A sister ship is a

ship A ship is a large watercraft, vessel that travels the world's oceans and other Waterway, navigable waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research and fishing. Ships are generally disti ...

of the same

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

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.

warship A warship or combatant ship is a naval ship that is used for naval warfare. Usually they belong to the navy branch of the armed forces of a nation, though they have also been operated by individuals, cooperatives and corporations. As well as b ...

s , , , and are all sister ships, each being an . Perhaps the most famous sister ships were the

White Star Line The White Star Line was a British shipping line. Founded out of the remains of a defunct Packet trade, packet company, it gradually grew to become one of the most prominent shipping companies in the world, providing passenger and cargo service ...

's s trio, consisting of , and . 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 Royal Caribbean International (RCI), formerly Royal Caribbean Cruise Line (RCCL), is a cruise line founded in 1968 in Norway and organized as a wholly owned subsidiary of Royal Caribbean Group since 1997. Based in Miami, Florida, it is the l ...

'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. 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 s of the

Second World War World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

that came in "long-hull" and "short-hull" versions. Notable

airships An airship, dirigible balloon or dirigible is a type of aerostat (lighter-than-air) aircraft that can navigate through the air flying under its own power. Aerostats use buoyancy from a lifting gas that is less dense than the surrounding ai ...

include the American sister ships and , and the German 's '' Hindenburg'' and '' Graf Zeppelin II''. The generally accepted commercial distinctions of a "sister ship" are the following: * Type: Identical main type (bulk, tank, RoRo, etc.) *

Dry weight Vehicle weight is a measurement of wheeled motor vehicles; either an actual measured weight of the vehicle under defined conditions or a gross weight rating for its weight carrying capacity. Curb or kerb weight Curb weight (American English) or k ...

(DWT): ± 10% on the DWT (If the ship is 100,000 DWT, 90,000 to 110,000 DWT) * Builder: Identical shipbuilding company name (not the ship yard location or the country of build) The critical overriding criterion is having the same hull design. For example, the popular TESS-57 standard design built by Tsunishi Shipbuilding are built in Japan, China, and the Philippines. All the ships of this design are classed as sister ships. The International Maritime Organization defined ''sister ship'' in IMO resolution MSC/Circ.1158 in 2006. Criteria included these: * A sister ship is a ship built by the same yard from the same plans. * The acceptable deviation of lightship displacement should be between 1 and 2% of the lightship displacement of the lead ship, depending on the length of the ship.

References

{{reflist Shipbuilding Ship classes