Ship & Boat International eNews: September/October 2020
Hydrofoils have transformed the America’s Cup. “Flying has spoiled us... the race won’t return to displacement boats now,” admits double Olympic gold medallist Iain Percy. Likewise, ‘flying’ may have the same effect on a swathe of commercial craft. “Water has almost 1,000 times the density of air,” says Percy, so a foil just a fraction of the size “will generate the same force as a huge aircraft wing”.
Percy is now leading Artemis Technologies, a tech spin-off of the racing venture, to apply the concept to an issue that’s previously held back all-electric vessels: range. The company is working with partners on a 400-pax ferry demonstrator to reduce congestion in Belfast, Northern Ireland, and the vessel will incorporate an electric foiling propulsion arrangement known as the eFoiler Propulsion System.
The eFoiler is a modular, scalable device. A wired databus and power cable to the batteries in the vessel above are the only connection: there’s no engine in the hull and no mechanical link. This allows the strut a profile of just a few centimetres, but, despite its lack of girth, the composite construction has the necessary strength. Bringing the hull out of the water can reduce the drag of fast craft by up to 70%, says Percy. This combined with an electric drivetrain can reduce the energy requirement by as much as 90%, significantly increasing the range of electric craft.
However, there’s a hitch: during foiling operation, traditionally located propulsion would no longer be in contact with the water. The alternative has to be located on the foil itself – so the design has to be extremely compact. Merged into the cross of the ‘T’ is a nacelle and propeller linked to a motor generator and gearbox. “It’s all about the diameter of a large coffee mug,” says Percy, as the eFoiler needs to sit inside a 25cm section. Despite this, it is still capable of around 250kW output apiece.
There’s more going on out of sight. The aileron flaps control the foil’s height in the water, and therefore need to be extremely responsive to maintain the balance between them while ‘flying’ the boat above. Altogether, it demands a highly automated active control system: another aerospace crossover. But, with this in place, it promises a smoother ride than more traditional high-speed boats. “Even though, say, a 1.3m wave might reach the hull, that would only be a tiny proportion of its volume,” explains Percy.
The eFoiler could prove “quite transformative” for vessels up to 120dwt, says Percy. For example, a 350-passenger foot ferry (similar to the one to be developed for Belfast), powered by a 2MW eFoiler system with energy storage somewhere around 4MW, will have a 100-mile, 35knot run on a single charge. It could therefore be used during peak times on a continuous duty cycle for suburb-to-city hops, with a top-up recharge during slower periods. That makes it very useful for the envisaged Belfast demonstrator but it’s also potentially attractive to other waterway cities. Percy points out that 35knots is typically “much faster than a car” when crossing a dense, urban area – such as New York, for example.
(Check out Ship & Boat International September/October 2020 for the full, in-depth article).