Turning Tides: The new wave of e-boats taking to the seas

7 min read

Electric boats of all shapes and sizes are beginning to make a splash in the maritime sector. Andrew Wade reports. 

It’s estimated that maritime greenhouse gas emissions account for around 2.5 per cent of the global total, marginally ahead of the two per cent that aviation contributes. If shipping were a country, it would be the sixth largest emitter of CO2 after China, the USA, India, Russia and Japan. International shipping – much like long-haul flight – will not be electrified any time soon, as the energy density of batteries simply cannot facilitate it. And for an industry that’s recently committed to halve its emissions by 2050, that’s a fairly significant problem.

While companies such as Maersk are looking to solutions like green methanol to power ocean-going ships, electrification is viable for boats that don’t venture too far from the shore and can recharge regularly. From speed boats to ferries and even small container ships, e-boats are beginning to make headway in the maritime sector, inspiring some cutting edge engineering to complement their green credentials.

The Candela 7 rides above the waves using submerged hydrofoils (Credit: Candela)

One outfit leading the way is Swedish company Candela. In 2014, founder Gustav Hasselskog had a eureka moment while boating in the Stockholm Archipelago. Realising that a round trip from the summer cabin to buy his children ice cream cost around 10 times more in fuel than it did in dairy, he set out to build a more energy-efficient, planet-friendly boat that wouldn’t cost the Earth to run.

“He realised that fossil fuel, planing powerboats are some of the most energy consuming craft in the world,” Mikael Mahlberg, Candela’s communications manager and occasional test pilot explained to The Engineer.

“Due to the high drag of planing boat hulls, they consume about 15 times as much energy as a normal family car. You can’t really electrify them using current batteries. Even the best lithium-ion batteries have about one-fifteenth of the energy density of gasoline.”

Candela 7's hull, deck and foils are all made from carbon fibre (Credit: Candela)

The solution Hasselskog came up with was submerged hydrofoils to lift the hull out of the water, all but eliminating the twin enemies of friction and drag. The result, after several years of engineering endeavour, is the Candela Seven, an electric foiling speedboat that flies above the waves. At its cruising speed of 20 knots, the boat can cover 50 nautical miles on a single charge – enough for a return jaunt to Calais from Dover. A top speed of 30 knots sees that range shrink to 32 nautical miles.

“Candela is really more similar to an aircraft than a boat,” said Mahlberg. “It’s unstable, it ‘flies’, and there’s a lot of software and computers to stabilise it. So the hydrodynamic part is pretty challenging.”

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Given the nature of the project, it’s not surprising that the company has brought in team members from Airbus Helicopters and Saab AB, as well as experience from America’s Cup racing, where hydrofoils have become de rigeur in recent editions of the fabled sailing competition. The Candela Seven has one small hydrofoil at the back, integrated with its rudder and propeller, and a second much larger retractable foil protruding from its belly that does the heavy lifting. It’s the latter of the two where the really clever stuff takes place, with hydraulic actuators operating at 100Hz, adjusting the struts on the hydrofoil 100 times per second to keep the vessel stable with the help of a flight computer and an impressive array of sensors.

“The secret to balancing this boat is that the wing doesn’t really have flaps or ailerons like an aircraft wing,” said Mahlberg. “Instead, we twist the whole wing using independent struts that move back and forth, bending and twisting the wing, which is flexible in the middle.

(Credit: Candela)

“The brain of the boat is the flight controller, which is basically a computer that we developed in-house. There are six different types of sensors. Ultrasonic sensors in the bow measure wave height in front of the boat at 100Hz, or 100 times per second, to angle the foil optimally and counter the waves. But since the boat is unstable, you have to balance it by using the foil at all times.”

Lifting a 7.7m boat from the water requires not only hydrofoils, but also some impressive lightweighting. The Seven’s entire body – hull, deck and foils - is fabricated locally in Sweden using carbon fibre, its 240kg weight coming in at around 50 per cent that of a fibreglass equivalent. The 40kWh battery pack – made using repurposed BMWi3 modules – adds significantly to the overall weight of 1350kg, but this is still about 30 per cent lighter than the Candela’s fossil-fuelled antecedents.

Out on the water, the boat essentially flies above waves up to four feet, with no slamming or pitching and virtually no noise or wake. It can accommodate a pilot plus 700kg, so six or seven passengers and a few pieces of luggage or a couple of well-stocked picnic baskets. The custom-built, slimline propeller hub is integrated with the secondary hydrofoil and enclosed in its own case, further aiding the boat’s hydrodynamics.

“This obviously helps with drag a lot,” said Mahlberg. “It’s probably about six or seven times more efficient than a traditional lower unit on a 25-foot powerboat…when we’re up and flying at 20 knots we use 24 horsepower on the motor and a traditional combustion engine boat will use maybe 120, so it’s a huge difference.”

(Credit: Candela)

Innovation comes at a price, however, and the Candela Seven will set you back in the region of £200,000. Not exactly small change, but a long way from the upper echelons of the boating world, and when it comes to cost per mile, owners can really start to claw some of their investment back. Three-phase 11kW chargers – common in most harbours and marinas – will take the Seven from flat to full in four hours at a cost of around £5, roughly 95 per cent less per mile than the e-boat’s gas-guzzling cousins.

“In the US, maybe the prices are a bit different when it comes to gasoline, but I would still say 90 per cent (savings),” said Mahlberg. “We have one owner in the Stockholm archipelago who says that he saves 10,000 euros per year driving the Candela versus his fossil fuel boat.”

For those shy of a spare £200k, a slightly less exhilarating but no less important e-boat project will soon be open for business on England’s south coast. E-Voyager, formally the diesel-powered Mermaid, is a 12-passenger ferry that has been converted into a fully electric vessel. Developed by Voyager Marine in partnership with Plymouth Boat Trips and E-Marine Solutions, it is set to become the UK’s first seagoing electric passenger vessel when it begins taking passengers this summer.

“E-Voyager was fitted with a 140kW motor in place of the old Ford 60hp (45kW) diesel engine,” Andy Hurley, project manager at Voyager Marine, told The Engineer. “The batteries are repurposed Nissan Leaf batteries enclosed within a purpose designed and built marine casing. The inverters and DC-DC converters are also designed to be durable and capable of operating in a maritime environment.”

The project has been funded through the Clean Maritime Call, a Maritime Research and Innovation UK (MarRI-UK) initiative backed by the Department for Transport, with academic support coming from the Universities of Exeter and Plymouth. During development, the team used duty cycle modelling and advanced simulation of propeller performance to make sure every bit of power could be efficiently wrung from the 20kWh battery module. At its cruising speed of five knots, e-Voyager will be able to operate for more than four hours, and 1kW of solar panels have recently been added in an effort to extend the range.

(Credit: e-Voyager)

When the sun alone isn’t up to the task, e-Voyager will top up as passengers board and disembark using one of two new 22kW Type 2 chargers recently installed by Plymouth City Council at the harbour’s landing stage. Given Voyager Marine’s plans for two significantly larger e-boats, Plymouth Council may well have to add to its quayside charging infrastructure in the near future.

“The first is the conversion of the Plymouth Princess, a domestic passenger vessel of approximately 16m and certified to carry over 100 passengers,” Hurley explained. “Much of the groundwork has been completed ready for the conversion in the autumn of 2021. She’ll be able to operate for up to 12 hours per day on a single overnight charge.

“We have also progressed with our design for a new-build ferry to operate on the famous Cremyll Ferry route linking Cornwall with Plymouth. This will be a catamaran with twin propellers driven by similar but larger motors to that used on the e-Voyager. The vessel will be 20m long and will be capable of carrying up to 150 passengers. We’re looking to build this vessel in 2022.”

What these latest plans from Voyager Marine prove is that e-boats are by no means limited to dinky leisure craft and retrofitted work vessels. Driven by cost-efficiency and environmental demands, the maritime sector is making genuine strides towards a cleaner, greener future, empowered by exceptional engineering and truly innovative technologies.

For a glimpse of what that future may look like, we can turn yet again to Scandinavia, where a pioneering project is combining electrification and autonomy on a scale beyond anything seen before. Set to begin operating later this year, the Yara Birkeland is the world’s first battery-powered autonomous vessel, a 120 TEU container ship designed to load up, navigate its course, then unload its cargo with virtually no human input.

(Credit: Yara International)

Developed for Norwegian agricultural company Yara International, the 80m vessel was conceived of as an alternative solution for transporting chemicals and fertiliser from the company’s base in Herøya to the nearby container port in Brevik. Yara exports around 20,000 containers annually from Herøya, meaning a total of 40,000 diesel-powered journeys are required to facilitate the short first leg of all cargo. The Yara Birkeland aims to replace those journeys with zero-emissions round trips as a feeder vessel for Brevik and also the deep sea port of Larvik, around 31 nautical miles away.

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The 3200 DWT vessel will be powered by a 7MWh battery which will drive two 900kW azimuth thruster pods and two 700kW tunnel thrusters, providing a top speed of 13 knots and a cruising speed of 6-7 knots. Norwegian technology firm Kongsberg has been responsible for the navigation, sensor, control, communication and electrical systems. After some frustrating delays, some of which were pandemic-related, the ship was finally delivered in November last year and is set for manned sea trials before taking up its autonomous mission later in 2021.

Autonomous landside logistics have delayed Yara Birkeland's rollout (Credit: Yara International)

A key part of that mission is still a work in progress, however. While the vessel itself appears ready for action, the autonomous logistics on land are proving a more difficult challenge, with Yara admitting to ongoing struggles with the electrified cargo solution for loading the Birkeland. These problems serve as a reminder that integrating complex, autonomous, electrified systems is not easy, despite the underlying technology maturing rapidly in recent times.

For world-first projects like the Yara Birkeland, some delays and hiccups are to be expected, and the challenges it has faced do not appear to have deterred others from adopting similar ideas. Norwegian grocery distributor ASKO is currently working with Kongsberg on two new battery electric freight ferries to operate in the Oslofjord, in an effort to reduce circuitous road haulage around the capital’s waterways. The two vessels will operate between Horten and Moss, a distance of about five nautical miles that will take roughly an hour to travel but which has the potential to significantly reduce the company’s emissions in and around Oslo.

It’s perhaps no surprise that Yara and ASKO - two non-shipping firms with unique and specific cargo requirements - are the first to dip their toes into the choppy waters of electrified, autonomous shipping. Designing exclusively for a certain route allows for hull and powertrain optimisation as well as the necessary charging and logistics infrastructure at either end of the journey. Whether they can serve as pathfinder projects for more widespread adoption remains to be seen, but if the shipping industry is serious about its impending emissions targets, then large vessels – as well as small ones – will need to ride the electrification wave.