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).
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 as a result - 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 tonnes 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 rowed across the Atlantic. In 1991, he achieved a similar feat, this time across the Pacific, becoming the first man to row single-handed across both oceans.
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.
But Turanor's journey did not end with its circumnavigation of the world. Its emission-free propulsion gives the scientific community a rare opportunity to conduct important deep-water research.
As Ms Bros de Puechredon says, once the potential of solar power had been demonstrated, "the boat needed another purpose".
To this end, PlanetSolar teamed up with the University of Geneva to lead an expedition to study the Gulf Stream - which runs up the east coast of America and across the Atlantic to Ireland - and how changes within it are affecting global temperatures.
"To be able to spend several months studying the whole length of the stream in such a boat is unique," says Prof Martin Beniston, chair for climate change at the university.
First, the boat enabled the team to "collect vital samples from the atmosphere uncontaminated by exhaust fumes", he says.
This clean data, focusing both on the role of organisms living in vortexes within the stream and on the impact of aerosols released by it, suggest that oceans may play a much greater role in climate change than previously thought.
The boat also helped "communicate the science to the general public", says Prof Beniston. "The interest generated [by Turanor] has been amazing - we really weren't expecting so much public and media attention."
The team has now signed a five-year contract with PlanetSolar and is planning to go back across the Atlantic to Rio to study the impact of Saharan dust on the ocean.
Without rising global temperatures, Turanor would never have been conceived, let alone built.
Shipping accounts for 2.7% of all global CO2 emissions, compared with less than 2% for aviation. According to the International Maritime Organization, emissions from ships could double by 2050.
The industry itself points out that the vast majority of the world's trade is conducted across water, so emissions per unit of cargo from ships are a fraction of those from planes.
That may be so, but ships themselves remain one of the dirtiest forms of transport, and some shipping companies are preparing for the inevitable legislation to curb emissions. Indeed the European Commission has already drafted proposals forcing all large ships using EU ports to measure and report their CO2 emissions.
Cleaner technologies are inevitable, and a number of companies are developing what they hope will be the future of large vessel propulsion.
One such firm is B9 Shipping, which has designed a 100m long cargo ship capable of carrying 4,000-5,000 tonnes of dry bulk, powered by sails and biogas engines fuelled by food waste.
With fuel being the single biggest operational cost for any ship operator, the company says simple economics, as well as the inevitable cost of carbon permits, is driving interest in the ships.
And while tried and trusted wind is leading the way, harnessing the sun's power remains an option for the future.
"Wind is the most sensible solution but there are areas where there is not much wind but a great deal of sun, and here solar can play a part," says B9's Diane Gilpin.
Large tankers crossing oceans powered solely by the sun is an unlikely scenario, but there is no reason why ferries and pleasure boats, as well as those designed for scientific expeditions, cannot rely on solar power.
As Reza Shaybani, chairman of the British Photovoltaic Association, says: "We have to be realistic and a huge amount of research and development is needed, but the journey has to begin somewhere."
By demonstrating that the sun alone can power a 100-tonne boat to circumnavigate the world, the MS Turanor PlanetSolar has proved just how far solar power has come.