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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 (places, constellations, monarchs) 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.
Perhaps the most famous sister ships were the White Star Line's Olympic-class ocean liners, consisting of 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.
The generally accepted commercial distinctions of a sister ship are the following:
- Type: Identical main type (bullk, tank, RoRo, etc.)
- Dry weight (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 criteria are 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.
- "The 'Olympic' Class Ships: Olympic, Titanic, Britannic" (PDF). 2012-01-03. Archived from the original (PDF) on 2019-04-04. Retrieved 2020-06-24.
- "DEVELOPMENT OF EXPLANATORY NOTES FOR HARMONIZED SOLAS CHAPTER II-1" (PDF). SUB-COMMITTEE ON STABILITY AND LOAD LINES AND ON FISHING VESSELS SAFETY. International Maritime Organization. 2 June 2006. p. 3. Archived from the original (PDF) on 5 March 2016. Retrieved 21 May 2015.