Tugboats lead U.S. maritime industry into greener future

The future Hydrogen One will run on methanol fuel that is converted to hydrogen. The hydrogen will run through fuel cells to create electricity.
The future Hydrogen One will run on methanol fuel that is converted to hydrogen. The hydrogen will run through fuel cells to create electricity.
The future Hydrogen One will run on methanol fuel that is converted to hydrogen. The hydrogen will run through fuel cells to create electricity.

Maritime propulsion companies are fielding a broad spectrum of powerplants to meet emerging market need, and tugboat designers and builders must make them work in the real world.

Tugboats have been assisting commercial ships safely into port for more than 200 years. Modern iterations of these workhorse vessels have been powered by fossils fuels. Now, under pressure from regulators, bankers and, increasingly, their customers, that’s changing fast. Builders are competing to meet the needs their customers define — and in some cases, trying to jump ahead of their competition.

In a manner almost as profound as the transition in shipping from sail to steam, tug designers now must wrestle with propulsion systems that can radically change the layout and performance of the vessel. They often occupy more volume than in the past while possessing differing mass, weight and power characteristics. The methods for ensuring safety are different, too. 

All of this has an impact on the bottom line. Just as a Tesla costs more than a Toyota, the costs of these new propulsion systems can easily rise to the point of being unaffordable, forcing designers to be more clever than ever before.

The Robert Allan Ltd. Electra design for a fully electric tugboat.
The Robert Allan Ltd. Electra design for a fully electric tugboat.

Lawren Best, director of design and development at Robert Allan Ltd., in Vancouver, British Columbia, said selecting a propulsion plant that fits the tugboat’s operating profile is the first job for a customer and naval architect. In the case of battery-electric propulsion, which is being rolled out on multiple tugs in 2023, the operational profile determines not only the motors needed but also the battery capacity. And those batteries become much more of a design factor than bunkerage was for traditional diesel vessels. 

“Going electric” cannot be an afterthought, Best stressed in a recent interview. And consideration also must be given to backup or emergency power and integrating control systems properly.

But unlike the great flexibility inherent in liquid fuels, batteries require more care and more planning. For starters, the hull form needs to be designed to handle the increased weight batteries represent while maintaining proper trim, metacentric height, stability and handling. “Particularly with high bollard-pull requirements, stability is especially critical,” he said.  

An additional issue, Best added, is that battery-electric systems are more sensitive to water and moisture than diesel powerplants. In other words, choosing an electric powerplant requires more than just modifying an existing hull or substituting one set of components for another.

The hybrid tugboat Spartan, designed by Robert Allan Ltd. and built by Master Boat Builders of Coden, Ala., is a case in point. The 98-by-43-foot vessel has a propulsion system that includes two EPA Tier 4-rated Caterpillar 3512E main engines, each producing 2,550 horsepower plus three Caterpillar generators and two ABB propulsion motors along with multiple variable frequency drives. (Titan, a sister tug to Spartan, is profiled on page 54.)

Coulston Van Gundy, vice president of Crowley Engineering Services, said his company faces those challenges and is actively working on vessel designs that consider the various alternative propulsion options. “Whether it is all-electric, methanol or hydrogen-powered, it is important to take into consideration the customer’s overall operational objective for the vessel and propose a sustainable solution based on those specific needs,” he said.

Put another way, the job the tugboat does and where it does it will influence what type of alternative propulsion system makes the most sense. 

Crowley’s fully electric tugboat eWolf will be delivered this year from Master Boat Builders.
Crowley’s fully electric tugboat eWolf will be delivered this year from Master Boat Builders.

Crowley is currently building eWolf, the nation’s first fully electric ship-assist tug and has designs under development for multiple alternative fuels, including hydrogen and methanol-powered ship-assist tugs. The tug is scheduled for delivery from Master Boat Builders this summer, and it will work in San Diego, Calif. 

Glosten, a naval architecture firm based in Seattle, is also delving into alternative fuel tugs. So far, the company favors methanol as a diesel alternative for many tug applications, according to Peter Soles, who does business development and marine operations for Glosten. 

The tradeoffs with electric propulsion come down to battery capacity. That will influence the vessel’s range, its speeds during transit and its ability to work at high power demands for extended periods. Extra space devoted to batteries and switchgear can also limit accommodation space, which is crucial for longer-duty operations. Finally, there is the question of whether necessary recharging capabilities will be available when and where they are needed. 

“Not all operators can return to the dock and charge, especially if you work in a busy port district where you are subject to ever-changing ship arrival and departure times,” Soles said.

“Can you live with the operational limitations and the added cost?” he added. For instance, if you need rapid charging at the dock, how long will it take and how much will that infrastructure cost to install? Perhaps more importantly, how do you manage the out-of-service time and the shoreside installations?  It is no longer just design, build and put the vessel into service. Operators also need the appropriate supporting infrastructure for alternative propulsion, “so for many operators, it isn’t a viable option,” Soles continued.

Those factors, Soles said, mean methanol is often a better choice — but it comes with its own design challenges. With roughly half the energy density of diesel, methanol fuel tanks must be much bigger to achieve the same range and endurance as an equivalent diesel-powered tug. But of course, enlarging a boat to accommodate bigger tanks can hinder its performance as well as make it more expensive. Additionally, the International Maritime Organization has adopted rules for methanol — a low flashpoint fuel — that could discourage some early adopters.

“The rules are similar to those for LNG-powered ships, including a requirement for 600-mm cofferdam spaces around methanol storage tanks. What this means, in practice, is… precisely when you need the fuel tanks to be as large as possible,” he explained,  “they generally have to get smaller.”

All about the powerplant
No matter how you slice it, the discussion is always really about the selection of propulsion methods, in the view of Michael Complita, principal in charge and vice president of strategic expansion at Elliott Bay Design Group in Seattle. 

His company is delving into hydrogen-based propulsion with its design for M/V Hydrogen One. The towboat, under construction for Maritime Partners at Intracoastal Iron Works, offers a glimpse into a low-carbon future. Its 90-by-43-foot hull contains batteries, hydrogen fuel cells and diesel engines. It also has berthing for nine crewmembers.

Hydrogen One uses ABB’s electrical power distribution and automation alongside e1 Marine’s methanol-to-hydrogen generators that supply fuel to the fuel cells.

It’s a design that Complita calls both optimized and scalable. It is optimized in that the elements of the complicated power system are appropriately balanced, and the needs of the crew and the mission are also optimized. It’s scalable in that the same approach can be ramped up to power larger tugs. 

The vessel, once complete, will be capable of pushing two barges between Houston and New Orleans. And methanol fuel can already be found at many large ports, eliminating uncertainty around bunkering.   

Separately, Elliot Bay is designing its own power system to comply with California emissions regulations in the form of a barge that can move to different ships within a port as needed. “It is barge-based and it is intended for wherever you can’t easily put in infrastructure or if you want the flexibility to move away from the docks,” Complita said.

Looking past the immediate challenges, Complita said the technology is essentially ready and commercially available, whether it is variations on combustion engines or methanol reformers, that have been used on land for the past 30 years. It is just a matter of getting equipment manufacturing to scale for the maritime industry.

Indeed, according to Van Gundy, there is a strong future for alternative propulsion, especially with the goal of decarbonization throughout the industry. As with many new and upcoming solutions, there are going to be challenges. It is no different with alternative propulsion for tugs, he believes, specifically with cost, infrastructure and availability. 

“But as the demand for sustainable solutions continues to grow,” he said, “those challenges will diminish, and the solutions will become widespread and attainable.”