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A sister ship is a ship with a hull design and appearance identical or very similar to another ship. The ships are built by the same company, although they may be made in different ship yards and by different crews, and it is possible to have more than two sister ships; a fleet, for example, may hold a number of ships of the same design. It is common for the same company to own all the ships, but this is not always the case. A company may sell off sister ships when downsizing, and the new owner will change their livery and may make other changes to their appearance.
Sister ships are all in the same class, and usually built within a few years of each other, following the same basic design. Battleships are commonly sister ships, as governments place an order for a block of ships rather than buying one at a time. While the hull design is the same, sister ships can have small variations, depending on their intended use. Facilities inside the sister ship may move around to suit the owner, and it could have radically different fittings.
Shipping companies, cruise lines, and militaries typically put out a request for bids when they need new ships. Multiple firms will present ideas, and the company will select the firm with the best design and price, commissioning a set of ships. As sister ships start to roll out of production facilities, the owner may request changes to future sister ship models in response to issues they identify. These could range from moving crew quarters around to changing engines or defense systems.
Sometimes sister ships have similar names, indicating that they are in the same generation. In other cases, they are known as sister ships because they are identified as members of the same class, like Nimitz class aircraft carriers in the United States. Each ship has its own captain and crew, although people may transfer between ships as part of their work for the company. It is common for a sister ship to have its own regular route, traveling between specific set locations on a schedule.
A famous sister ship trio, the Titanic, Olympic, and Britannic, were built for Cunard lines at the turn of the 20th century. The Titanic fell victim to one of the most famous maritime disasters in history, while her sister ships were later commandeered for use in the First World War, the Britannic as a hospital ship and the Olympic as a troop carrier.
One reason designers use the sister ship concept is so once they find a design that works, they can stick with it. It's much more expensive to have to design a completely new ship every time, rather than drawing on what works, design-wise, for the original ship. If the builders know they have a good hull and steering array, for example, they can continue using those features in future sister ships, since they have been proven to work well.
A couple of other famous sister ships are the Lusitania, whose sinking essentially brought the U.S. into World War I, and her sister ship, the Mauritania.
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