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Technology

Surf and fly simultaneously
The Audi e-foil is more than a surfboard – it's a kind of seaplane. With electric drive, of course.

Swept by constant winds, caught between the steep mountainsides and accelerated by a venturi-like effect – the northern end of Lake Garda, between Torbole and Malcesine, is a hotspot for windsurfers. But what happens when the southerly wind, known as the Ora, subsides as evening falls? That’s when Franz Hofmann and his Audi e-foil take over.

The word “foil” is part of a sailing megatrend. Racing craft have been flying across the water for years on nothing but a narrow blade. The same goes for the Olympic Nacra 17 catamaran class competed in by the Audi e-tron sailing team of Johannes Polgar and Carolina Werner. Kite surfers and windsurfers use foils, too. The principle is always the same: as soon as the boat or board reaches a certain speed, the foil generates sufficient upward force to lift the hull out of the water. This causes a massive drop in drag, enabling extremely high speeds from very little wind.

Franz Hofmann is an expert in lightweight engineering

33-year-old Audi engineer Franz Hofmann loves the thrill of kite surfing with a foil. In his day job he develops hydrogen tanks made from carbon-fiber reinforced polymer (CFRP) in Ingolstadt and is an expert in lightweight engineering. No surprise then that he didn’t rate the first kite foils when they appeared in stores several years ago – too heavy.

Hofmann got in touch with Christian Rößler, whom he knew from his youth in the Upper Franconia region of southern Germany. Rößler works in the aerospace systems faculty at the Technische Universität München and knows just about all there is to know about fluid dynamics. He built initial computational models and simulations and used them to create his very own hydrofoil. Hofmann and Rößler cut molds at the TU, sanded and smoothed them to the correct shape at home in their living rooms and then laminated the first CFRP foil in the basement. The kite foil was finished in 2015. “From a fluid dynamics standpoint, data comparisons tell us that it was one of the best and most efficient foils out there,” says Hofmann with a certain degree of pride.

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Surfing? Gliding? Flying?
Franz Hofmann tries out the Audi e-foil at the northern end of Lake Garda, against the backdrop of Malcesine.

 

 

Widespread support from co-workers

His work at Audi caused Hofmann to think further on the matter of propulsion – beyond wind. A foil is extremely efficient – what could be a better fit than to combine it with a clean electric drive just like the Audi e-tron models? Hofmann began putting the idea to various colleagues within the company: Innovation Management, Audi Design, the experts in 3D printing, the Audi Lightweight Engineering Center in Neckarsulm and several workshops. He was greeted with substantial interest and sound practical support at every turn.

Hofmann launched the first prototype of the Audi e-foil in early 2018. It was a surfboard strapped to a battery pack inside a waterproof case. The next evolution was equipped with a battery compartment integrated into the board. The first pre-production boards were on the beach ready for the Kiel Week sailing regatta that very summer. “Many of my colleagues are now involved with this,” says Hofmann, “which means the foil and the board are perfectly equipped, starting with the electronic connections and ending with the casing for the jet propulsion.”

Balance takes practice

How hard is it to ride the Audi e-foil? According to Hofmann, “A couple of days of practice is all it takes for just about anyone to go racing across the water.” In fact, when Hofmann himself stands on the board, he makes it all look incredibly easy, almost child’s play. He holds a remote control in his right hand. When it starts up, the board is lying flat on the water. But as soon as Hofmann curls his index finger a little tighter, it lifts up onto its foil and only the carbon mast cuts through the water. Hofmann increases the power and flies out over the wide expanse of Lake Garda, silently, with zero emissions and absolutely no wash – heading for the setting sun and into the subsiding wind.

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“Revolution on lake and ocean”
Franz Hofmann believes firmly in the future of electric drive on the water.

 

Five questions for
Franz Hofmann

Hi Franz, how does it feel to ride the Audi e-foil?
Hofmann: Totally amazing, because it’s a completely unique experience – a mixture of surfing and flying, silent, efficient and sporty. The Audi e-foil is more than an electric surfboard, it’s a one-person seaplane.

What makes the Audi e-foil better than its competitors?
Hofmann: We didn’t invent the e-foil, there are hundreds of projects like this all over the world. But we have combined our high-performance foil with electric jet propulsion. This technology package is hugely efficient and extremely stable in operation.

What does the Audi e-foil have in common with Audi cars?
Hofmann: It runs silently and with zero emissions, which makes it the perfect match for the new Audi e-tron and its strengths such as aerodynamics, sustainability and efficiency. On water, on land or in the air – electric mobility is forcing us to explore the limits of physics in all areas, simply because the amount of energy we can carry with us in future will be lower.

Will e-foils establish themselves in the market?
Hofmann: Yes, I’m absolutely certain they will. Just like with bicycles, electric drive will become widespread on water, too. Battery-powered foils will mark a true revolution on the world’s lakes and seas.

Is your project likely to make it into production?
Hofmann: It’s my vision to build the first Audi e-foils in low volumes in 2019 and then offer them to our customers to test drive.

The Audi e-foil in action.

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High-tech hydrodynamics –
the Audi e-foil

The board is around 1.7 meters in length and incorporates the battery and electronics. A 100-centimeter mast connects it with the hydrofoil, the jet propulsion unit and the rudder. Aside from the drive itself, all the components are made from carbon-fiber reinforced polymer (CFRP) – guaranteeing exceptional strength and stiffness paired with low weight.

The casing for the jet propulsion unit is made from aluminum and the technical components cooled by the surrounding water. An impeller – a small shrouded propeller – sucks in the water, accelerates it into a flow channel and ejects it back out through a jet. A stator in the outlet nozzle removes the rotation from the stream, thus ensuring the water flows in a straight line.

It takes between 3 and 5 kW to power the Audi e-foil, depending upon the weight of the rider. The battery runs for about one hour, which is sufficient to cover more than 30 kilometers. At around 17 km/h, the hydrofoil lifts the board up to one meter out of the water and enables a top speed of approximately 45 km/h.

Technical Data
Audi e-foil

Length

1.70 meters

Power

ca. 5 kW

Battery runtime

ca. 1 hour

Range

ca. 30 km at 30 km/h

Top speed

45 km/h

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A matter of balance
Hofmann floats on the foil around one meter above the surface of Lake Garda.

 

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