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Boats and Ultra-Low-Sulfur Diesel

For anyone fond of breathing, and fond of their engines, there's a lot to like about ULSD.

Less than two decades ago, diesel fuel contained up to 5,000 parts per million (ppm) of sulfur. That dropped to 500 ppm in 1993. Ultra-low-sulfur diesel (ULSD), with only 15 ppm or less of sulfur, was mandated for use in most vehicles, boats, and machinery in December 2010. As a practical matter, however, ULSD has been around a lot longer, since late 2006, when oil companies had to begin selling ULSD for use in model year 2007 diesel vehicles. (The latter have advanced emissions control devices that require ULSD in order to work properly.) A spokesman for one of the oil companies said that refiners don't have spare tanks available to offer two types of diesel fuel, so once ULSD was mandated for newer vehicles, it was sold to everyone. A survey of service stations by the Environmental Protection Agency (EPA) found that in September 2006, 85 percent of service stations were already selling ULSD (although less than half of the pumps were labeled correctly). By the fall of 2007, the number had grown to 94.7 percent. And while the EPA didn't compile statistics for marinas, refiners don't store "marine" and "vehicle" fuel separately; whatever was going in trucks and automobiles was distributed to marinas.

Given the problems that arose with ethanol, the obvious question from anyone who owns an older, pre-2007 diesel is, "What is ULSD fuel liable to do to my engine?" After talking to numerous repairers, marina managers, engine manufacturers, and oil company technicians, the answer is likely to be: very little. Below are the potential trouble areas and what, if anything, could be in store for your marine diesel engine.

Diesel boat engine


The most immediate concern with ULSD, certainly the one that has received the most publicity, is lubricity. When diesel fuel is refined to make ULSD, it is reacted with hydrogen to remove the sulfur. This process also removes much of the fuel's lubricity. (Many people mistakenly believe that it's only the sulfur that supplies lubricity.) Lubricity is essential in diesel fuel to reduce friction at finely machined pumps and injectors; without sufficient lubricity, a diesel would grind itself to a premature death.

Minimum lubricity is a requirement of the ASTM-D975 standard, which means oil companies must use either soy biodiesel or a synthetic additive to return fuel to its pre-ULSD lubricity levels. Soy biodiesel has the potential to loosen built-up gunk in the tank and clog filters, but not to the extent that ethanol does with gasoline tanks. Note, however, that a synthetic additive is more likely to be used, as it’s less expensive and more stable than soy.


Diesels rely on compression (and not a spark) to ignite the fuel. A higher cetane number means the fuel will ignite more readily, run smoother, and produce less smoke. All diesel fuel must have a cetane rating of at least 40. Most regular diesel fuel has a cetane rating of 43 to 45, which should be fine for most boat engines. The good news is that the cetane numbers remained the same with ULSD.

Using an additive to boost lubricity may help your engine, but be aware that an independent study of 19 additives sold to improve an engine’s lubricity and/or cetane rating found that five had no significant effect on the fuel’s lubricity rating and four additives significantly lowered the lubricity content. Engines performed worse when the additive was added.


Since ULSD was introduced in 2006, some suppliers have reported accelerated corrosion in underground steel storage tanks. The cause is unclear and no one is certain whether it's related to ULSD, an additive, or something that occurred during the transition from LSD to ULSD. (Curiously, in Europe, where ULSD has been in use for much longer, there have been no corrosion issues.) The only solution, thus far, is to keep tanks as clean as possible with no water in the bottom so there's nothing with which the fuel can react. Boat repairers we talked to haven’t seen any indications of corrosion problems.

New diesel boat engine

Leaking Gaskets

When the transition was made to LSD in 1993, there were problems with leaking gaskets. Newer gaskets that resist leaking were developed, but there were some fears that the gaskets might not stand up to ULSD. After talking to numerous marina owners and engine manufacturers, leaking gaskets don't appear to be a problem.

Water And "Bugs"

Microbial growth — bugs — needs water to grow and that's always been a concern with diesel fuel. ULSD holds less water than older, higher-sulfur fuels. While that sounds like good news, it's not; any water that finds its way into a boat's tank is less likely to be absorbed into the fuel and more likely to wind up at the bottom of the tank, where it can help spawn the dreaded microbial "bugs." Biocides kill bugs (as will freezing temperatures), but their tiny little carcasses accumulate at the bottom of the tank and form a funereal goo. It's possible that tanks may need to be cleaned more often to prevent filter clogging and corrosion.

The best defense is to keep water out of your fuel by keeping the tank topped off to reduce condensation, only buying fuel from a reliable source, and checking your water separator routinely. If water starts to appear regularly, you'll have to take steps to clean your tank and "polish" the fuel. (There are several ways to polish your fuel. You can have it done professionally, or purchase an onboard system. The principle is the same, however: A pump pulls the fuel from your tank, filters the microorganisms and moisture from it, and returns it to the tank.)

Cold Weather

The refining process used to lower the sulfur content of ULSD also can affect the content of naturally occurring paraffin (wax) in diesel fuel, which causes it to gel more readily in cold weather. For the vast majority of boat owners who lay up their boats over the winter, cold-weather starting isn't a concern. For anyone who plans to use their diesel in winter, fuel distributors compensate for colder temperatures by selling a winter blend. If you still have a summer blend in the engine, you'll need to use a cold-weather additive and follow the instructions. Use only the recommended dose, as too much additive may make gel problems worse.

Fuel Source

A reliable source for your fuel is (and has always been) very important. A high-volume dealer is far more likely to have fresh fuel than a sleepy, backwater marina. As a general rule, diesel fuel can be expected to remain "healthy" for at least a year. The major oil companies or distributors will sometimes use their own additives (antioxidants and biocides). If the fuel has been treated and stored in a clean (no rust), water-free tank that is in a cool (or underground) climate, diesel fuel can last as long as three years.

Bob Adriance

Bob Adriance is the former editor of Seaworthy, which is now incorporated into BoatUS Magazine and called BoatUS Reports. The stories and lessons learned are based on real-life BoatUS Marine Insurance claim files.