Electric hydrofoils are boats that use submerged wings (hydrofoils) and electric drivetrains to lift their hulls out of the water, dramatically reducing drag and energy consumption. Their proponents see them as a way to shift urban passenger traffic from congested roads to underused waterways.
The hydrofoil concept dates to at least 1869, and the first successful example was built in 1904. Hydrofoil ferries have long operated in many parts of the world, but older designs used "surface piercing" foils that gave a bumpy ride. Modern electric hydrofoils use fully submerged wings, which reduce drag and improve stability but require constant digital adjustment—tiny corrections made many times per second using sensors and control systems borrowed from smartphones, drones and autonomous cars.
The convergence of three technologies has made the modern electric hydrofoil viable: advanced materials (particularly cheaper carbon fibre from aerospace), electric drivetrains (piggybacking on the EV industry's batteries and fast-chargers) and digital flight-control systems. Because the power needed is directly proportional to mass, minimising weight through carbon-fibre construction is critical.
Existing urban ferries use 15-30 times more fuel per passenger mile than buses. They must be large to handle peak demand, then run at low occupancy throughout the day. A fleet of smaller, more efficient electric hydrofoils can provide more frequent service. Because hydrofoils produce little wake, they could be permitted to travel faster than conventional boats in speed-limited urban waterways.
Nearly half the world's population lives in coastal regions. Advocates speak of "blue highways" or "forgotten highways"—waterborne routes that cities used before the introduction of cars.
An international array of startups is developing the technology. Candela (Sweden) is in serial production of both leisure boats and 30-passenger ferries. Other firms include Artemis Technologies (Northern Ireland), which has developed a 150-person electric hydrofoil ferry called the EF-24; MobyFly (Switzerland); Navier (America), founded by Sampriti Bhattacharyya; SeaBubbles (France); and Vessev (New Zealand).
The maximum size of hydrofoiling vessels is constrained by physics: mass increases with the cube of length, but passenger capacity increases only with the square. Large conventional ferries carrying 1,750 passengers and their cars will continue to dominate high-traffic routes. Electric hydrofoils are best suited to short, passenger-only routes in cities.
"I'd love to go out with you, but I never go out on days that end in `Y.'"