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

There are a few mechanisms included in the hydrofoil’s design to enable banked turning. The first are the lock- pins, one in each of the foil’s two struts, which are activated electromechanically to enable the actuators pushing on the struts to make them swing back and forth. To steer left, for example, the left strut is pointed backwards, the right strut is pointed forwards, and the wing twists accordingly. The foil system shares similar traits with the Wright brothers’ first aircraft, the Flyer, which relied on a twisting wing rather than ailerons. Also, a proprietary holding device joins each strut to the foil to allow some limited axial freedom of movement, and to ensure no structural stress is imparted to the joints that could cause either of them to break. “The middle of the carbon fibre foil flexes and bends, which makes the foil function much as an aileron to roll the boat, with the rudder at the rear assisting in steering,” Mahlberg says. “We didn’t want to use flaps underwater, as they would just create more potential points of failure and increase the potential risks of biofouling or similar.” The foil is by design not indestructible – if it hits a submerged rock at cruising speed or above, for example, it will snap off for safety reasons, rather than breaking the whole boat. This is a big advantage compared with non- hydrofoil boats, says Mahlberg. The shafts that hold the struts are designed to function as the strongest points in the hull, so despite the loss of the foil, the rest of the boat will still function normally, enabling a safe (albeit less energy-efficient) return to port. The foil can then be easily replaced. “I had the opportunity to try this in my first week on the job,” Mahlberg recalls. “When foiling into a harbour, I hit a submerged chain strapped across the entrance. The landing was totally undramatic, but when I looked down I couldn’t see the foil – it had come off. We replaced it in less than an hour and were out foiling soon after,” says Mahlberg. Powertrain The core of the thruster used to propel the C-7 is a Deep Blue 50 R from Torqeedo. This is an 80 bhp electrically powered outboard thruster weighing 139 kg, with a top propeller output speed of 2400 rpm and a maximum torque of 198 Nm. The shaft running down from the electric motor and gearbox to the propeller is 51 cm long. Its tilting and trimming surfaces are designed without hydraulics, and its structure is cooled using seawater. The thruster is lifted in and out of the water by hydraulic actuators, in a movement resembling that with a normal IC outboard engine, but with an added pitching movement that eases access to the motor and gearbox for maintenance. When navigating shallow waters, the pilot manually selects a ‘shallow mode’ that retracts the foil to a safe position behind the keel The thruster’s e-motor and inverter combination produce up to 55 kW, achieving peak eɉciency at around Rnots Autumn 2021 | E-Mobility Engineering 25 Dossier | Candela C 7 speedboat