'We are at the mercy of the weather," smiles Don Scott, standing on a slender steel hull. "If the sea is like glass it could take two weeks, but the north Atlantic weather is going to have a lot to say about that. Here on a hillside industrial park above Plymouth, Britain's boldest foray into robot shipping is taking shape. On Sept 16, the Mayflower Autonomous Ship, a 15-metre-long steel trimaran, will slip into Plymouth Sound to prepare for a pioneering voyage: the first unmanned transatlantic crossing powered by artificial intelligence and solar energy. Barring an accident, no human beings will be involved in the 3,220-mile trip, which will mark the 400th anniversary of the original Mayflower crossing in 1620.