The Microtransant Challenge is a autonomous robot sailing race across the Atlantic. They’ve had two warm up competitions in which the boats variously didn’t arrive in time or components failed. In the second competition in 2007 the INNOC (Austrian Association for Innovative Computer Science) team managed to sail for 24 hours and were pronounced the winners.
When I first arrived at the SRG three years ago we discussed doing something similar on the pond at UCD. An autonomous yacht would be a fun thing to build. Perhaps that is a summer project for us.