HUMBugs vs. the Pristine Tank
What is the crud that, if left to have its way, can foul your filters, starve the fuel pump, and bring your diesel engine to a shuddering stop? Collectively, they're just microrganisms, primitive creatures that live in water and feed on oil. They're informally called "HUMbugs." The HUM stands for Hydrocarbon Utilizing Microorganisms.
HUMbugs are nasty, maybe even worse than France's violent, brilliant, evil, eloquent, scheming, cruel Charles II, King of Narvarre, an original humbug of the 14th century. Remember Lady Caroline Lamb's description of Lord Byron—"mad, bad, and dangerous to know"? Well, Charles II was worse. Rotten to the core, he was, but he died a deserving death in 1387. Withered by premature aging and miserably sick with chills, he was, on his doctor's orders, put to bed each night in cloth wrapped and sewn around his body and then soaked in brandy. One night his valet was careless with a candle...
There are many kinds of HUMbugs—bacteria and fungus (mold, yeast, algae, etc.). The ones that need oxygen are aerobic bacteria; those that survive without oxygen are anaerobic; those that can handle either situation are facultative. They all create thick slime, which helps them stick to surfaces and protect themselves from attack. Many are airborne. Some thrive best in freshwater, some in salt. It's unlikely that you can buy diesel fuel without getting some.
They have proper names like Klebsiella spp., Corynebacterium spp., Alcaligenes spp., Pseudomonas aeruginosa, and dozens of others.
How fast these creatures grow depends heavily on temperature, but also on the amount of water present and the chemical make-up (Ph) of the water. The experts with whom Practical Sailor talked said that in as little as a week they can foul the fuel enough to start collecting in the secondary filters. In a month, the gunk can clog a common filter. The bacteria create a slime mat, with bodies laced together and long tendrils that can be seen with the naked eye.
The experts said most biocides are just alcohol, and useless. They don't kill all the HUMbugs, nor do they clean the slime-covered tank walls. The biocides must be used continuously, which encourages an accumulation of crud and promotes corrosion.
The good biocides aren't often sold over the counter. You need to find a serious source, such as the trucking industry and industrial or agricultural refrigeration suppliers. (Hospitals that use diesel engines on back-up electrical systems usually have fuel "polishers" that circulate the stored fuel through expensive filters at required intervals.)
The U.S. Enviromental Protection Administration offers on the Internet a 52-page registry of hundreds of fuel additives—including, believe it or not, several made by the Bayer Corporation, which is better know for aspirin.
What can a boat owner do to avoid the HUMbug plague? It's very simple. No water = No HUMbugs.
Be careful where you buy fuel. The critters live in large numbers in the bottom of big storage tanks and fuel dock tanks that, being underground, produce lots of condensation. Keep your own tank as full as possible (to reduce condensation) and filter the fuel as it comes aboard.
If the filler tube permits, it would help perhaps once a season to run a small hose down to the bottom of the tank, pump out a glassful, and let it settle to see if what's down there is water or fuel oil. (Some tanks have spigots, but usually it's impossible to reach them.) If you find water, especially dirty-looking water, keep pumping until you get clean, clear fuel.
Depending on how much you use your boat, drain and clean the tank every year or two. The more you use the boat, the less often you need to do this. Fuel left to sit gets bad.
By the way, PS is working on a story about fuel additives, including HUMbug-killers. Please stand by.