Another ship, Solar Sailor, ferries passengers in Australia's Sydney Harbour using solar panels that can also catch the wind.
A larger version of Solar Sailor will soon be coming to the United States to carry passengers in the San Francisco Bay, one of the country's busiest ports.
But no one has made a solar-powered voyage across the Atlantic before, and no solar boat has carried more than one passenger on such a long trip.
One adventurer, Japanese sailor Ken-ichi Horie, made a solo trip across the Pacific in 1996. It took him five months to cross that ocean in his submarine-shaped solar boat, made from recovered aluminum cans.
Sun21 will move faster, about 7 miles an hour (11 kilometers an hour), and take about four months to cross the Atlantic.
The crew could make it to the U.S. in less time, but they're taking time to stop at the Canary Islands off Morocco (map), the Cape Verde islands off northwestern Africa, and Martinique in the Caribbean Sea to spread their renewable-energy message.
Once they reach the U.S. the crew will cruise up the East Coast from Florida to Boston, stopping at ports to make presentations along the way.
After the sea journey Vosseler will trade his sea legs for walking shoes to trek the width of the United States.
"I would like to walk to demonstrate that you can get long distances without fossil fuels," Vosseler said.
"The U.S. is still the highest consumer of energy and has the biggest ecological footprint [on] this planet—but to me, also the biggest potential for innovation," Vosseler said.
He added, "If people in the United States really see something that is a chance and is necessary, developments can take off quite quickly. And that is, for me, a big encouragement."
The sun21 mariners aren't the only explorers aiming for green glory.
"I believe that all the adventurers that can promote renewable energies should be supporters" of projects such as the sun21 trip, Swiss explorer and physician Bertrand Piccard said.
In 1999 Piccard and British pilot Brian Jones became the first hot-air balloonists to circle the globe without stopping. Now Piccard is working on a solar plane that can circle the globe. That journey is planned for 2011.
In addition, two groups so far have signed on for the World Solar Navigation Challenge.
Inspired by the Jules Verne novel Around the World in 80 Days, the challenge is to see who can be first to circle the globe in a solar-powered boat.
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