worst part of the 1980’s, but riddled with problems and now phased out) you should have it replaced (regardless of fuel used).

Unlike automobiles, marine fuel tanks are vented. This means that fuel stored in a marine fuel tank is always partially exposed to the air. If your boat is near the water (and why would’t it be?) then the ethanol in your gas may absorb moisture. This can be a problem if you don’t have a fuel/water separating filter in your fuel line, as water can enter your motor. A fuel/water separating filter is a good idea, regardless of fuel type, if you want your motor to last as long as possible, but some smaller boats don’t have them. If you have a removable fuel tank, store it somewhere dry when not in use. The other problem with moisture absorption is that if your fuel mixture reaches about 0.5% water, it will undergo phase separation and you will have a layer of gas floating over a layer of ethanol and water. If this happens, you could be drawing nothing but alcohol and water into your engine.

There are some easy ways to avoid this problem. If you trailer your boat, driving to the ramp will be enough to return the alcohol and water to solution. Pay attention to your fuel use, and don’t store more fuel than you need. Instead of topping off your tank at the end of the day, top it off at the beginning of each trip. That way you’ll be adding fresh, well-mixed fuel beforehand instead of storing old, moisture laden fuel.

The final point is the easiest to avoid. Ethanol and MTBE don’t mix well, and can form clumps that block your fuel line. Don’t mix ethanol and non-ethanol gas.

As one final note of caution: there were cases early in the decade of gas stations improperly mixing ethanol in with their fuel, resulting in situations where the gas at the pump could be up to 50% ethanol. This causes massive damage to vehicles that weren’t equipped to handle fuel with greater than a 10% concentration. As ethanol has become more common and the mixing process standardized, this is much less of a concern.

So don’t let the doomsayers keep you from filling up your tank or force you to buy expensive gasoline. Be aware of the issues that can arise with ethanol, but don’t let that keep you off the water.


Disclaimer: I’m a marine biologist, not a mechanic, but I have spent my life on the water, built several boats, and rebuilt multiple outboard engines. I talked to several local boat and small engine mechanics while writing this piece.


Deep-sea biologist, population/conservation geneticist, backyard farm advocate. The deep sea is Earth's last great wilderness.


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