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

Developing the hydrofoil The first version of the C-7 featured a vee-shaped hydrofoil, whose design was based on extensive computer simulations. Nonetheless, when it came to real-world testing it proved to be insufficiently stable at speed. That was not unexpected: the C-7 has to deal with airflow around its hull as well as the flow of water around its foil and thruster as it moves, and Candela’s engineers were acutely aware that not many software programs were capable of accurately simulating the dynamics of both air and sea at once. That meant much of 2016 and part of 2017 was spent redesigning the foil and developing the software. The foil’s width is close to that of the beam, with the hydrodynamic optimisation of the cross-section (and the rest of the foil shape) being a trade-off between multiple conflicting factors. “For example, if you want more lift you could just make the wing much longer, but if the foil protrudes past the draft of the hull, it’ll become a pain when you’re trying to dock,” Mahlberg says. “We didn’t want any compromises in how the C-7 compared with a normal IC-engined boat; it had to be able to dock normally, traverse shallow waters, be transportable on a road trailer, and so on.” To ensure these latter two capabilities, the wing is held in place by two retractable struts that are mounted vertically into the boat hull via two holding shafts. Each strut is connected to an electric motor that serves to either lower the foil about 80 cm below the keel line or raise it about 20 cm above it (where it tucks in safely behind the keel). From start-up to about 16 knots, the C-7 operates like a normal speedboat, using just its outboard thruster; when commanded via the throttle lever to go faster, the hull starts to ‘fly’, foilborne. When dropping below 16 knots, the foil will not produce enough lift to sustain flight, so conventional operations are resumed. When flying, the foil struts are locked in their most extended position by a robust bronze lock-pin. In flight, electronic actuators push on the struts to make them pivot around the lock- pins. When the struts pivot back and forth, they cause the foil to change its angle of attack. “Engineering the system for banked turns proved to be a challenge that nobody had solved before on a foiling leisure boat,” Mahlberg says. “Most hydrofoils in the past were built such that when they turned, it really felt like a go-kart: the passengers would feel as if they were being pushed to one side from the g-forces, because the boats weren’t engineered to bank when turning. “Performing a banked turn, in which the whole vessel leans into the curves like a motorcyclist, is really important not just for a comfortable ride but to minimise the mechanical stresses that the boat puts onto the foil and its beams.” And although perfecting the dynamics and timing of the foil’s actuators via the control algorithms has been a key part of the C-7’s turning ability, its success has also come from designing the wing to physically twist during the boat’s turns. There is no actuator inside the wing: the carbon composite from which it is made has been selected because it can be twisted back and forth without cracking. Earlier versions used aluminium, but the stresses imparted on them from turning proved unsustainable. Above 16 knots, an electric motor engages with the teeth on the foil’s struts to lower them, and a bronze lock-pin holds them in place for steering Hydrofoiling A hydrofoil is a system or shape that works below the waves much as an aerofoil does in air. It is akin to a wing, generating lift when it moves through water; a vessel designed to use a hydrofoil can also be referred to as a hydrofoil, or simply as a ‘hydrofoiling boat’. Hydrofoils were irst patented in the late 1800s, and prototypes were starting to be developed and tested around the beginning of the 20th century. There are broadly two types of hydrofoil. One is the surface-piercing hydrofoil, which has a vee-like shape (part of which protrudes above the surface of the water during foiling) and acts rather like water skis to produce lift. It has been used successfully for many years, for example in the Voskhod-type river ferries manufactured irst in the former Soviet Union and now in Ukraine. The other is the fully submerged hydrofoil, which as its name suggests keeps its entire foil beneath the water. Comparatively fewer of these have been made, but those that have often take an inverted-T shape, with beams extending vertically down from the hull to mount two separate – or one collective – lat horizontal hydrofoil surfaces. Surface-piercing hydrofoils have to contend with the shapes and forces of waves, so boats that use them move with less stability and energy e iciency compared with those using fully submerged hydrofoils. However, the latter are not self-stabilising, unlike surface-piercing hydrofoils, and therefore need an accurate control system capable of constantly adjusting and rebalancing their vessels. That has made the design and scale production of such systems far more challenging. 24 Autumn 2021 | E-Mobility Engineering