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

In the C-7, however, the mechanics and electronics of the Deep Blue 50 R have been repackaged into a design with similar dimensions to the foil and forward struts, to achieve comparable hydrodynamics. Its beams, vertical rudder and horizontal rudder – which resembles a wing, sitting above the prop and extending about 35 cm either side of it – are therefore about 2 cm thick, far thinner than a conventional outboard and about 5-6 times more drag-efficient. Once repackaged into the C-7’s architecture, the 400 V e-motor and inverter combination produces up to 55 kW. It consumes about 17-18 kW when the boat is moving from 5 to 20 knots, and its efficiency peaks at around 22 knots. By contrast, an equivalent traditional IC-hydraulic outboard would consume around 100 kW to produce the same net power. During operation, a pump in the motor-gearbox assembly draws seawater up through a hose-like channel to directly cool the motor and battery pack. When the C-7 isn’t moving, the thruster can be hoisted out of the water, much like any outboard system. The hydraulic actuators serve to raise it at slower speeds in shallow waters – although this ‘shallow mode’ must be activated via the pilot’s interface – and to lower it during foiling. As a result of foiling, the maximum wake wash from the C-7 produces waves of between 7.5 and 10 cm in height during cruise, meaning minimal disruption to other boats in busy waters. The battery pack used is a 40 kWh BMW i3 system, bought off the shelf and selected largely for its 8-year service guarantee and for being a widely used, well-proven battery using energy-dense lithium-ion NMC prismatic cells. The popularity of the battery (styled by Torqeedo as the Deep Blue Battery) among maritime users comes in no small part from the various safety elements in its engineering. For example, if excess pressure should accumulate in its cells, a pressure valve disc opens to release any build-up of gases, minimising the chances of explosions or water ingress in the IP67-rated pack. Also, its BMS is rated to the ASIL-C automotive safety standard, which Torqeedo cites as being a more rigorous level of safety compliance than is typical in boating, and a compressor is integrated to enable uniform air-based cooling throughout the pack without needing the weight of a liquid-cooling circuit. Internally, the pack’s cells have laser-welded cell connections for consistency and reduced complexity in their manufacture. It is charged at a maximum rate of 9.9 kW, with charging managed by the standard-issue BMW i3 BMS, although future Candela boats will have fast DC charging. When the foil is retracted and the C-7 is operating in normal displacement mode, the battery also helps by acting as ballast, its 350 kg of weight keeping the centre of balance in the approximate dead centre of the boat. Therefore, if the user chooses to drive slowly through heavy waves or to allow seven or eight people on board, the boat’s weight distribution remains largely the same. “Conventional, planing motorboats built for the open sea tend to be rather unstable at slow speeds if you load them unevenly, owing to their sharp vee-hulls. By contrast, the C-7 has a totally flat aft hull, since it relies on foils for seakeeping. So the whole crew – up to seven – can stand on the railing and it won’t heel over that much. The light controller The control system by which the C-7 successfully foils and balances works in a similar principle to a self- driving car or quadcopter UAV. A series of motion reference sensors feeds various data on position and A 40 kWh BMW i3 battery pack is installed in the C-7, and can be AC-charged at up to 9.9 kW; fast DC charging is expected in the future 26 Autumn 2021 | E-Mobility Engineering Dossier | Candela C 7 speedboat