All-electric tugs: the drive for maximum efficiency

For an industry on a mission to cut costs and carbon emissions, all-electric tugboats have some serious pulling power. With cheaper running and maintenance costs, and lower (or zero) local emissions compared with their diesel counterparts, it is easy to see the appeal. 

And a tug’s operational profile is perfect for all-electric propulsion. It spends most of the time at low load, with short bursts of higher power demand and very rare peaks of max power. 

The responsiveness and ability to quickly generate thrust that comes with all-electric propulsion can be crucial when manoeuvring in tight spaces and tricky ship-assist situations.

However, fitting batteries into the small space available on tugs presents a challenge, as does endurance concerns. As demonstrated in numerous recent projects, through close collaboration between designers, shipyards and equipment suppliers, it is possible to make the batteries fit. 

The endurance concerns are addressed by dimensioning batteries and planning charging infrastructure to fit the operations. 

They can also be resolved through the mix of tugs in a particular port, ensuring the fleet can handle the odd day with extremely bad weather and an unusually high number of ship assists. 

The key to success is to make sure the entire power and propulsion system is as efficient as possible and designed with a thorough understanding of the real operating profile. 

Most electric tugs are based on direct current (DC) rather than alternating current (AC) power distribution to reduce losses when integrating energy storage and numerous drives. They are typically liquid cooled to maximise power density, reliability and efficiency, which ultimately leads to reduced downtime and lower maintenance costs.

Liquid cooling takes care of 98% of a drive’s heat losses, with no need for additional filtered air cooling. This allows liquid-cooled drives to be compact and powerful, making them the optimal solution for tugs in demanding marine environments, where space savings and reliability are a must.

Indeed, several series of new all-electric harbour tugs are currently under construction around the world. ABB drives have been selected to power more than 13 electric tugs, with flexibility being key. 

These tugs are being built to seven different designs at four different shipyards, in collaboration with three different system integrators to at least six different owners.