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Annual ‘physical’ spares yacht’s diesel a preventable ‘illness’


By Capt. Jeff Werner

We all understand the importance of completing an annual physical with our doctor. It is longevity. We all want to live as long as possible.

Parts of our annual physical are tests ordered by our doctor to check our levels of cholesterol and blood sugar. We also do biometric screening such as measuring our blood pressure and bio mass index. The goal is to identify potential health issues, even before we experience symptoms, since many issues that develop over time can be prevented.

Diesel fuel is no different.

We all want longevity for the engines on board the yachts we work on, so our jobs can run smoother. Fewer breakdowns mean fewer headaches. Since diesel fuel is the lifeblood of our engines, screening the fuel to make sure it is “in spec” should be part of all yachts’ fuel preventive maintenance program.

The American Society of Testing and Materials (ASTM) has developed 2,400 standards for diesel fuel. In the petroleum industry, if fuel doesn’t comply with those specifications, it is “out of spec” and is considered “bad fuel.”

Of the 2,400 ASTM specs for diesel, there are only seven that an engineer or captain should be concerned about for everyday use, and they should be tested for on a quarterly basis:

  • 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.
  • Water determination by Karl Fischer titration measures the concentration of water entrained 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 sh


    elf life.

If testing and analysis determine the fuel is out of spec and this is not addressed by fuel polishing, tank cleaning and the use of proper additives, the results can be detrimental for an engine.

The distillation analysis determines the boiling range characteristics of the fuel. This affects the injection, ignition and combustion phases of the diesel engine cycle. It is an important predictor for smoke, exhaust odor, cylinder and piston deposits, fuel economy and, ultimately, power output.

The analysis of the remaining six tests all have an impact on a yacht’s fuel delivery system: fuel storage, fuel filters, high pressure fuel pumps and the injectors. If any of these six tests are out of spec, then problems with tank sludge, corrosion, fuel flow, filter plugging, pump wear and injector wear are likely.

These seven tests are typically performed for a package price by a dedicated fuel testing laboratory using a fuel sample provided by the yacht. Any crew member can be taught to draw the sample needed from each tank.

If you suspect bad fuel, or conditions that accelerate the natural loss of fuel quality, then these tests should be run more frequently than four times a year.

Samples should be drawn from two locations within the tank, the bottom and center. Water, dirt, rust, and other debris will settle to the bottom of a tank. It is important to detect and correct for these contaminants. However, the bottom sample may not be representative of the overall fuel characteristics. Therefore, a mid-fuel level or center sample should be drawn to represent the general properties and characteristics of the tank’s contents.



For smaller tanks, samples can be taken with a specially manufactured, inexpensive hand pump that collects the fuel in a plastic bottle. For larger capacity tanks, a metal “fuel thief” or “bacon bomb” is used. These samplers are much more expensive than the plastic hand pump variety. For a bottom sample, the fuel thief is lowered into the tank until the sampler’s plunger contacts the tank’s bottom. That opens the plunger assembly, admitting fuel into the housing. To take a mid-level fuel sample, a tug on a pull chain attached to the plunger opens the housing.

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


Capt. Jeff Werner has been in yachting for almost 25 years, and is the owner of Diesel Doctor ( All Triton readers receive a 10 percent discount on online orders. Comments are welcome below.

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One thought on “Annual ‘physical’ spares yacht’s diesel a preventable ‘illness’

  1. Capt. Juan alvarez

    Thank you Captain Jeff for your post. You’ve just opened my mind. It’s really a good idea to test the fuel on board. I definitely will.

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