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Law intended to clean exhaust unintentionally hurts engines


The worldwide effort to combat global warming is part and parcel to the daily operations aboard a yacht. Both the International Maritime Organization (IMO) and the U.S. Environmental Protection Agency (EPA) give yacht captains and engineers diesel engine standards and guidelines to follow.

IMO MARPOL Annex VI limits the main air pollutants contained in marine engine exhaust gases. Engine manufacturers comply by adding advanced technology to newly manufactured diesel engines that will reduce pollution to mandated levels. They have tweaked the combustion process as much as possible, and tacked on new “after engine” treatments to achieve the necessary results.

There is one variable left to fine tune in the equation for cleaner emissions, which is diesel fuel quality. To the rescue comes ASTM, an international organization that develops and publishes a range of technical standards, including those for diesel fuels. A little over six years ago, ASTM revised D975, better known as “Standard Specification for Diesel Fuel Oils”, which was adopted by most oil companies and their refineries selling to U.S. markets.

For yacht engines and generators, Grade No. 2-D is the fuel that is used on board. It is defined as a “general purpose, middle distillate fuel for use in diesel engines … especially useful in applications with conditions of varying speed and load”.

A close look at the fine print of D975 reveals the following:

Grade No. 2-D diesel can contain up to 5 percent biodiesel and still be labeled as 2-D. Therefore the buyer does not know that biodiesel has been mixed in. Biodiesel is added to help increase fuel lubricity that was lost when most sulfur was removed from fuel blends.

Grade No. 2-D diesel can contain up to 500 parts per million of water and sediment. That means, if a yacht bunkers 10,000 gallons of fuel, it can contain up to five gallons of water and still be sold legally as meeting specifications.

Doug Haugh, the president of Mansfield Energy, one of the largest diesel suppliers in the United States, writes, “The challenge with using increasing amounts of biodiesel is that, at the temperature and pressures of modern diesel engines, there are likely to be deposits formed in the engine… .

“With sulfur removed, diesel fuel can hold far less water in solution. When that water drops out of solution in tanks, the bacteria and algae that feed on the hydrocarbons while living in the water start having quite a feast. In the process these organisms are creating a hideous mess inside diesel tanks of all shapes and sizes … .

“So if you are dumping nearly a five-gallon pail of water in your diesel tank every time you get a load of diesel and that low-sulfur diesel no longer holds water in suspension, let’s guess what we find in our tanks at the end of the year. I am not even going to talk about the sediment part of the specification. Let’s just assume running dirt through an engine with two to four micron clearances is, on the face of it, a really bad idea.”

In July, the EPA released a study on the increasing presence of substantial corrosion in diesel fuel tanks. The results showed that 83 percent of diesel tank systems exhibited moderate-to-severe corrosion, and 75 percent of those responsible for maintaining the fuel tanks had no idea it was happening. Corrosion develops in both steel tanks and the metal components of fiberglass tanks.

Not only does water in fuel contribute to corrosion, so do the caustic waste products of the microbes living in the diesel fuel.

The key to living with ASTM D975 is establishing a scheduled maintenance program for the fuel stored in a yacht’s fuel tanks. This program includes:

* Visually inspecting inside the fuel tank for fuel color, odors and corrosion.

* Inspecting all fittings and orifices for rust.

* Using a water-finding paste to check for the presence of water.

* Sampling fuel in the tank and testing its quality against standard benchmarks.

* Treating fuel with a fuel additive.

* Regular fuel polishing.

With this program, diesel engines will run trouble-free longer, which means less repair costs and downtime, and more time for the yacht owner to enjoy cruising.
Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain on private and charter yachts, both sail and power, and a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor ( Comments are welcome at

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One thought on “Law intended to clean exhaust unintentionally hurts engines

  1. Richard Boggs, Owner, EnerYacht

    Law intended to clean exhaust unintentionally hurts engines
    Wait a minute… let’s put the content of this article in perspective. To start, while the maximum amount of dissolved water allowed in diesel fuel per ASTM D975 is 500ppm, the actual water content in fuel, as delivered, is usually well under 200ppm and the norm might be 100ppm or less. 500ppm is the U.S. standard, European diesel fuel standard EN 590 limits water content to 200ppm.
    Substandard fuel storage procedures and facilities located in remote outposts, might increase the risk of higher dissolved water content but a minimal amount of attention by the yacht’s engineer before and during fuel loading can greatly reduce the risk. Water content in concentrations of around 150ppm can be seen as a slight cloudiness when viewed in a clear sample bottle in bright light. Ensure that the delivery truck water drains are checked for free water and take regular samples during loading, especially after changing tank truck compartments.
    Reference to the EPA study on diesel fuel tank corrosion is slightly misleading. The subject of the study was underground fuel storage tanks (USTs) and the corrosion problems noted occurred in the vapor space above the fuel level. No increase was found in corrosion attributed to water in tank bottoms or in areas normally covered by fuel. While the EPA and the UST industry state clearly that minimizing water content is critical, they do not attribute the problems to dissolved water in the fuel.
    Centrifugal separators and coalescing filters used in combination between fuel tanks and engines should relieve yacht engineers of concern about a problem that more likely than not simply does not exist to the degree the article implies.

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