Captain Erwann Le Rouzic is chasing the sun at sea. The Frenchman is manning the world's first solar marine expedition on the Turanor PlanetSolar, a boat that runs entirely on energy from the sun, and, at 90 tonnes, is the largest solar boat ever built. As part of a global voyage that has spanned 38,000 kilometres since leaving Monaco last September, the ship berthed at Ocean Terminal in Tsim Sha Tsui yesterday. It has more ports to visit and a further 22,000 kilometres or so to go before it ends its trip in Portugal. A crew of just four - the captain, an engineer, a maintenance man, and the US$25 million expedition's founder - is making the voyage to demonstrate the possibilities of silent, solar-powered boats. 'In 15 years, it may be possible for you to buy a solar boat ... we have the technology,' said the founder, Raphael Domjan, a Swiss native and self-styled eco-explorer. Though solar power alone may not work for much bigger boats, he said commercial boats could use hybrid technology. With 537 square metres of solar panels on its wide, flat deck, the vessel - a central hull borne by two pontoons which also carry the propellers - provides all its own energy for the motors and on-board electrical equipment, including the crew's refrigerator and coffee-maker. The boat can travel for 60 hours once its solar batteries are charged. With an average cruising speed of five knots - nine kilometres an hour - it is slow by power boat standards but comparable with most sailing yachts. It can travel at up to nine knots, but the sailors are careful: higher speeds mean more energy use and they want to use no more than what they can generate from the sun. Domjan described this mentality as like a microcosm for the planet and its rapidly increasing use of limited fossil fuels. 'If we try to use more energy than what we get, then it's not sustainable,' he said.