"Like all good conspiracy theories, this one is incredibly complex, based on several supposedly questionable events and apparent coincidences that occurred (or supposedly did so) between the Titanic’s launch on May 31, 1911, and the ten-second encounter with the iceberg at 11:40 PM on April 14, 1912. The starting premise of Gardiner’s thesis is that the two ships, the Olympic and Titanic, were designed to be as identical sisters, as alike as two peas in a pod. By the time the Titanic was launched, this was no longer true, of course, as several modifications had been made to the details of her design before she was even put into the water, details which subtly but unmistakably differentiated the two ships. Not to be put off by this inconvenient fact, Gardiner blithely sails on to describe how the Olympic, which had gone into White Star Line service in June 1911, turned out to be a rather unlucky ship with a spotty service record, and eventually became such a liability that her owners decided to dispose of her. In the most sneaky manner possible, of course."
the two liners was done to perpetrate an insurance fraud on a near-unimaginable scale. The “Titanic”–actually the crippled and irreparable Olympic–would be lost at sea, and an insurance claim for her total value, some $15,000,000 in 1912, would be filed and of necessity honored.
"How this was to be brought about is the most entertaining–and mindlessly absurd–aspect of the entire theory. It was intended for the “Titanic” to steam into an area where several ships would be waiting nearby, the better to take off the passengers and crew of the stricken liner, where a collision would be staged, the “Titanic’s” sea cocks opened, and the ship scuttled, the “victim” of the “damage” suffered in the collision. Some, though not all, of the ship’s senior officers, including Captain Smith and First Officer Murdoch, were in on the scheme and were tasked with carrying out the “accident” and overseeing a successful evacuation of the ship. Any loss of lives, no matter how few, was never part of the plan."
"Despite all of the readily apparent holes in Gardiner’s theory, it has gained a surprising amount of traction among the credulous and easily-led, as well as that fringe element that always seems to prefer embracing a conspiracy theory, no matter how far-fetched, to the truth. Sadly for them, any credence that might have resided in Gardiner’s “insurance-fraud switch theory” was eliminated by a simple three-digit number: 401. You see, the hull number assigned to the Olympic by her builder, Harland and Wolff, was 400; the number assigned to the Titanic was 401. This number was cut, stamped, engraved, burned, or painted onto every major component and piece of machinery for each ship, in order to make sure that the correct parts went to the right vessel. A component for Hull No. 400 would never be used in Hull No. 401, and vice-versa. And as nearly-identical as the two ships were, as shown earlier, the Olympic and the Titanic were not exact twins."
The Titanic and Olympic were “switched” in an insurance-fraud scheme | RMS Titanic Remembered