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Diesel bug contamination in boat fuel not only has the potential to be dangerous, starving your boat engine of fuel just when you need it most, but is also a frustrating waste of both time and money, wrecking engines and corroding fuel tanks in both boats and yachts. Unfortunately, with the introduction of biodiesel, which provides the ideal environment for the germination and procreation of diesel bug, it is becoming ever harder to avoid. So, what is this bug, and why is diesel contamination on the increase?
What is diesel bug?
‘Diesel Bug’ is the term commonly used for a number of contaminants that include microbial bacteria, fungi and algae. They can originate from the air or moisture, or during tank filling, as well as the expansion and contraction of storage tanks, and can lie dormant in boat fuel tanks and boat fuel systems, usually in pits and crevices, covering themselves in a protective slime to fend off biocides. When as little as a droplet of water is introduced, and the temperature is within the right range (5-70o C), the microbes germinate and begin to reproduce at the interface between fuel and water, feeding on the nutrients and hydrocarbons that bio-fuels contain.
Microbes have a very short life, but multiply at an alarming rate; a single cell weighing only one millionth of a gram can grow to a biomass of 10 kilograms in just 12 hours resulting in a biomass several centimetres thick across the fuel/water interface. Before dying, the microbes produce waste deposits which descend to the bottom of the boat fuel tank, and due to their rapid reproduction the build up of dead microbes and waste can be considerable.
When dead diesel bugs, sludge or water is sucked into the boat fuel lines the resulting filter and injector blockages can cause engine failure and an extended and dirty tank cleaning exercise, with the added annoyance and expense of lost contaminated fuel inside the tank. Additionally, corrosion can occur wherever the bio-film settles, resulting in expensive replacement of the boat fuel pumps and injectors. Once contaminated diesel has entered the boat fuel system it can be difficult to eradicate.
So why is boat diesel contamination on the increase? Biodiesel is diesel fuel made from plants and other biological materials. The most common form is FAME (Fatty Acid Methyl Ester). Biodiesel is added to petrochemical diesel to make the fuels we currently use. B numbers are used to designate how much biodiesel the fuel contains: the B7 biodiesel (fuel standard BS EN590 standard) currently used in the UK and Europe contains 7% biodiesel. However, due to environmental concerns and the fact that it is cheaper to produce biofuels, it is expected that the percentage will keep on rising with the introduction of fuel with 12% bio content not being far away.
Biodiesel may have environmental advantages but due to its higher water content, it is more vulnerable to fuel contamination than petrochemical diesel. Not only does it contain more water when it leaves the refinery, but because it is hygroscopic it can also attract even more water if stored badly. When stored, biodiesel also suffers from particulate contamination even more so than petrochemical diesel. Rust, dirt, wax, sand, clay, asphaltenes, microbial growth and colloid carbon all reduce the fuel quality. If fuels with a 12% bio content are introduced it is likely that they will have a shelf-life of only six months once in your boat fuel tank.
Avoiding diesel bug
How can you prevent the boat diesel fuel bug? The best form of defence is preventing water from entering your boat fuel tank. Diesel bug cannot survive without water. However, it is virtually impossible to prevent water entering your boat fuel tank; tanks will suffer from condensation at the very least.
Boat Diesel Fuel Bug Treatment
Good boat fuel management to reduce the risk of contamination includes storing fuel properly, i.e. at a stable temperature of around 5-10OC, and using fuel before the end of its shelf-life. It is also advisable to buy fuel from a reputable supplier who has a reasonably high turnover. For boats or yachts carrying 15 gallons or less of fuel, it may be cheaper to dispose of the fuel at the end of the boating season and to then give the tank a really good clean out. The emptier a tank is, the greater the volume of damp air at the top, and hence the likelihood of condensation.
For further tips on staying clear of the risk of the boat diesel fuel bug you may wish to read How to Avoid Diesel Bug article in Yachting Monthly, or Practical Boat Owner’s article on Diesel Bug Treatments.
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