ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

made by boat,” explains Candela’s communications manager Mikael Mahlberg. “Gustav had looked into electric boats but found existing hulls consumed too much energy to get decent range at high speeds. “For reference, a conventional 25 ft speedboat consumes about 15 times the energy per kilometre of a standard family car. And lithium-ion batteries contain just one-fifteenth of the energy of gasoline.” Anyone hoping to make an electric boat with a conventional hull hits something of an engineering wall. If they want high speeds, they get very short ranges, even with the highest-quality lithium-ion batteries currently available. The optimum with present technology is around 120 kWh of energy for roughly 20-25 nautical miles of range, since integrating any additional battery packs will increase the vessel’s weight and drag to offset any potential gains in range, until eventually the boat simply sinks. Faced with this hurdle, Hasselskog opted to create an electric hydrofoiling system that would lift a boat’s hull above the water and remove the problem of hydro drag altogether. He therefore began gathering a development team in 2014 that soon became Candela Speed Boat. Building the first prototype began in 2016, the final prototype followed in 2018, and serial production started in 2019. With the boat and its unique technology now highly optimised and matured, Candela has started manufacturing and selling the C-7, making it the first electric hydrofoiling vessel to enter serial production; about 30 boats are produced and sold each year. That is currently the maximum capacity of the 50-person company’s production capacity, but Candela plans to scale up production, its next target being 400 units a year by 2024. There is one last technology in the C-7 worth noting. Green City Ferries’ latest version of its fast commuter electric ferry design now does without the surface-effect air bubble technology in favour of a hydrofoil-assisted catamaran architecture, making the hydrofoil a de facto blueprint for the future of maritime e-mobility. System overview Compared with a conventional powerboat, the C-7 uses 80% less energy when in motion – around 0.85 kWh is consumed per nautical mile when cruising at 22 knots. As mentioned, almost all this energy efficiency comes from having lifted the boat out of the water, although not having to bear the mechanical and thermal losses of an IC engine helps as well. Foiling successfully relies on a few key points of the boat’s architecture. First, it needs to be light enough to be held above the waterline by the hydrofoil’s lift. The heavier the boat, the wider the foil must be, which increases its contact area and reduces the drag efficiency and hence range. The C-7’s hull design and engineering have been handled largely by structural engineer Olivier Philippot, who was chosen partly for his extensive experience with Eurocopter Group (now called Airbus Helicopters) of producing safe, lightweight, low-drag structures for attack helicopters. The hull and structure have therefore been manufactured almost entirely from carbon fibre composite rather than fibreglass, the traditional hull material of most speedboats. It features thin outer carbon skins and decks, with spars and ribs glued throughout the interior for strengthening, like those inside the wings of passenger aircraft. When it comes to piloting the boat, the key problem with submerged hydrofoil vessels is that they are inherently unstable: like a learner cyclist on a two-wheeled bike, they ‘want’ to fall to one side or the other. This imbalance must constantly at 22 knots, at which speed it has a range of 50 nautical miles, or 92.6 km (not including an additional 14.8 km the system automatically ‘sets aside’ for limping home at 5 knots) between charges of its 40 kWh battery pack. Its carbon fibre hull is 2.4 m wide, and its draft is 0.7 m when stationary or moving slowly. The draft extends to 1 m when moving at speed and foiling, and reaches a maximum of 1.5 m when the hydrofoil is extended but the boat is not foiling. History of the C 7 Candela was founded in 2014 by now-CEO Gustav Hasselskog after a holiday in the Stockholm Archipelago. While various factors motivated him to spearhead the development of a clean and energy-efficient speedboat, initially it was the high fuel costs incurred by repeated trips to and from the mainland during the holiday. “Many people in northern Europe live either in island towns, island suburbs or similar, where regular commutes or other short trips are most conveniently A fully submerged hydrofoil enables the sWeedboat to ºfly» above tOe waterline 0t is Tade froT a carbon coTWosite tOat can be twisted for performing banked turns Autumn 2021 | E-Mobility Engineering 21 Dossier | Candela C 7 speedboat