RMS Olympic to Belfast where the passengers disembarked. Following "Olympic"s return to Britain, the White Star Line intended to lay her up in Belfast until the war was over, but in May 1915 she was requisitioned by the Admiralty, to be used as a troop transport, along with the Cunard liners and . The Admiralty had initially been reluctant to use large ocean liners as troop transports because of their vulnerability to enemy attack, however a shortage of ships gave them little choice. At the same time, "Olympic"s other sister ship "Britannic", which had not yet been completed, was requisitioned as a
Sister ship an . The most famous sister ships were the White Star Line's , and . As with some other liners, the sisters worked as running mates. Other sister ships include the Royal Caribbean International's and . "Half-sister" refers to a ship of the same class but with some significant differences. One example of half-sisters are the First World War-era British s where the first two ships had four guns, but the last ship, , had two guns instead. Another example is the American s of the Second World War that came in "long-hull" and "short-hull" versions. Notable airships include the