THREE FEET ABOVE the waves, the Candela P-12 sprints across Lake Mälaren near Stockholm, Sweden. With only its hydrofoils cutting through the water, the boat leaves virtually no wake, noise, or emissions—a sea change from the hulking diesel-powered ferries that currently haul commuters through the archipelago that makes up the Swedish capital.
So far, it’s a water-bound fantasy: While Swedish startup Candela is already manufacturing leisure versions of its electric flying boats, the P-12 hasn’t yet been built. Candela CEO Gustav Hasselskog says the boat is in the “design for manufacturing stage” ahead of a November launch that will be followed by a trial next year. The aim is to have the flying ferry form a part of Stockholm’s public transport fleet.
Cutting carbon emissions from ferries is a priority for a city surrounded by water. The city’s existing fleet of 60 ferries emits 40,000 tons of carbon dioxide annually, making up 8 percent of total shipping emissions in Sweden—and they’re spewing that air pollution in cities, raising public health concerns. “Shipping has to stop using fossil fuel, fast,” says Simon Bullock, a researcher at the University of Manchester’s Tyndall Centre for Climate Change Research. “For short journeys, electric ships can be a big part of the solution.”
On that point Sweden is ahead of the curve, with Stockholm working toward emissions-free ferries by 2025. Electric ferries have previously been trialed in the Swedish capital, with local authorities testing another model from Green City Ferries alongside the flying P-12. Norway uses electric passenger ferries to tour its fjords, Belfast in Northern Ireland is trialing a similar “flying” style boat, and a project at the University of Plymouth in the UK is converting diesel ferries to electric. That’s good news given that ferries, most of which are powered by diesel, are a major environmental headache: EU data shows ferries represent 3 percent of all vessels but make up 10 percent of carbon emissions, while more than 95 percent of US ferries are powered by diesel.
But Candela believes there’s more to cleaning up Stockholm’s commuter traffic than emissions-free energy: making ferries quick enough to persuade more people to ditch cars. Traveling from the suburb of Tappström to central Stockholm takes 50 minutes by car during rush hour, but the P-12, which can hit 30 mph, could navigate the waterways between the two in 25 minutes, Hasselskog says. Waxholmsbolaget, the agency that runs public transport boats in the archipelago, carries 1.2 million passengers annually, but that’s compared to 780,000 commuting trips by other forms of public transport each day in the city—in short, there’s room to get more Swedes in the sea.
The problem with powering any form of transport with electricity is it requires heavy batteries. That’s a particular problem for boats, as they suffer drag in the water. To address this Candela uses hydrofoils, legs that extend down into the water and act like wings, propelling the boat up into the air as it picks up speed like an aircraft during takeoff. “In harbor the foils are fully retracted, so they’re protected,” Hasselskog says. “But then you lower the foils and hit the throttle and off it goes. The control system takes care of the entire takeoff sequence, it’s like an airplane.”
Hydrofoil boats aren’t new, but electric power and automated controls are. The carbon-fiber Candela P-12 will have twin propulsion systems powered by 180-kWh batteries, letting it run three hours before requiring charging. At 12 meters in length and 4.5 meters across, the 8.5 metric ton boat will carry 30 seated passengers.
A superfast flying boat sounds like a surefire way to lose your breakfast on the morning commute, but the Candela has sensors that feed into an automated control system to adjust the height and roll and pitch up to 100 times a second to ensure a smooth ride regardless of the weather. “Through the control system we can cut out any vertical movements of the boat,” Hasselskog says, which is what tends to cause seasickness. “So far nobody has gotten seasick on our boats.”
All of that means the Candela P-12, when built, should use less energy per passenger than a hybrid electric bus, go faster than a car, and bring down fuel and maintenance costs by 40 percent. And as it glides above the water it’s less disruptive to the local environment both above and beneath the water.
Candela couldn’t simply upsize its existing boat to build the P-12—regulations require a thicker hull, fire safety systems for the batteries, and, confusingly, separate toilets for passengers and the single member of crew, who will be driving all the time.
Toilets aside there’s another regulatory challenge: Speed limits on inland waterways tend to be as low as six knots (7 mph), but hydrofoil boats are most efficient at top speed. Such speed limits are for safety and to reduce wake, which boats like the P-12 don’t cause. “The solution is working with port authorities and ferry operators to get dispensation,” says Charles Haskell, decarbonization program manager at maritime consultancy Lloyd’s Register. Around Stockholm that limit is 12 knots, though Candela has a temporary exemption during the trial.
Not all cities can use waterways as highways like this, but it could be an appealing idea for coastal conurbations. Rival flying boat maker Artemis is testing its version in Belfast, while Hasselskog has held talks with authorities in Istanbul and across the Middle East. Reps from the Water Emergency Transportation Authority (WETA), which operates ferry services in the San Francisco Bay Area, have visited Stockholm to see how the Candela P-12 works.
For coastal cities like Stockholm, ferries could become the watery equivalent of trams without having to lay infrastructure like rail, though charging systems will be needed. “If it’s acting like a sea-based light rail facilitating hundreds of people who would have gone by car, then that’s what we need more of,” says Paul Chatterton, professor of urban futures at the University of Leeds. “The speed is a red herring … in a big urban river environment you need big large crafts that can take a lot of people short distances.”
Hasselskog argues that a large fleet of smaller boats offers more flexibility than larger ferries and could mean they’re used on demand, ditching the need for timetables or fixed stops. The idea is also being touted by hydrogen-powered hydrofoil water taxis made by SeaBubbles, which have been trialed in Lyon, France. Smaller boats have another use: ferrying maintenance staff and supplies out to offshore wind farms, says Haskell, solving a problem of getting staff to locations many miles offshore without them arriving seasick.
Even without top speeds, water taxis and boat buses offer promise to cities with waterways, Chatteron says, pointing to the popularity of Venice’s vaporettos. And beyond passenger transport, slow, electric canal barges could take freight off of roads. “You can move a lot of things with little or no energy,” Chatterton says, “and a lot of European cities have canals.” Whether its electric-powered flying ferries or low-energy barges, making better use of urban waterways makes sense for sustainability, says Hasselskog. “You don’t need any special infrastructure, the water is just there,” he says. “That’s probably why they were used back in the day—you just go.”
Wired, 14 July 2022