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

entirely from the best composite material Candela could find. Philippot says, “The only downside with a carbon fibre hull is in fact the expense of building it, to make the boat affordable for serial production. Carbon composite is around half the weight of fibreglass composite while being stronger and stiffer. Every gram added would have meant we’d need a more powerful, heavy and expensive powertrain, which would risk reducing our range.” Although carbon composite is pricier than fibreglass, Candela has striven to use thin carbon parts to reduce the overall bill of materials in the structure. Philippot points out that the labour cost of carbon fibre is a bigger concern than the material cost. To optimise the value for money in the factory processes, vacuum infusion is used to embed the resins into the carbon fibres for the larger hull sections. The advantages of this process were also espoused by Green City Ferries: by enclosing the fibre and resin within airtight soft plastic bagging, and forcing the latter through the former at -1 bar, it ensures a uniform spread and density of resin. This is used for the large hull and deck sections, while the struts and rudders (each being around 2 cm thick) were made using a different carbon composite and an undisclosed manufacturing process to ensure they could handle the loads and stresses coming from the waves, from turning, and from carrying the 1.3 t boat, as was the foil to ensure it could twist and deform as needed for turning the boat. “We searched for a long time for suppliers who could produce the parts with the strengths we needed at a reasonable price, and we did a lot of testing to make sure those thin, lightweight parts could safely handle and transmit huge loads while maintaining the stability of the boat, to be certain we were choosing the right manufacturers,” Philippot notes. The deck is also topped with teak for aesthetic reasons, and is manufactured with hatches for maintenance workers to access the battery pack, flight controller and other parts. Beneath the deck is a network of aerospace- style ribs and spars, to maximise the structural strength of the boat while minimising weight. “If you look inside an Airbus or Boeing aircraft structure, you have a rib, spar or string every 10 cm or so, and the The C Pod As well as developing the C 7 and additional, larger vessels for public transport applications, Candela has unveiled a new electric pod thruster, the C Pod, which will be used on its new water taxis, ferries and other future vessels. At the time of writing, the C Pod was intended solely for use on Candela’s future craft. Pod thrusters such as the C Pod are inherently more e icient than outboards, as they have no gears or transmission shaft. By directly driving the propeller, they require no oil and greatly reduce parasitic losses. They will be mounted in line on Candela’s upcoming P 12 water taxi and P 30 ferry – one on the former and two on the latter, sitting and operating beneath the water without emitting any noise noticeable to passengers or people on the shore, and without needing gearboxes or z-drive azimuth thrusters. Each C Pod is built around two permanent magnet synchronous motors that together produce a cumulative 60 kW at peak. This will enable the P 12 to cruise at 20 knots over a range of 45 nautical miles, and the P 30 to cruise at the same speed over 60 nautical miles. Candela’s communications manager Mikael Mahlberg says, “As the irst obstacle encountered by any new, small e-motor design is heat losses, the packaging of the C Pod has been designed to get the low of seawater as close to the stator as possible.” At the time of writing, the size of the C Pod had not been publicly released, but Mahlberg estimates that its output (enabled through its high heat tolerance) gives its motor a power-to-volume ratio several times that of the best COTS motor. As output power is a function of speed and torque, and torque is limited by the size of the motor, the C Pod’s engineers needed a way of increasing the system’s power without encroaching on the limitations that propeller tip speeds encounter in water. At around 45 m/s, the pressure in the water low around each prop tip drops to the point of cavitation, leading to ine iciencies and potential damage. To avoid that, two counter-rotating props are used to push overall propeller e iciency to more than 80%. “That gives the C Pod several times the power density compared with the best available COTS pod thrusters,” Mahlberg adds. A structure of thin carbon parts, reminiscent of an airliner wing, has been vital for making the C-7 light enouNO to foil eɈectively and to Wrevent increasinN tOe boat»s Wrice Autumn 2021 | E-Mobility Engineering 29 Dossier | Candela C 7 speedboat