Did you know you could navigate 100 tons 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 was simple - the sun.
Add some design expertise from New Zealand and you have the MS Turanor PlanetSolar, the world's largest solar-powered boat.
"The idea was to demonstrate the enormous potential of solar power by circumnavigating the globe," says Rachel Bros de Puechredon from PlanetSolar.
And with 37,000 miles successfully navigated, the team has 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).
On its journey, the boat averaged just five knots as the five-man crew charted a course around the equator to maximise exposure to the sun. They were at sea for 585 days - somewhat longer than the record 45 days for sailing round the world.
To boost power when the sun is weak or hiding, the boat holds eight tons of lithium ion batteries, capable of powering the vessel for three days when dark clouds shade the ocean skies.
The most important numbers, however, are the smallest - the Turanor uses zero gallons of fuel and emits almost no carbon dioxide.
"Captaining the Turanor is a little bit special," says skipper Gerard d'Aboville. "You have to use a lot of foresight, constantly checking the weather and choosing your speed to coincide with the sun. You must always think well in advance. It is different [from other boats], more interesting," he says.
His latest voyage is a world first for a solar-powered boat, but Mr D'Aboville is already familiar with breaking records. In 1980, he became the first man to row single-handed across the Atlantic. In 1991, he achieved a similar feat, this time across the Pacific. Now he has circumnavigated the world, albeit with a little help from nature.
"As I get older, my personal energy is less sustainable, so it's good to have a partner like the sun," he says.
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