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The capital ships of a navy are its most important warships; they generally possess the heaviest firepower and armor and are traditionally much larger than other naval vessels. A capital ship is generally a leading or a primary ship in a naval fleet.
There is usually no formal criterion for the classification, but it is a useful concept in naval strategy; for example, it permits comparisons between relative naval strengths in a theatre of operations without the need for considering specific details of tonnage or gun diameters.
A notable example of this is the Mahanian doctrine, which was applied in the planning of the defence of Singapore in World War II, where the Royal Navy had to decide the allocation of their battleships and battlecruisers between the Atlantic and Pacific theatres. The Mahanian doctrine was also applied by the Imperial Japanese Navy, leading to their preventive move to attack Pearl Harbor and the battleships of the U.S. Pacific Fleet. The naval nature of the Pacific Theater of Operations, more commonly referred to as the Pacific War, necessitated the United States Navy to deploy its battleships and aircraft carriers in the Pacific. The war in Europe was primarily a land war, consequently, Germany's surface fleet was small, and the escort ships used in the Second Battle of the Atlantic were mostly destroyers and destroyer escorts to counter the U-boat threat.
Era of sail
Before the advent of the all-steel navy in the late 19th century, a capital ship was generally understood as a ship that conformed to the Royal Navy's rating system of a ship of the line as being of the first, second, third or fourth rates:
- First rate: 100 or more guns, typically carried on three or four decks. Four-deckers suffered in rough seas, and the lowest deck could seldom fire except in calm conditions.
- Second rate: 90–98 guns.
- Third rate: 64 to 80 guns (although 64-gun third-raters were small and not very numerous in any era).
- Fourth rate: 46 to 60 guns. By 1756, these ships were acknowledged to be too weak to stand in the line of battle and were relegated to ancillary duties, although they also served in the shallow North Sea and American littorals where larger ships of the line could not sail.
Frigates were ships of the fifth rate; sixth rates comprised small frigates and corvettes. Towards the end of the Napoleonic Wars and into the late 19th century some larger and more powerful frigates were classified as fourth rates.
Battleship / Battlecruiser
The definition of "capital ship" was formalized in the limitation treaties of the 1920s and 30s in the Washington Naval Treaty, London Naval Treaty, and Second London Naval Treaty. This applied mainly to ships resulting from the dreadnought revolution; dreadnought battleships (also known first as dreadnoughts and later as battleships) and battlecruisers.
In the 20th century, especially in World Wars I and II, typical capital ships would be battleships and battlecruisers. All of the above ships were close to 20,000 tons displacement or heavier, with large caliber guns and heavy armor protection.
Heavy cruisers, despite being important ships, were not considered capital ships. An exception to the above in World War II was the Deutschland-class cruiser. Though this class was technically similar to a heavy cruiser, albeit with considerably heavier guns, they were regarded by some as capital ships (hence the British label "Pocket battleship") since they were one of the few heavy surface units of the Kriegsmarine. The Alaska-class cruisers, despite being oversized heavy cruisers and not true battleships/battlecruisers, were also considered by some[who?] to be capital ships.
During the Cold War, a Soviet Kirov-class large missile cruiser had a displacement great enough to rival WWII-era battleships and battlecruisers, perhaps defining a new capital ship for that era. In regard to technical design, however, the Kirov is simply a supersized guided-missile cruiser with nuclear propulsion.
It took until late 1942 before aircraft carriers were universally considered capital ships. The U.S. Navy was forced to rely primarily on their aircraft carriers after the attack on Pearl Harbor sank or damaged eight of their Pacific Fleet battleships.
In the 21st century, the aircraft carrier is the last remaining capital ship, with capability defined in decks available and aircraft per deck, rather than in guns and calibers. The United States possesses supremacy, in both categories of aircraft carriers, possessing not only 10 active duty supercarriers each capable of carrying and launching nearly 100 tactical aircraft, but an additional 12 amphibious assault ships as capable (in the "Sea Control Ship" configuration) as the light VSTOL carriers of other nations.
Despite their significance to modern fleets, the US Navy has never named aircraft carriers after US states as was the practice when battleships (e.g. Iowa class) were considered capital ships. Instead US state names are today applied to nuclear submarines.
Ballistic missile submarines (or "boomers"), while important ships and similar in tonnage to early battleships, are usually counted as part of a nation's nuclear deterrent force and do not share the sea control mission of traditional capital ships. Nevertheless, many navies, including the Royal Navy and the United States Navy, consider these ships to be capital ships.
Beginning with the first class of Trident equipped ballistic missile submarines (i.e. the Ohio class) state names have been applied to US nuclear submarines indicating their status as capital ships. Previous ballistic missile submarines (e.g. Poseidon missile equipped submarines) had not been named for states. After the completion of the last Ohio class ballistic missile submarine, state names were applied to attack submarines (e.g. Virginia class). Earlier attack submarines had been named for major cities (e.g. Los Angeles class) - as was previously the practice for cruisers (e.g. USS Indianapolis)
Some navies reserve specific names for their capital ships. Names reserved for capital ships include chiefs of state (e.g. Bismarck), important places (e.g. Australia), historically important naval officers or admiralty (e.g. De Ruyter), historical events or objects (e.g. USS Constitution), traditional names (e.g. HMS Ark Royal). However there are some exceptions to the rule.
Beginning with the USS Texas (BB-1) (the first US battleship), US capital ships have traditionally been named after US states. Lesser vessels are named after US territories (e.g. Alaska class battlecruisers), major US cities (e.g. cruisers), or US presidents (e.g. early attack submarines and late aircraft carriers). Prior to and during WWII the Imperial Japanese Navy also followed the practice of naming battleships after provinces (e.g. IJN Yamato).
Beginning with the Ohio class, US state names were applied to ballistic missile submarines indicating their rising status as capital ships equipped with a complement of doomsday nuclear weapons. After the last delivery of Ohio class ballistic missile submarines, the US Navy began applying state names to attack submarines (e.g. the Virginia class).
The term has been adopted into science fiction literature and culture to describe large spaceships used in military contexts, particularly where other naval terms have also been adopted in similar fashion; for example, sci-fi capital spaceships are often "carriers", that carry small fighters analogous to the way the real world naval equivalent carries fighter aircraft, as well as functioning as "battleships".
- Ship of the line
- Aircraft carrier
- Amphibious Assault Ship