Chinese sailor Guo Chuan has gone missing while trying to break the record for crossing the Pacific alone.
The US Coast Guard is now searching waters off Hawaii, where his boat was spotted without him onboard.
Mr Guo's support team had lost contact with him at about 07:00 GMT on Tuesday.
The sailor, who was the first Chinese person to sail solo around the world, left San Francisco on 18 October with the aim of reaching Shanghai within 20 days.
Shortly after his team lost contact with him, a search plane was sent to his last co-ordinates, about 600 miles (1,000 km) off the Hawaiian island of Oahu.
Chinese state news agency Xinhua reported that the boat, the Qingdao China, was spotted with its main sail snapped off and in the water.
Mr Guo was not seen onboard the 97ft (26.5m) trimaran.
US ships are now heading to the boat. A Coast Guard spokesman told AP news agency that Mr Guo had been in good health and was an experienced sailor, "so that gives us some hope".
Mr Guo was trying to break the current speed record for his journey, which is 21 days.
He was the first Chinese person to sail around the world alone when he completed his trans-navigation of the globe in a 12m yacht in 2013.