While most of our customers are aware of the problems that ethanol-based fuels can cause in their valuable boats, we’ve noticed that a number of people who do their own engine maintenance often don’t realise that they could be doing significant, avoidable damage to their engines every single time they take them out.

It’s all about the valves

During the combustion cycle in an engine, valves need to open to let in air or release gases, and then close again, creating a seal.  Problems with an engine’s valves quickly lead to problems with the whole engine –the engine loses compression and runs poorly, stalls, and won’t start reliably.

Normal fuels cushion the valves

Premium unleaded fuels provide a ‘cushion’ around each valve that stops the force of movement from damaging the valve.  This allows valves to keep their shape and run at maximum efficiency for longer.

Ethanol-based fuels don’t cushion the valves

When valves aren’t cushioned while the engine is running, they quickly get misshapen and will no longer seal correctly.  And we saw earlier that if the valves aren’t working at peak efficiency, neither is the engine.  This is the primary reason that we recommend so strongly against the use of ethanol-based fuels – because the damage to a marine engine can be substantial even over a single tank of fuel at high RPM.

Can’t I just retune the engine?

It’s possible to retune a car engine to run on ethanol with minimal damage to its valves.  However, this doesn’t translate too well to marine engines for a number of reasons:

  • Boat engines run at much higher RPMs – up to 3,000 rpm – than car engines.
  • Boat engines typically run under much higher loads than car engines.
  • Boat engines have only one gear.

These differences between car and marine engines mean that it isn’t feasible to simply ‘retune’ a marine engine to cope with ethanol-based fuels.

Condensation concerns

Because a boat operates in the water, condensation is also an issue.  Ethanol absorbs moisture faster than other fuels – and this increases your chances of getting water in the engine.  We’ll tell you more about how this affects the engine in a future blog post.


Your marine engine isn’t designed for ethanol-based fuels, and unlike a car engine, it can suffer severe damage quite quickly from them.  Avoid these fuels like the plague!