The yacht, named the Alamshar, belongs to His Highness The Aga Khan, who leads the world's 20 million Ismaili Muslims.
Built by Babcock Marine, it is rumoured that The Aga Kahn flew in by helicopter this morning to pick up his purchase.
Weighing in at 400 tonnes, the yacht will be moved from the dockyard out to sea at 2.30pm this afternoon.
The 164ft yacht - named Project 305 - was commissioned in the 1990s in the hope that it might break the world speed record for super-yachts as well as beat the transatlantic record of two days, ten hours and 34 minutes.
Built under a veil of secrecy, the Alamshar was intended to be a superstar among super-yachts, powered by engines capable of blasting it to more than 60 knots.
However the boat, named after the Aga Khan's racehorse, returned to the dockyard in 2008 after sea trials off Plymouth showed it could reach only 30 knots.
At the time a source said: "This was supposed to reach 60 knots but it can go only half that speed, so the Aga Khan is taking legal advice. He's not a happy man."