Sister ship

A sister ship is a ship of the same class as or of virtually identical design to another ship. Such vessels share a nearly identical hull and superstructure layout, similar displacement, and roughly comparable features and equipment. Often, sisters become more differentiated during their service as their equipment (in the case of military ships, their armament) are separately altered.

For instance, the U.S. warships USS Iowa, USS New Jersey, USS Missouri, and USS Wisconsin are all sister ships, each being an Iowa-class battleship.

The most famous sister ships were the White Star Line's RMS Olympic, RMS Titanic and HMHS Britannic. As with some other liners, the sisters worked as running mates. Other sister ships include the Royal Caribbean International's Explorer of the Seas and Adventure of the Seas.

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 Courageous-class battlecruisers where the first two ships had four 15-inch (381 mm) guns, but the last ship, HMS Furious, had two 18-inch (457 mm) guns instead. Another example is the American Essex-class aircraft carriers of the Second World War that came in "long-hull" and "short-hull" versions.

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