Understanding Diesel Fuel

Understanding Diesel Fuel

Trawler and motor yacht owners love the sound of their diesels. It is  like music to our ears. The diesel power plants are the heart of any vessel. But problems can occur with diesel fuel stored in your used trawler’s fuel tanks, affecting those diesels.

It is important to know what diesel fuel is to begin with. Diesel fuel is refined from crude oil. In oil refineries, crude oil is heated, and various hydrocarbon molecules are extracted to create the fuel. Diesel is much heavier than gasoline and evaporates much more slowly. It’s often referred to as “fuel oil” because of its consistency. Often forgotten, diesel is an organic matter.

“Give a used trawler diesel engine clean fuel and it will run forever.” This old saying is less of an exaggeration than you might think. Repair statistics show that 90% of used trawler diesel engine problems stem from contaminated fuel. The promise of eliminating 9 out of 10 potential failures should put fuel-system maintenance at the top of your list.

The most common signs of fuel contamination in a trawler are clogged filters, and reduced engine performance. However, the absence of these conditions does not necessarily imply that your fuel is not contaminated. In fact, it is likely that every trawler’s fuel is somewhat contaminated. Fuel pick-up tubes, where the engine draws fuel from the tank, typically sit about three-quarters of an inch off the bottom of your fuel tank floor. This placement is designed to protect the engine from contamination that has settled to the bottom of the tank. Consequently, you may never realize that you have a fuel contamination problem until the fuel is agitated in some way – such as when you are in rough waters in your trawler.


Water can get into trawler fuel storage tanks in several ways.  One is by condensation of humid outside air.  Another is during transportation from refineries to distributors.  Also by leakage through faulty fill pipes or vents and by careless handling. Water can cause injector nozzle and pump corrosion.  It can also cause microorganism growth and fuel filter plugging.    Your trawler’s water filters should be checked frequently for water and drained as necessary. In cold northern winters, ice formation in fuels containing water creates severe fuel line and filter plugging problems. Regularly removing the water is the most effective means of preventing this problem.  However small quantities of alcohol may be used on an emergency basis to prevent fuel line and filter freeze-ups.

Frequent trawler diesel fuel filter changes and the expensive and time consuming task of cleaning diesel fuel tanks have become acceptable periodic maintenance instead of a warning signal for diesel engine failure. Diesel fuel filter elements should last several hundred hours or more and injectors some 15,000 hours. However, since diesel fuel is inherently unstable, solids begin to form and the accumulating tank sludge will eventually clog your diesel fuel filters, ruin your injectors and cause diesel engines to smoke.

Fuel stored in trawler tanks for long periods (6 months and more) requires special attention. This fuel suffers from multiple problems that influence its quality. The presence of free water provides the medium for microbiological growth that result in the formation of slime and acids causing corrosion of metal surfaces such as storage tanks, pumps, injectors, etc. Left unattended this water layer will trap sludge and become the breeding ground for microbes, fungus, yeast and more. This toxic mix produces acids that compromise the integrity of your tank, lines, pumps, fittings and worse, diesel machinery; microbes live in the water and feed on the fuel.

Microbes in the form of bacteria and fungus are present in all diesel fuels. Long periods of fuel storage can create ideal opportunities for microbes to grow in fuel tanks. The first indication of microbial contamination is mucous-like accumulations on fuel-filters and increased requirements for fuel-filter replacement. Microbes can only be removed from the fuel system by polishing or by preventing their occurrences by use of a diesel fuel biocide such as BioBor. I highly recommend using it. You can see how I polished my trawler’s fuel at this link.

Another key factor leading to fuel deterioration is mechanical stress caused by the heat and pressure of pumps. Since most diesel engines return considerable amounts of fuel back to the tank, it is easy to see that the engine itself contributes to fuel deterioration.

Should you use diesel fuel additives? There are plenty of additives on the market today that are designed to improve the performance and efficiency of diesel fuel. I prefer to use Marvel Mystery Oil. However, if you use the right additives, you can achieve peak performance from your fuel every time.

Most used trawlers fuel systems have primary and secondary filtering systems installed; Racor is a good example. Each time the engine is operated, the diesel is polished by filtering. A separate polishing system can also be installed that filters the fuel independently of the engine operating.


  1. Alan V.Cecil says

    Thanks Mike! Enjoyed the straightforward guide. I’ve boated for years on everything frok a 19′ cedar planked Lightening, to a 20′ Chincoteague Scow, to a 52′ Chesapeake Bay oyster buy-boat (built in 1912) sailboats in the USVI, to 1968 31′ Chris Craft Commander (currently nonoperational).

    One of my aspirations is the AGL. I’m going to their Spring Reuni8n that is here in Norfolk next week. (I currently live in Norfolk on Willoughby Spit.

    In the past two years I’ve electronically viewed 100s of trawler type boats with the AGL in mind. Realistically my benchmark dream might have to be trimmed down to Cruising the Chesapeake Bay and North Carolina sounds and their numerous river systems because of finances and other matters.

    It doesn’t preclude me in my zeal for the AGLCA because ok believe that one doesn’t have to be a New York Yankee to be a fan or follow their games!

    What a novel and professional offer you make to the readers of your article on Diesel Fuel!

    Thank you,

    Alan V. Cecil
    Norfolk, VA

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