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

be corrected – whether it be at high speed, cruise or turning – and it must be accomplished in a way that keeps the boat’s pilot and passenger(s) comfortable, or else no-one will want to sit in the resulting product. The C-7’s hydrofoil is thus designed to be electromechanically shifted in order to rebalance the boat across the full range of motion it can perform, and in the pitch, roll, and yaw axes. It must also be able to maintain its height with great accuracy, or else risk repeatedly slamming into waves. To enable the boat to understand its own dynamics and surroundings well enough to consistently keep the foil 50- 60 cm beneath the surface of the water – given that piercing the surface will induce a loss of lift and cause the boat to settle on the water’s surface – a comprehensive suite of sensors has been arranged. This includes inertial sensors for measuring changes in the vehicle’s position and orientation, as well as barometers for gauging the altitude of the boat, and ultrasonics to give readings on the distance between the hull and the water. Mahlberg notes that Candela is not the first to attempt a control system such as this. Before them, Boeing produced its 929 Jetfoil ferry, which was launched in 1974 and used a control architecture based on the aerospace giant’s jet aircraft to keep the large, 115 t watercraft balanced. “The development of Boeing’s system originated from their military technology,” Mahlberg says. “Their flight controller is actually pretty similar to ours in terms of its overall architectural concept, and I imagine it was picked from their jets. “The dynamics of a hydrofoiling boat aren’t too dissimilar from a fighter jet. You have a lot of speed, a lot of imbalance, so you need a control system that keeps the craft from constantly rolling or dipping too far in one direction, and does so without overcompensating.” That vessel was powered by twin gas turbine engines and could achieve up to 45 knots. Boeing built a handful of them, although after 1985 it opted out of the hydrofoil industry and sold its licence for the 929 to Kawasaki Heavy Industries, who built only 16 of them between 1998 and 2020. As mentioned, however, Candela has been able to manufacture and sell far more boats than this, with many more on the way. Its production costs relative to the 929 have been helped in no small part by the widespread availability of much smaller, cheaper and more lightweight electronics for making the kind of control architecture necessary for a hydrofoil. Also, the measurability and controllability using sensors and processors are helped by using an all-electric, fully digitalised powertrain, rather than a twin gas turbine arrangement. “Advances in those components have helped us so much,” Mahlberg adds. “We’re selling a €300,000 pleasure boat, and the precision of driving and balancing from our electronics would have cost tens of millions of dollars just 20 years ago. I don’t want to think how much it must have cost Boeing in the 1970s. “And we’ve refined the overall concept too, particularly for energy efficiency. If you look at the 929 Jetfoil, it’s actually a super-inefficient vessel – I think it consumed about 1600 litres/ hour of fuel.” In addition to needing various sensors and other electronics, the drive for energy saving and motion stability has spurred a lot of work on the propulsion mechanisms, both the hydrofoil and the thruster at the back. These systems comprise the three beams, or “struts” as they are called internally, that extend from below the C-7’s hull as it flies above the waves – that is, two forward struts attached to the foil, and one rear strut holding the thruster. Candela emphasises that the real revolution with the C-7 is the foil system rather than the electric motor and battery pack, but the way the latter are packaged and used are still worth noting, particularly in terms of the thruster’s hydrodynamics. Both the thruster and the hydrofoil have been engineered to minimise drag along with any potential points of failure that could harm the ride quality. Dossier | Candela C 7 speedboat Candela has repackaged the Torqeedo Deep Blue 50 R into a more slender thruster design, to better match the hydrodynamics of the C-7 hydrofoil’s wing and struts 22 Autumn 2021 | E-Mobility Engineering