The menu of choices for propelling your vessel cleanly and efficiently is long and growing. Flexibility is key in choosing.

The hydrogen fuel cell / electric battery-powered pushboat Elektra (illustration courtesy Hoppecke)

Published Jan 10, 2020 7:44 PM by Paul Benecki

(Article originally published in Nov/Dec 2019 edition.)

For more than a century, shipping has relied on diesel engines and petroleum fuel, a combination that’s become synonymous with marine propulsion. It’s efficient, cost-effective and reliable, and will remain an essential part of the energy mix for many years to come.

It’s far from the only option, however. Wind power, fuel cells, hybrid electric systems, biofuels and efficiency improvements all have the potential to reduce or replace the oil-based products that fill almost every ship's fuel tanks. As emissions rules tighten and the IMO aims for decarbonization, the future of propulsion looks like a more diverse collection of solutions.

The only certain thing about the future of marine power is that no one knows exactly what the dominant option (or options) will be in 10 or 20 or 30 years. Given the range of possibilities, expert advisors like DNV GL and Lloyd’s Register are cautioning shipowners to build flexibility into their propulsion systems to accommodate future changes.

Electric Drive

One of the most promising ways to gain flexibility is to design the ship’s systems around an electrical bus – especially a DC main bus, which can easily handle the addition of batteries or fuel cells.

When combined with podded propulsion, an electric drive system has already become an established option for icebreakers, offshore vessels, windfarm service ships, research vessels, cruise ships and other classes requiring maneuverability and redundancy. It’s ideal for complex, compartmentalized vessels since it eliminates long shaft lines and opens up more options for locating the main engines (or other power sources).

Volvo Penta, the engine maker known for its integrated propulsion systems, sees electric drive as the future for yachts and small craft power. In collaboration with French luxury yacht builder Fountaine-Pajot, it recently debuted a prototype battery-electric propulsion system for sailing yachts. The batteries yield an all-electric motoring range of more than 25 nautical miles when under way and up to 12 hours of all-electric power for auxiliary systems when moored.

“In the mid-term, battery-electric applications are certainly becoming more feasible,” says Peter Granqvist, Chief Technology Officer for Volvo Penta. “For some applications, they’re already feasible today. Long-term, we predict electrification powered by either significantly improved batteries or other environmentally friendly energy sources will increase significantly.”

Two new prototype vessels illustrate what electrification might look like on the working waterfront. The first, the new German workboat Elektra, will be the first all-electric pushboat in the world when delivered next year. This $14 million hybrid uses a multifaceted approach to renewable power, motoring along with a combination of hydrogen fuel cells and electric batteries. German propulsion company Schottel is supplying its electric Rudderpropeller azimuthing drives for this flagship project, giving the Elektra both power and maneuverability on inland canals.

The Elektra is currently under construction at a yard in Derben, Germany, and will enter initial service in 2020 with expanded operations slated for 2022. "It demonstrates – not only as a pushboat, but in particular as a model for electrical energy concepts for many maritime areas – that an energy turnaround is quite possible," says Professor Gerd Holbach, Project Manager at Technische Universität Berlin's Marine Systems Department, which developed Elektra's design.

The second example, a new concept vessel developed by Japanese tugboat operator Tokyo Kisen, is an all-electric harbor tug with a combination of battery power storage, hydrogen fuel cells and an auxiliary generator. Electric azimuthing drives give it the 50 tons of bollard pull it needs to carry out ship-assist work in Tokyo Bay. It’s presently in the design stage, but Tokyo Kisen and technology partner e5 Lab are aiming for delivery of a prototype in 2022.

Liquid Biofuels

For large, oceangoing ships, many industry players are betting on liquid biofuels or bio-LNG, which can be used as drop-in replacements for petroleum energy sources. With biofuel, the future of propulsion would look much like the present – the same proven diesel engines and the same bunkering infrastructure, but with fuel made from plants, wood chips or waste.

It’s a well-established idea: MAN Energy Solutions has been using biofuels in its two-stroke and four-stroke engines for more than a decade and publishes OEM biofuel specifications for ready-to-go use.

Most of today’s diesel engine biofuels are based on vegetable oils and other natural fats, but this may not be the case in the decades to come. Maersk Line, Wallenius Wilhelmsen and a coalition of big-name shippers are sponsoring research on a new drop-in biofuel based on a mixture of ethanol and lignin, a viscous byproduct of the paper industry with few current uses. The project is underway now at Copenhagen University, and first engine testing is planned for the middle of next year.

Enhancing Efficiency

In addition to changes in shipping’s fuel supply, efficiency will be a critical part of any transition to low- or no-carbon propulsion. According to a new study from Maersk and Lloyd's Register, "significantly more expensive" fuel sources will be the biggest cost in decarbonizing ship operations, not the capital cost of new propulsion systems.

Even setting aside future carbon-neutral fuels, the imminent IMO 2020 sulfur rule is expected to dramatically raise the cost of bunker fuel for most vessels. If energy will be more expensive, using less of it will be more important than ever for the operator’s bottom line.

Luckily, there are plenty of affordable modifications that improve ship efficiency. Among other simple and effective options, speed reductions can cut fuel consumption by as much as 10 percent per knot. Advanced bottom coatings can sharply reduce biofouling, saving one to four percent, and simple modifications to improve flow over the propeller and rudder can save another six to eight percent.

Becker Marine Systems is well-known for its Becker Mewis Duct®, an energy-saving device fitted in front of the ship's propeller to induce a pre-swirl in the water flow. Each one is custom-designed using computational fluid dynamics (CFD) to minimize drag over the propeller and rudder system.

According to Mike Pevey, Sales Director for Becker's North American division, the company has sold over a thousand units worldwide since introducing the system in 2009. Over the entirety of this fleet, the Becker Mewis Duct's power savings average nearly seven percent, according to the company’s data. Add one of Becker's high-efficiency rudders and it gets better: With a unified duct-and-rudder package, "you can push up to 10 percent power savings," Pevey says.

The Becker Mewis Duct isn't suitable for workboats, but Becker's rudder optimization can still produce major improvements for smaller vessels. Becker recently launched a specially-tailored flap rudder for U.S. inland waterway service, a high-lift design that produces more turning force at low rudder angles.

This gives towboats substantially more maneuverability at low speed, which is important on downriver voyages when the tow is moving with the river. In the shifting currents and close quarters of the Mississippi, more maneuverability equals more safety.

The first towboat fitted with the new rudder system is currently on its first commercial voyage to New Orleans, and Pevey says it’s showing promise: "The feedback from the vessel is they're still trying to get used to the added maneuverability, and it's actually the smoothest the vessel has ever run. There's very little vibration or cavitation when they make hard turns. Thus far the crew appears to be very happy with it."

As a welcome side effect, the rudder design also improves fuel economy. Less rudder angle means less resistance when holding a course and less engine load during turns. This can add up to a measurable improvement in the bottom line. One inland towboat customer reports annual fuel savings in the seven figures using Becker flap rudders, says Pevey, adding that "The river's currents are unpredictable, and it’s very difficult to get an exact understanding of how much less fuel they're using on any given trip. But all the operators say they get significant savings."

He notes that any fuel savings claim has to come with a caveat: The customer gets to decide what to do with a more efficient vessel. If the operator chooses, he or she could use the extra efficiency from a Becker device to boost speed rather than save on fuel. "If the captain leaves the throttle in the same place and goes half a knot faster than before, you won't see any fuel savings," Pevey explains, though there may still be a commercial advantage from shortening transit times or pushing a few extra barges.

Flexibility Is Key

While it’s hard to predict the future with precision, experts and suppliers expect that the different requirements of each shipping sector will lead to a diversity of fuel and propulsion arrangements. Every ship is different, as the saying goes, and this may be more true than ever in the years to come.

For deep-draft ships, the transition may be relatively simple as many existing large-bore engines are well-suited to liquid biofuel and bio-LNG. For smaller working vessels, electric drive systems can easily accommodate new power sources, especially if the naval architect designs the ship with adaptability in mind.

And for any ship, more efficiency means that less power (and less money) is needed to get to the destination. – MarEx  

Paul Benecki is the magazine’s Americas/Europe Editor.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.