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The world's first hybrid electric tugboat, Foss Maritime's Carolyn Dorothy which plies Southern California's San Pedro Bay, emits 73 per cent less soot, 51 per cent fewer nitrogen oxides and 27 per cent 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.

The Port of Long Beach, which handles more cargo and containers than any other US port, contributed $500,000 to the cost of $8m tug boat built by Foss. Founded in 1889, the Seattle-based firm operates one of the larger tug and barge fleets on the US West Coast.

Foss developed the hybrid tug to help cut pollution from merchant vessels and improve their fuel economy. The Long Beach and Los Angeles ports are the largest contributors to air pollution in California's South Coast Basin, according to UC Riverside.

The design for the Carolyn Dorothy received the EPA's Clean Air Excellence Award for Clean Air Technology in 2008. Last May, Foss received an Environmental Excellence Award for Green Enterprising Technologies from the Association of Washington Business. The tug boat also has received attention beyond environmental circles -- the vessel was featured in an April 2010 episode of The History Channel's "Modern Marvels" TV program.