Sister ship

A sister ship is a ship of the same class or of virtually identical design to another ship. Such vessels share a nearly identical hull and superstructure layout, similar size, and roughly comparable features and equipment. They often share a common naming theme, either being named after the same type of thing or person (places, constellations, heads of state) or with some kind of alliteration. Typically the ship class is named for the first ship of that class. Often, sisters become more differentiated during their service as their equipment (in the case of naval vessels, 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. A notable group of sister ships were the White Star Line's Olympic-class ocean liners trio, consisting of RMS Olympic, RMS Titanic, and HMHS Britannic. As with some other liners, the sisters worked as running mates. Of the three sister ships, Titanic and Britannic would both sink within a year of being launched, while RMS Olympic's career spanned 24 years. 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. All three were converted into aircraft carriers, with Furious again differing from her half-sisters by initially being flush-decked and later by having a much smaller island. Another example is the American Essex-class aircraft carriers of the Second World War that came in "long-hull" and "short-hull" versions. Notable airships include the...