Diesel bug is a slime of bacteria and fungi living where water meets fuel in the tank. On a boat it has everything it wants: fuel that sits for months, condensation, warmth, and a filler on deck. It blocks the filter, and the engine stops, usually when the sea is rough enough to stir up the tank.
Why boats get it
Ben Sutcliffe-Davies, a marine surveyor writing in Yachting Monthly, says the organisms like 5 to 70°C and thrive around 30°C, and over 100 variants have been identified. SeaHelp, which runs breakdown boats around the Mediterranean, says fuel problems were one of the main reasons for its call-outs, and puts it down to fuel stored through idle months with moisture, heat and condensation. Both point at the biodiesel in road diesel: up to 7% under the EN 590 fuel standard, which holds water and feeds the bug.
How a surveyor checks it
The filter bowl. Look into the primary filter's bowl before the engine runs. Clear fuel is good. Water at the bottom, dark sediment or a brown jelly are the signs. In PBO, Richard Barnard found his filter "jammed solid with a brown, jelly-like substance" after a rough sea.
A sample. Draw a little from the tank drain or the filter into a clear jar and let it settle.
The deck filler. Sutcliffe-Davies often finds filler caps that trap a little water, which then drops down the filler pipe, and cockpit-sole fillers that leak. SeaHelp also blames defective caps and seals.
The tank. Can it be inspected? Barnard's 250 litre tank had no access hatches and had probably never been cleaned. Half an inch of sludge lay on the bottom.
The sea trial. Run at cruising revs long enough to see whether the engine hesitates, smokes or loses power.
What decides the grade
A clogged filter or an engine that falters is an A: the next stop could be in a harbour entrance. Water and sediment with the engine running well is a B. Filler and treatment points are C.