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I have a diesel powered sailboat. What are effective ways to avoid microbial contamination of the diesel fuel? And what do you do if the fuel becomes contaminated?

[This question originally asked by Curd on the Sailing Yachting and Seamanship proposal]

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Bacteria and fungi can only grow in water, not in the fuel itself. If a fuel tank is unused and resting for a longer time, the small water content in the fuel can separate at the bottom and provide a viable environment.

Usually, the fuel will be extracted at the lowest point of the tank, so that accumulated water will be removed whenever the engine runs and separated in the fuel filter.

So if you have a well-constructed tank, and run the engine and clean and drain the fuel filter regularly, you should be fine. It also helps to keep the tank full to prevent condensation from the air inside it.

If you cannot run the engine regularly, I think there are additives to prevent microbial growth.

Once you have significant contamination, you probably have to drain and clean the tank as well as the filter and fuel pipes.

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Michael Bogwart's answer revealed already relevant points. I experienced contamination 5 years ago, which resulted in multiple engine failure (sometimes while entering marinas), so here is my personal resolution:

Best is a big inspection hatch to clean the fuel tank, but this is also a possible point of failure / leakage. I only have a 4cm hole in mine, where the fuel gauge is installed, and wanted to keep it this way. My "minimally invasive" approach was as follows:

  • filter all the fuel in the tank (I used a double action air pump as a "vacuum cleaner" with hoses on in- and outlet, tried to suck up all the big chunks of algae, worked quite well)
  • clean fuel water absorber (algaes were growing in the transparent bottom part), change integrated fuel pre-filter (different filter mesh size did not matter too much as far as I can tell, rather use a finer one, so that main fuel filter lasts a bit longer - my new filter was really black after 20-30 hours the first time after the incident)
  • use a fuel additive, choose well, as some do work only as a prophylactic security to stop bacteria from developing, while others even seem to degrade algae to some extent. I had good experience with Grotamar, not so much with Yachticon.
  • check your reserve tanks, use a filter before filling them into main tank, add additive to reserve as well.

Prevention tips:

  • keep tank and reserve always filled up completely and as close to water, so that the temperature does not change much and condensation water is not such a problem
  • check water absorber and filter every month
  • think about a small backup tank in case main tank gets clogged

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