Reliable Diesel Fuel Matters

Written by Capt. Jeff Werner

Diesel fuel is the lifeblood of the main engines and generators aboard every yacht. If left unmanaged, diesel fuel quality can degrade rather quickly rendering it harmful rather than helpful to an engine. The first step in assuring reliable fuel is developing a three-part fuel preventive maintenance program: sampling, testing and polishing. As with all preventive maintenance programs, a small amount of time used up front will stop problems and costly repairs from developing in the future. Consider that maintaining a yacht’s fuel supply is just an inexpensive insurance policy for the fuel system and engines.

Diesel fuel is hygroscopic by nature, meaning the fuel has the ability to attract water from the environment and hold it in solution. Water is the most damaging contaminant found in diesel fuel, and it is the primary cause of additional fuel breakdown. Water can come from humid air, condensation on the wall of a fuel tank or be mixed in with the fuel when it is purchased at a marina. ere is one basic rule when it comes to water in diesel fuel, if it is present, remove it. If not, microbial growth will occur.

Since water is denser than diesel fuel, the dissolved water will eventually settle at the bottom of the fuel tank. This free water serves as a perfect medium for microorganisms to live in. And the hydrocarbons in the fuel are a tasty food source for many species of microbes. The result is a proliferation of bacteria and fungi feeding at the fuel/water interface and large colonies floating in the free water below that interface. In addition, microbes can adhere to the tanks’ walls and can grow fast enough to quickly coat those walls with a slime of organisms.


Samples should be drawn from two locations within the tank: the bottom and center. Since water, dirt, rust and other debris will settle to the bottom of a tank, it is important to detect and correct for those contaminants. However, the bottom sample may not be representative of the overall fuel quality. erefore, a mid-fuel level or a center tank sample should be drawn to represent the general properties and characteristics of the tank’s contents. Samples can be taken using a specially manufactured, inexpensive hand pump that collects the fuel in a plastic bottle.


Once the samples are collected, they are sent to a fuel testing laboratory for analysis. There are seven standard tests that a yacht owner should be concerned about for determining if the fuel stored in the boat’s tanks is in specification:

  • Distillation gives information on the behavior of fuel during storage and use.
  • Microbial growth determines the presence of bacteria, mold and fungi and the size of the colonies.
  • Flash point indicates contamination of diesel fuel by other fuels such as gasoline, which may occur when fuel is delivered from the refinery.
  • Water determination by Karl Fischer titration measures the concentration of water dissolved in the fuel.
  • Water and sediment determines the volume of free water and particulate matter in the fuel.
  • Appearance using a visual inspection procedure to determine the clarity of fuel.
  • Stability and accelerated aging measures the oxidation of fuel that affects its shelf life. These tests should be completed on a semiannual basis and the analyses used to determine the fuel polishing regimen and proper additives to use to cure any problems with the fuel that is out of spec.


In general, fuel polishing is a multi-phase process. First the fuel is passed through a magnetic conditioner that breaks apart large clusters of hydrocarbon contaminants that form as fuel ages. Permanent magnets generate a very weak electrical eld that separates the bonds holding the large wax and asphaltene molecules together so they are more combustible.

The next phase is a water separator or centrifuge that removes free water. is step is followed by circulating the fuel through a disposable particulate filter that removes organic and inorganic matter greater than 30 microns in size.

The last phase is final filtration in which particles down to a diameter of three microns or less are removed. is is the most critical step in the fuel-cleaning process since particulate contamination is the number one cause of failure in electronically controlled common rail fuel systems in newer diesel engines.

During the fuel-polishing process an additive is used to dissolve sludge and slime that accumulates on the walls and baffles of the tanks. is procedure assures that mold and bacteria are removed from all tank surfaces. Once the tank is cleaned and the fuel is polished, an additional dose of fuel additive is mixed into the tank to help guard against future microbial contamination. Use of a full spectrum additive during this step will also add lubricity to the diesel fuel, prevent corrosion, reduce carbon build-up and stabilize the fuel.

Ideally, fuel should be polished and the tanks cleaned whenever the yacht is refueled or on a monthly basis if the yacht is not fueled frequently.

Implementing and strictly adhering to a fuel preventive maintenance program is a simple and inexpensive alternative to the cost of repairing severely damaged high-pressure pumps, injectors and pistons.