The six-day race covers a 135-mile course on a network of canals, rivers and lakes in the north of the Netherlands. Speed limits on narrow waterways have been temporarily waived for the boats, the fastest of which can go nearly 19 mph.
Sunlight is not the most obvious source of renewable energy in the rain-soaked Netherlands, but organizers say the threat of poor weather will spark creative design.
Participants met that challenge with technologies that included water-cooled solar cells, carbon fiber propellers, and mathematically-optimized designs to reduce drag.
The Technical University of Delft, which won last year's event, has outfitted its boat with gallium arsenide solar panels, and the hull was professionally engineered by the Marin Research Institute Netherlands.
The craft weighs less than 200 pounds without the skipper.
Delft also is a perennially strong contender in a similar solar car race in Australia.
Winning isn't the only objective. Many racers are intent on other goals, ranging from promoting solar energy to learning how to design the most efficient trash collection.
Ronaldo Fazanelli Migueis of the Federal University in Rio de Janeiro, Brazil, said his team's interests lie in making a lightweight boat with a simple design for mass production.
"Our goal is to use this kind of technology for small boats in Guanabara Bay in our zero garbage program," he said. The boats, to be solar-powered in the future, will be used to pick up floating trash in the bay next to Rio de Janeiro.
Competitors include technical students just out of high school, doctoral students, and even independently wealthy hobbyists.
"Our secret weapon is our propeller," Joop Steenman, a private enthusiast, said of his large, yellow boat. Its precisely-engineered carbon fiber blades cost $8,500.
Steenman, owner of a gas turbine company, said he spent $186,000 on his boat, which is shaped like an aircraft carrier and has a single hull and a large deck covered with a thin layer of black solar cells. The cells can generate a combined 1.6 kilowatts under ideal conditions, Steenman said.
Steenman hired the former students from Delft University who won the 2006 race to design the craft.
"The only thing I did was select the color," he said.