The northern end of Lake Garda, between Torbole and Malcesine, is a hotspot for windsurfers, but  it’s when the southerly wind, known as the Ora, subsides that Franz Hofmann and his Audi e-foil come into their own.

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 and kite surfers and windsurfers use foils, too. The principle is that 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 a 33-year-old engineer with Audi who develops hydrogen tanks made from carbon-fibre reinforced polymer (CFRP) in Ingolstadt and is an expert in lightweight engineering. He is also an avid kite surfer, who was less than impressed with the first kite foils because of their tremendous weight.

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 moulds 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. 

But Hofmann wasn’t satisfied with this and began to think about other mean 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? He began putting the idea to various colleagues within the company: Innovation Management, Audi Design, the experts in 3D printing, the Audi Lightweight Engineering Centre 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 and the first pre-production boards were on the beach ready for the Kiel Week sailing regatta that very summer. 

“A couple of days of practice is all it takes for just about anyone to go racing across the water,” says Hofmann.

A hand held remote starts the board which rises out of the water onto its foil as pressure is tightened on the trigger.

The board is around 1.7 metres long and incorporates the battery and electronics. A 100-centimetre 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-fibre reinforced polymer (CFRP) – guaranteeing exceptional strength and stiffness paired with low weight.

The casing for the jet propulsion unit is made from aluminium 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 5kW 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 kilometres. At around 17 km/h, the hydrofoil lifts the board up to one metre out of the water and enables a top speed of up to 45km/h.