I was lining up my course to the inlet, returning from a perfect day of offshore mackerel fishing, when my engine developed a slight but obvious hesitation. I eased off the throttle and rpm, let the engine cool and inspected the Racor filter. My Caterpillar 3208 wasn’t due for a fuel-filter switch, but I had a hunch it’s where I’d find the problem.
Indeed, I was surprised to see a dark blotch of something floating in the Racor globe. Then, I realized that the day’s fishing conditions had been in rough water. If anything had been living in the boat’s fuel tank, the sloppy, bouncy ride could have stirred it up from the bottom.
As a precaution, I changed not only that filter, but also the secondary fuel filter, the one closest to the fuel pump. I was happy to see nothing other than clean fuel coming out. Nonetheless, my boat’s hesitation was a warning that too many skippers ignore, often leading to far bigger problems.
Bacteria that lives and thrives in fuel tanks can resemble a dark or greenish slime. It has a habit of appearing without warning, exposing itself via a sudden loss of engine power. Worst-case scenario, it can clog the fuel line, shut down the engine and make restarting iffy at best. If you are lucky enough to get a warning before losing an engine, do not look that gift horse in the mouth. When the bacteria gets past the boat’s filtration systems, it can damage the fuel injection pump, and you are dead in the water.
Microbial growth flourishes in fresh water. It finds its path into your fuel supply in a number of ways, likely beginning at your fuel source. It may start at the refinery, transfer into the fuel-supply truck and the marina, and then reach your boat’s tank. Although most fuel docks have some sort of filtration system, the bacteria thrives in the condensation on fuel tank tops and sides that are exposed to air—especially metal tanks that fluctuate with temperature changes day and night, or seasonally. Fresh water also can enter your fuel tank from rain or washdown water if the fill-cap gasket is worn or missing, or if the cap is loose. Keeping the fuel tank full can help prevent condensation, providing less real estate for the bacteria to grow.
Eventually, the bacteria will fall from the tank walls into the fuel supply. From there, it will drift to the bottom, where the fuel pickup tubes are located. In rough conditions, when fuel is washing the tank sides, there is a party going on with a host of unwanted guests.
Ideally, the engine’s fuel-water separator filter will attract and collect the bacteria, but the filter is only as good as its condition—hence the need for regular inspection. My Racor filter has a globe at the bottom for visual inspection. It also has a drain valve to remove water or impurities that may collect in the globe. If your engine room is poorly lit, shining a flashlight behind the clear globe helps to verify your system’s status.
My primary Racor uses a 30-micron element, with secondary element filters down to 2 microns (one micron is the thickness of a single human hair). I change both fuel filters at the same time, although some skippers will change the secondary at every other interval. I have learned that if any dirt or bacteria shows up in the primary, there is no doubt my fuel supply has been compromised. I don’t take chances; changing filters while at sea is never fun. Savvy skippers always carry a stock of spare filters.
Controlling microbial growth in fuel is an ongoing maintenance imperative. You could go for years and never have an issue, and then suddenly, one batch of bad fuel or an eruption of bacteria leads to nonstop havoc. In serious cases, it may be necessary to hire a professional fuel-polishing team to purge the bacteria and sludge from the fuel, with multiple runs through a network of filters. This process could take an entire day or longer, often with no guarantee of success.
What has worked for me is to add a capful of Biobor JF microbiocide to my tank each time I fuel up. The additive is a metabolic poison for microorganisms. It’s especially effective in small, regular applications. I started using it when I was delivering yachts between the Northeast and Florida because I never knew the condition of the fuel in the boat I was operating on long ocean legs. Although some fuel additives claim to work in single, massive doses, I do not recommend such treatment. It is akin to starting a controlled burn that can destroy the entire forest.
In addition to using suitable biocides and running the engines so the fuel transfers all the debris into the filter housing for removal, I recommend checking fuel filters regularly at the beginning of the season, and throughout the warm summer months. In addition, get in the habit of running your engine at full throttle for a minute or two at the end of each trip to verify it can turn up to its rated rpm specifications. An engine that does not turn properly often suffers from a fuel-related issue that needs attention.
This article was originally published in the May/June 2023 issue.
