How do you propel 100 tonnes of metal around the world's oceans without fuel or even a sail?
For a pioneering group of Swiss investors and German engineers, the answer is simple - the sun.
Add some design expertise from New Zealand and you have the MS Turanor PlanetSolar, the world's largest solar-powered boat and a striking glimpse into the future of marine travel.
"The idea was to demonstrate the enormous potential of solar power by circumnavigating the globe," says Rachel Bros de Puechredon from PlanetSolar.
And with 60,000km (37,000 miles) successfully navigated, the team have achieved precisely that.
The Turanor uses energy harnessed from more than 500 sq m of solar panels to drive two, 60kW electric engines, each in turn driving a standard propeller. They are capable of pushing the 35m catamaran to a top speed of 14 knots (26km/h, 16mph).