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Diesel bug: what it does to tank and engine
Short answer
Microbial growth at the water line inside the tank blocks filters and stops engines. The clean-up and the engine damage are two separate bills, and a hull wording treats them very differently.
Diesel bug is not a disease of the fuel, it is life inside it. Bacteria, yeasts and moulds settle at the boundary between condensation water and diesel, and they leave a black sludge that blocks filters, coats lines and pits the tank from within. The biodiesel share in modern pump fuel encourages the process because it holds water, and a tank left half empty over a British or Dutch winter gives the colony everything it needs: water, time, and warmth in spring.
For insurance purposes the case splits in two, and it is worth knowing the line before the invoices arrive. Cleaning or polishing the tank removes a condition that developed gradually. Gradual deterioration and maintenance are routinely excluded from hull policies, in the British market as much as on the continent, because a property contract answers for sudden events and not for ageing. The tank bill therefore tends to stay with the owner, whatever else the wording says.
The consequential damage is the part worth checking in the wording. If a filter chokes under load, the engine stops, and the stoppage wrecks the injection system or the turbocharger, that is a machinery question, and machinery damage sits in a separate clause of many hull contracts rather than in the basic hull section. Read whether the contract has such a clause at all, what it says about wear, and what it expects by way of servicing. Then read the exclusion for gradual deterioration, because between them those two paragraphs decide a case like this. Our own machinery cover, available from Basis Plus, expressly excludes damage caused by diesel bug.
Diesel bug symptoms: how the engine tells you
Diesel bug rarely announces itself at the pontoon. The typical pattern is an engine that runs well at idle and in gentle motoring, then hunts, loses revs or stops once it is asked to work hard in a seaway. Rough water stirs up the sludge at the bottom of the tank, the pre filter clogs, and the engine is starved of fuel at exactly the moment you need it most, often while entering a harbour or motoring off a lee shore.
Other signs are easier to spot in calm conditions. A pre filter bowl that turns dark or shows a layer of slime, filters that need changing far more often than the service schedule suggests, a sour or musty smell from the fuel, and black flecks in a sample drawn from the bottom of the tank all point in the same direction. If you see them, treat the problem before the next passage rather than carrying a box of spare filters and hoping.
Diesel bug treatment: biocide, filters and fuel polishing
A biocide kills the bacteria and fungi living in the fuel, but it does not remove them. The dead matter stays in the tank, settles and is drawn into the filters over the following hours of running. Anyone treating diesel bug should therefore expect to change filters several times after dosing, and keep spares on board until the fuel stays clear. Follow the dosage on the product and the engine maker's guidance, because more is not better.
Fuel polishing goes a step further. The fuel is pumped out of the tank, through a series of fine filters and a water separator, and back in, often for several hours, until it comes out clean. Some boats have a permanent polishing system; others hire a specialist who brings a mobile unit to the berth. Polishing deals with the fuel, while a heavily contaminated tank usually needs to be opened and cleaned as well.
Cleaning a diesel tank on a boat: when polishing is not enough
Where the sludge has formed a layer on the tank walls and baffles, circulating the fuel will not shift it. The tank has to be emptied, opened through an inspection hatch and cleaned by hand, and on many older boats that hatch does not exist and has to be cut in. That is a job for a yard or a specialist, both because of the fuel handling and because a badly fitted hatch is the next leak. Ask for photographs of the inside of the tank before and after the work.
Keep the invoice, the photographs and a note of the filters changed. They show when the problem was found and how it was treated, which is exactly what anyone will ask if an engine failure follows later. After cleaning, the prevention routine matters more than ever: a full tank over the winter, a water separator that is checked and drained, and a sample jar taken from the bottom of the tank at the start of the season.
Petrol in an outboard: phase separation rather than diesel bug
Petrol engines do not suffer from diesel bug in the same way, but modern petrol has its own problem. The ethanol it contains attracts water, and once there is enough, the ethanol and water separate out from the petrol and sink to the bottom of the tank or the carburettor bowl. An outboard that has stood for months may then start badly, run lean or refuse to start at all, and the varnish left behind can block jets and injectors.
The routine for an outboard is simple. Use fresh fuel, add a stabiliser before a long lay up, and either run the carburettor dry or drain it at the end of the season. Keep portable tanks out of the sun and with the vent closed when not in use, and do not top up last summer's fuel with new. A small outboard that will not start in spring is very often a fuel problem rather than an engine problem.
Worth remembering
The engine damage
follows from a sudden stoppage
The tank clean
removes something that grew slowly
One cause, two invoices, and hull wordings put them on opposite sides of the line.
Questions
What owners ask us about this
Who pays for cleaning the tank?
In practice the owner does. A tank clean removes a condition that built up slowly, and that counts as maintenance rather than a sudden loss in the hull wordings we reviewed.
What stops it happening?
Fill the tank before the winter so no air space is left for condensation, drain the water separator regularly, and use a biocide at the engine maker’s dosage during long lay-ups.
How do I spot it early?
Draw a sample from the sump into a clear jar. Cloudy streaks, a dark interface or black flakes are the warning, and they show up long before the engine starts to hunt.
Should I keep the invoice for a tank clean?
Yes. If an engine claim follows later, the receipt shows that the condition was found and treated, which is precisely what a surveyor will ask about.
Does a biocide on its own get rid of diesel bug?
It kills the growth but leaves the dead matter in the tank. Expect several filter changes afterwards, and on a badly contaminated tank fuel polishing or a manual tank clean as well.
Can petrol in an outboard go off in the same way?
Not through microbial growth in the same way, but ethanol in petrol attracts water and can separate out over a lay up. Fresh fuel, a stabiliser and a drained carburettor are the usual answer.
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