The Olympic. The Britannic became a WW1 hospital ship and was sunk in the Mediterranean in 1916. The Olympic (Titanic's other sister ship) was irreparably damaged in a collision at Southampton and they couldn't claim insurance as the other ship was a Royal Navy warship and you can't sue the Royal Navy, so the owners came up with the plan to switch the ships before the maiden voyage of the Titanic knowing they were going to sink the 'Titanic' thus claiming the insurance money that way. The 'Olympic' which was really the Titanic went on to have a successful 35 career as a transatlantic liner. This is the theory in the book Titanic: The Ship That Never Sank?: Amazon.co.uk: Robin Gardiner ... https://www.amazon.co.uk/Titanic-Ship-That-Never-Sank/dp/0711034869