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What You Should Know About Your Boat’s Fuel

Boats with either gasoline or diesel engines can suffer poor performance and damage if their fuel is contaminated. If you look out and there’s black smoke pouring out of the exhaust, that’s a pretty good indication that something’s gone awry! You may also experience excessive smoke, lack of power, hesitation in acceleration, or outright stalling. No one wants to be stranded, so let’s look at what you should know about your boat’s fuel, how it gets contaminated and how best to prevent contamination.

Water is the most common contaminant found in both gas and diesel fuel.

Water is the most common contaminant found in both gas and diesel fuel. Water in gas, particularly ethanol-boosted gas that attracts water, is quite common in pleasure boats that go unused for prolonged periods. Air in less-than-full fuel tanks contains moisture. Over time this moisture condenses in the tank and is one reason why, regardless of fuel type, tanks should be topped off during storage periods, leaving less room for condensation. Water can also intrude storage tanks through poorly sealed fuel caps and vents during boat fueling.

Less likely, but not unknown, is the fact that water can be introduced into your fuel directly from a supplier. Occasionally, adding a simple “dry gas” additive  when fueling your boat during the season is a good idea.

Whatever the source, it is estimated that in diesel engines, 90 percent of all problems are fuel-related. Fuel system inspection should be top-of-mind with boaters.

Find the Problem

If your boat experiences any of the symptoms of contaminated fuel listed above, you should start by examining your boat’s fuel filters. Small outboards with external tanks will have an inline filter; larger boats with internal tanks may have more elaborate filters between the engine and tank, such as a Racor water separator.  Sometimes, simply cleaning and replacing filters will eliminate the small amount of water or contaminant present in your fuel system, but a clogged filter should lead you to a more thorough visual inspection of the inside of tanks if possible–look for evidence of rust, corrosion or sediment.

The only real cures for fuel contamination are to completely replace the fuel or hire a company to do fuel polishing (filtration) along with a thorough tank cleaning.

Water or debris in the fuel can also lead to deeper problems, namely damaged fuel tanks and blown engines. Microbes that feed on hydrocarbons cause sludge, but can only flourish if water is present. These same algae emit sulphuric acid as a waste product which further corrodes metal tanks, pumps, and injectors. Water vaporizing in an engine’s cylinders can cause permanent damage—which is why we have all these filters to begin with.

The only real cures for fuel contamination are to completely replace the fuel or hire a company to do fuel polishing (filtration) along with a thorough tank cleaning. The presence of algae, discoloration, or sediment should prompt any boat owner to act quickly.

Simply adding products like Biobor to fight algae in an already fouled system will only bring the problem past the filters and into the engine.  Biobor and similar products should be used as preventatives to algae growth, not cures.

Near the end of the boating season, it’s recommended to:

  • Add a fuel stabilizer, specific to your fuel type, and run your engine long enough to draw the enhanced fuel into the engine—this will keep the fuel from breaking down over time and gumming up your carburetor or fuel injectors.
  • Inspect your fuel filters annually—lay-up time is a good time for this. Again, if you have water or contaminants in your filters check for further problems. A water indicating paste can be used to “stick” your tanks if visual access is limited.
  • Top your tanks off with fuel. This will minimize air and condensation from getting in your fuel.

Clean, fresh fuel will maximize engine performance and give you peace of mind to enjoy boating.