The world's first hybrid electric tugboat, Foss Maritime's Carolyn Dorothy which plies Southern California's San Pedro Bay, emits 73 percent less soot, 51 percent fewer nitrogen oxides and 27 percent less carbon dioxide than a standard tug of comparable size, according to a study by the University of California, Riverside.

Researchers from UC Riverside's College of Engineering Center for Environmental Research and Technology charted the performance of the Carolyn Dorothy against that of the tugboat Alta June for the study released this week.

Both vessels are Foss "dolphin class" tugs and were tested in the ports of Los Angeles and Long Beach, where the Carolyn Dorothy operates. The Alta June runs on four diesel engines; the Carolyn Dorothy, on four diesel engines and 126 batteries. All the engines meet EPA emissions standards for Tier 2 certification.

Researchers found that the Carolyn Dorothy's diesel electric drive train, rather than the vessel's batteries, was chiefly responsible for producing the emissions benefits. The researchers recommended that future studies include more running time without batteries to test the initial finding.

The team also said the tug's plug-in capabilities should be tested. The tug did not operate as a plug-in for the study because of insufficient shore power. As a result, the tug was plugged in only for about a third of its time in dock.

The Carolyn Dorothy started working the San Pedro Bay in January 2009. Here is the Port of Long Beach's YouTube video about the vessel's arrival:

Next Page: Backstory on the Carolyn Dorothy.