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Diesel fuel algae might be sign that water has entered tank


One of the first things a tyro engineer learns when working aboard a yacht is to watch out for algae growing in the yacht’s diesel fuel. Diesel fuel algae is really a misnomer that’s been passed down from generation to generation of diesel mechanics. Algae are what grow in the yacht owner’s aquarium in the main salon, but not in his fuel tanks. Algae require sunlight to grow, and of course, that’s impossible within a fuel tank. Diesel fuel algae received its name because of its appearance, not because of what it truly is.
Some engineers also call this algae “diesel bug.” And that label is incorrect as well, since it’s neither an insect nor a disease. Just what is it? It is actually a bloom of microbial growth within the fuel tank. This slimy bloom of organic contaminants can be composed of bacteria, mold, yeast or other types of fungi. There is one simple rule when it comes to microbial growth in a diesel fuel tank, it cannot exist without the presence of water. There are a number of ways water can enter a tank:
Freshly refined fuel contains some water. As the fuel cools down after cracking, this water separates out.
l During transport from the refinery via tankers and barges, ballast water can get mixed with the fuel.
l Condensation from humid air in a partially filled tank.
l Brittle, rotted seals at a tank’s inspection port or fuel intake port.
l Rain, sea spray and water from washdowns can enter deck fuel fillers and tank vents.
Aside from the damaging side effects water can have on an engine, its components, and the tank, free water found at the bottom of a tank is a breeding ground for microbial growth.
The microbes live between the water and the fuel in an area called the fuel/water interface. The bacteria and fungi feed off the fuel while living in the heavier water that sits just below the fuel. As the bloom grows, it binds with other forms of fuel contamination to form sludge. To make matters worse, the bloom can have an acidic by-product that promotes tank corrosion. Many yacht engineers never truly treat the cause of the problem, but only treat the symptoms of the problem. Biocides are the most common component in the large number of products on the market that just treat those symptoms. Used alone, biocides just make the problem worse. Although biocides kill the bacteria, mold and yeast, these dead microorganisms then settle to the bottom of the fuel tank and are mixed into the brew of sludge that clogs the fuel filters. To treat the cause of the problem, the only method that guarantees prevention of microbial growth is to remove the water and stop the contamination in the first place.
The key to preventing microbial growth requires establishing a maintenance program for the fuel stored in the yacht’s fuel tanks. This program includes:

  • Visually inspecting inside the fuel tank for fuel color and odors.
  • Inspecting all fittings and orifices for dry rot and rust.
  • Using a water finding paste to check for the presence of water.
  • Sampling fuel in the tank and testing its quality against standard benchmarks.
  • Treating fuel with a fuel additive.
  • Regular fuel polishing.

A yacht’s fuel preventive maintenance program should be followed monthly, or whenever new fuel is bunkered. Fuel polishing, which is the process of filtering out contamination, removing water, and conditioning the fuel to keep it in specification, is the most important method of removing existing and new growth contaminants. Then, following up with a high quality fuel additive, will ensure the fuel can be stored for an extended period of time whether the yacht is at the dock or hauled out at a shipyard. As with all preventive maintenance programs, the small amount of time used up front during an engineer’s monthly routine will stop problems and costly repairs from developing in the future.

Capt. Jeff Werner has been in the yachting industry for 25 years, and is the owner of Diesel Doctor ( All Triton readers receive a 10% discount on online orders. Contact him at

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3 thoughts on “Diesel fuel algae might be sign that water has entered tank

  1. Scott McDowell

    Great article! I incurred diesel fuel problems from contaminated tanks after purchasing my motor yacht which had been in the Caribbean for 10+ years. About 2 inches of brown sludge (looked like cow manure) in the bottom of each tank. Pumped out and steam cleaned to solve the problem. Took on new fuel that proved to be bad quality with excessive additives (potassium, etc.) and flash point problems; laboratory tests were stamped: “FAIL” Quite a lesson.

  2. Leo Lindstrand

    Since some water will invariably end up in the bottom of all tanks, it helps to design the tank plumbing in a way that allows removal of ALL water. Typically the fuel suction pipes stop about 1-3 inches above the bottom, sometimes even higher. That means that unless there is a drain on the very bottom, there will always be a few inches of water, even after you polish the fuel. A well though out fuel system will have a “stripping” pipe (or valve), i.e. one that sucks out (or drains) from within 1/8″ of the bottom. These are often present on commercial vessels and some older yachts, but unfortunately seem to have been forgotten on the current yacht fleet. The stripping procedure might be to open a drain valve, or to pump out (sometimes with a manual pump) into a glass jar on a regular basis, especially after bunkering and after a laid-up period. By having NO water in the tanks, there should be no more growth!

  3. Elizabeth Ann Smith

    In my experience, no one thinks to install water removing ports or pipes in fuel tank much less install access to tanks for fuel tank cleaning and fuel polishing.
    On another note, if salt water gets into your fuel then it is waste. Has anyone discovered a way to remove salt water from fuel? I would be very interested to hear from them if so.

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