Ultra-low-sulfur diesel (ULSD) was mandated for use in most vehicles, boats and machinery as of December 2010 to reduce particulate matter but it has been around since late 2006. ULSD has only 15 ppm or less of sulfur was first produced when the 2007 model year vehicles rolled out with their more advanced emissions control devices. Most refiners only have the capacity to offer one type of diesel so ULSD has been sold at marinas ever since.
Boaters have been keeping a close eye on the effects of this environmentally-friendly fuel for any potential problems. Recently the BoatUS Damage Avoidance Program investigated the issues and has these findings to share:
Lubricity: In diesel engines, having enough "lubricity" in the fuel is critical - without it, the engine would grind itself to a premature death. A lot of publicity has been given to ULSD because the process of removing sulfur from diesel fuel also removes much of the fuel's lubricity. Contrary to what some have said, however, lubricity is not a problem with ULSD. Minimum lubricity is a requirement of the ASTM-D975 diesel fuel standard and oil companies typically use a synthetic additive to return fuel to its pre-ULSD lubricity levels.
Cetane: 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.
Gaskets: When the transition was made to low-sulfur diesel (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 - need water to grow and have always been a concern with diesel fuel. ULSD holds less water than older, higher-sulfur fuels, which means that any water entering the tank is less likely to be absorbed and instead more likely to become a breeding ground for bugs. Biocides (and cold weather) kill the bugs but their tiny little carcasses pile up in funereal goo at the bottom of the tank. Tanks may need to be cleaned more often to prevent clogged filters and corrosion. The best defense is to keep tanks as full as possible (especially over winter storage) and keep a routine eye on the water separator.