World's first hybrid tugboat in Long Beach





The world's first hybrid tugboat was welcomed to southern California on 23 January 2009. The vessel of the Los Angeles fire department christened "Carolyn Dorothy" as a greeting in the Long Beach port.


The world's first hybrid tugboat was taken into use in the Long Beach port in California in January 2009. The tugboat "Carolyn Dorothy" is efficient, green, and quiet. It will probably have lots of followers, because hybrid tugboats are expected to be a major trend in the marine industry. In the hybrid propulsion system, frequency converters manufactured by Vacon are used.

Harbour tugs perform a variety of tasks, but have a most inflexible power production system. This results in inefficiency, high use costs, and a negative impact on the environment. Located in Seattle and specialised in marine transport and logistics, Foss Marine concluded that the best way to solve the tugboat problems would be hybrid technology. 

Foss partnered with Aspin Kemp & Associates (AKA) and XeroPoint to develop a unique energy transformation and power management system, which would be environmentally friendly without sacrificing power or functionality. An efficient combination of batteries, generators, and main engine produced a hybrid tugboat with better performance, lower fuel consumption, maintenance costs, and emissions than traditional tugboats. When running on battery, a hybrid is also quieter than traditional tugboats. Foss thinks that markets are open to "green" hybrids, and demand for them will grow in the coming years.  

The first hybrid tugboat "Carolyn Dorothy" will serve alongside traditional tugboats at the two largest US container ports, Los Angeles and Long Beach. The 5000 hp Carolyn Dorothy is mostly used for support tasks – to move vessels like tankers and container carriers to the port, to the pier, and out of the port.

Carolyn Dorothy is part of the Clean Air Action plan for the ports in the area. The goal of the plan is to reduce the health risks caused by the emissions of port related vessels, trains, trucks, port equipment and harbour crafts. The hybrid tugboat was granted the Clean Air Excellence Award for Clean Air Technology award in 2008. Arnold Schwarzenegger, Governor of California, congratulated Foss Maritime by letter on the day of the unveiling of the hybrid. "My deepest gratitude goes to all the people who made this project possible.  Applause for the hard work and thank you for your help in keeping this state green".  

Transformability as an asset of "green" technology

A tugboat spends up to 50 % of its time idling, with the main engine running and ready for action, but the power is used in reality. In a hybrid tugboat, the energy for idling and moving around the harbour is taken from batteries or smaller diesel generators, so the main engines are only needed for heavy work.  

A hybrid corresponds to a traditional Dolphin tugboat as regards power and towing power, but it uses current control, which keeps the most important engines running almost at their design point performance and switches them off when they are not needed. Compared to a Dolphin tugboat, a hybrid has smaller diesel engines and bigger diesel generators, a principal axis driven motor/generator, innovative power transformation and control technique, and a rechargeable battery. The vessel can work with diesel, diesel-electricity, diesel-plus-electricity, or mere electric power.  

A hybrid tugboat is designed so that the current tugboats can also be transformed to hybrids, following the example. Using new energy forms, like biodiesel and  ethanol is also possible. Batteries can be charged with the cheaper electricity onshore.


  • Length: 23.77 m
  • Width: 10.36 m
  • Maximum draft: 5 m 
  • Towing force: 60+ tons
  • Power: 5,080 hp/3,790 kW

Frequency converter technique as part of a hybrid motor system

Vacon frequency converter technique is used as part of the Carolyn Dorothy hybrid motor system. Vacon participates in the project through its long-term partner Eaton Corporation, which operates in the North American market. 
In the hybrid motor system there are two Vacon frequency converters manufactured under the Eaton trademark. They are 750 kW liquid cooled LCX9000 frequency converters (Vacon NXP series 525-690 V). The frequency converters control the motors/generators of the main drive, assisted by two 325 kW Active Front-End (AFE) units (525-690 V).   

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