Seaworthy: From The BoatUS Insurance Files


Boats And Ultra-Low-Sulfur Diesel

By Bob Adriance
Published: June/July 2012

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.

Photo of diesel 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.

1 | 2 | Next


BoatUS Magazine Is A Benefit Of BoatUS Membership

Membership Also Provides:

  • Subscription to the print version of BoatUS Magazine
  • 4% back on purchases from West Marine stores or online at
  • Discounts on fuel, transient slips, repairs and more at over 1,000 businesses
  • Deals on cruises, charters, car rentals, hotel stays and much more ...
  • All For Only $24 A Year!

Join Today!