Hull classification symbol The United States Navy , United States Coast Guard , and United States National Oceanic and Atmospheric Administration use hull classification symbols ( sometimes called hull codes or hull numbers ) to identify their ship types and each individual ship within each type . The system is somewhat analogous to a system of pennant number s the Royal Navy and some European and Commonwealth navies ( 19 in total ) use . History . The U . S . Navy began to assign unique Naval Registry Identification Number s to its ships in the 1890s . The system was a simple one in which each ship received a number which was appended to its ship type , fully spelled out , and added parenthetically after the ship ' s name when deemed necessary to avoid confusion between ships . Under this system , for example , the battleship " Indiana " was USS " Indiana " ( Battleship No . 1 ) , the cruiser " Olympia " was USS " Olympia " ( Cruiser No . 6 ) , and so on . Beginning in 1907 , some ships also were referred to alternatively by single - letter or three - letter codes — for example , USS " Indiana " ( Battleship No . 1 ) could be referred to as USS " Indiana " ( B - 1 ) and USS " Olympia " ( Cruiser No 6 ) could also be referred to as USS " Olympia " ( C - 6 ) , while USS " Pennsylvania " ( Armored Cruiser No . 4 ) could be referred to as USS " Pennsylvania " ( ACR - 4 ) . However , rather than replacing it , these codes coexisted and were used interchangeably with the older system until the modern system was instituted on 17 July 1920 . The United States Revenue Cutter Service , which merged with the United States Lifesaving Service in January 1915 to form the modern Coast Guard , began following the Navy ' s lead in the 1890s , with its cutters having parenthetical numbers called Naval Registry Identification Number s following their names , such as ( Cutter No . 1 ) , etc . This persisted until the Navy ' s modern hull classification system ' s introduction in 1920 , which included Coast Guard ships and craft . During World War I , the United States Navy acquired large numbers of privately owned and commercial ships and craft for use as patrol vessel s , mine warfare vessels , and various types of naval auxiliary ship s , some of them with identical names . To keep track of them all , the Navy assigned unique identifying numbers to them . Those deemed appropriate for patrol work received section patrol numbers ( SP ) , while those intended for other purposes received " identification numbers " , generally abbreviated " Id . No . " or " ID ; " some ships and craft changed from an SP to an ID number or vice - versa during their careers , without their unique numbers themselves changing , and some ships and craft assigned numbers in anticipation of naval service never were acquired by the Navy . The SP / ID numbering sequence was unified and continuous , with no SP number repeated in the ID series or vice versa so that there could not be , for example , both an " SP - 435 " and an " Id . No 435 " . The SP and ID numbers were used parenthetically after each boat ' s or ship ' s name to identify it ; although this system pre - dated the modern hull classification system and its numbers were not referred to at the time as " hull codes " or " hull numbers , " it was used in a similar manner to today ' s system and can be considered its precursor . The modern hull classification system . United States Navy . The U . S . Navy instituted its modern hull classification system on 17 July 1920 , doing away with section patrol numbers , " identification numbers " , and the other numbering systems described above . In the new system , all hull classification symbols are at least two letters ; for basic types the symbol is the first letter of the type name , doubled , except for aircraft carriers . The combination of symbol and hull number identify a modern Navy ship uniquely . A heavily modified or re - purposed ship may receive a new symbol , and either retain the hull number or receive a new one . For example , gun cruiser was converted to a gun / missile cruiser , changing the hull number to CAG - 1 . Also , the system of symbols has changed a number of times both since it was introduced in 1907 and since the modern system was instituted in 1920 , so ships ' symbols sometimes change without anything being done to the physical ship . Hull numbers are assigned by classification . Duplication between , but not within , classifications is permitted . Hence , CV - 1 was the aircraft carrier and BB - 1 was the battleship . Ship types and classifications have come and gone over the years , and many of the symbols listed below are not presently in use . The Naval Vessel Register maintains an online database of U . S . Navy ships showing which symbols are presently in use . After World War II and until 1975 , the U . S . Navy defined a " frigate " as a type of surface warship larger than a destroyer and smaller than a cruiser ; in other navies , such a ship generally was referred to as a " flotilla leader " , or " destroyer leader " — hence the U . S . Navy ' s use of " DL " for " frigate " prior to 1975 — while " frigates " in other navies were smaller than destroyers and more like what the U . S . Navy termed a " destroyer escort " , " ocean escort " , or " DE " . The United States Navy 1975 ship reclassification of cruiser s , frigates , and ocean escorts brought U . S . Navy classifications into line with other nations ' classifications , and at least cosmetically . i . e . , in terms of terminology , eliminated the perceived " cruiser gap " with the Soviet Navy by redesignating the former " frigates " as " cruisers " . United States Coast Guard . If a ship ' s hull classification symbol begins with " W " , it is a ship of the United States Coast Guard . Until 1965 , the Coast Guard used U . S . Navy hull classification codes , prepending a " W " to their beginning . In 1965 , it retired some of the less mission - appropriate Navy - based classifications and developed new ones of its own , most notably WHEC for " high endurance cutter " and WMEC for " medium endurance cutter " . Military Sealift Command . If a ship ' s hull classification symbol begins with " T - " , it is part of the Military Sealift Command , has a primarily civilian crew , and is a United States Naval Ship ( USNS ) in non - commissioned service — as opposed to a commissioned United States Ship ( USS ) . National Oceanic and Atmospheric Administration . The National Oceanic and Atmospheric Administration ( NOAA ) , a component of the United States Department of Commerce , also uses a hull classification symbol system , which it also calls " hull numbers , " for its fleet . In its system , the NOAA fleet is divided into two broad categories , research ship s and survey ships . The research ships , which include oceanographic and fisheries research vessels , are given hull numbers beginning with " R " , while the survey ships , generally hydrographic survey vessels , receive hull numbers beginning with " S " . The letter is followed by a three - digit number ; the first digit indicates the NOAA " class " ( i . e . , size ) of the vessel , which NOAA assigns based on the ship ' s gross tonnage and horsepower , while the next two digits combine with the first digit to create a unique three - digit identifying number for the ship . Generally , each NOAA hull number is written with a space between the letter and the three - digit number , as in , for example , or . Once an older NOAA ship leaves service , a newer one can be given the same hull number ; for example , " S 222 " was assigned to , then assigned to NOAAS " Thomas Jefferson " ( S 222 ) , which entered NOAA service after " Mount Mitchell " was stricken . United States Navy hull classification codes . The U . S . Navy ' s system of alpha - numeric ship designators , and its associated hull numbers , have been for several decades a unique method of categorizing ships of all types : combatants , auxiliaries and district craft . Though considerably changed in detail and expanded over the years , this system remains essentially the same as when formally implemented in 1920 . It is a very useful tool for organizing and keeping track of naval vessels , and also provides the basis for the identification numbers painted on the bows ( and frequently the sterns ) of most U . S . Navy ships . The ship designator and hull number system ' s roots extend back to the late 1880s , when ship type serial numbers were assigned to most of the new - construction warships of the emerging " Steel Navy " . During the course of the next thirty years , these same numbers were combined with filing codes used by the Navy ' s clerks to create an informal version of the system that was put in place in 1920 . Limited usage of ship numbers goes back even earlier , most notably to the " Jeffersonian Gunboats " of the early 1800s and the " Tinclad " river gunboats of the Civil War Mississippi Squadron . It is important to understand that hull number letter prefixes are not acronyms , and should not be carelessly treated as abbreviations of ship type classifications . Thus , " DD " does not stand for anything more than " Destroyer " . " SS " simply means " Submarine " . And " FF " is the post - 1975 type code for " Frigate . " The hull classification codes for ships in active duty in the United States Navy are governed under Secretary of the Navy Instruction 5030 . 8A ( SECNAVINST 5030 . 8A ) . Warships . Warships are designed to participate in combat operations . The origin of the 2 letter code derives from the need to distinguish various cruiser subtypes . Aircraft carrier type . Aircraft carriers are ships designed primarily for the purpose of conducting combat operations by aircraft which engage in attacks against airborne , surface , sub - surface and shore targets . Contrary to popular belief , the " CV " hull classification symbol does not stand for " Carrier Vessel " . The " CV " designation was originally derived from cruisers , since aircraft carriers were seen as an extension of the sea control and denial mission of cruisers . The " V " designation for heavier - than - air craft comes from the French verb " voler " ( to fly ) . Since 1935 , " CV " has been a two - letter , unitary hull classification symbol meaning " aircraft carrier " . Aircraft carriers are designated in two sequences : the first sequence runs from CV - 1 USS " Langley " to the very latest ships , and the second sequence , " CVE " for escort carriers , ran from CVE - 1 " Long Island " to CVE - 128 " Okinawa " before being discontinued . Surface combatant type . Surface combatants are ships which are designed primarily to engage enemy forces on the high seas . The primary surface combatants are battleships , cruisers and destroyers . Battleships are very heavily armed and armored ; cruisers moderately so ; destroyers and smaller warships , less so . Before 1920 , ships were called " < type > no . X " , with the type fully pronounced . The types were commonly abbreviated in ship lists to " B - X " , " C - X " , " D - X " et cetera — for example , before 1920 , would have been called " USS " Minnesota " , Battleship number 22 " verbally and " USS " Minnesota " , B - 22 " in writing . After 1920 , the ship ' s name would have been both written and pronounced " USS " Minnesota " ( BB - 22 ) " . In generally decreasing size , the types are : Submarine type . Submarines are all self - propelled submersible types ( usually started with SS ) regardless of whether employed as combatant , auxiliary , or research and development vehicles which have at least a residual combat capability . While some classes , including all diesel - electric submarines , are retired from USN service , non - U . S . navies continue to employ SS , SSA , SSAN , SSB , SSC , SSG , SSM , and SST types . With the advent of new Air Independent Propulsion / Power ( AIP ) systems , both SSI and SSP are used to distinguish the types within the USN , but SSP has been declared the preferred term . SSK , retired by the USN , continues to be used colloquially and interchangeably with SS for diesel - electric attack / patrol submarines within the USN , and more formally by the Royal Navy and British firms such as Jane ' s Information Group . Patrol combatant type . Patrol combatants are ships whose mission may extend beyond coastal duties and whose characteristics include adequate endurance and sea keeping , providing a capability for operations exceeding 48 hours on the high seas without support . This notably included Brown Water Navy / Riverine Forces during the Vietnam War . Few of these ships are in service today . Amphibious warfare type . Amphibious warfare vessels include all ships having organic capability for amphibious warfare and which have characteristics enabling long duration operations on the high seas . There are two classifications of craft : amphibious warfare ships which are built to cross oceans , and landing craft , which are designed to take troops from ship to shore in an invasion . Ships Landing Craft Combat Logistics Type . Ships which have the capability to provide underway replenishment to fleet units . Mine warfare type . Mine warfare ships are those ships whose primary function is mine warfare on the high seas . Coastal defense type . Coastal defense ships are those whose primary function is coastal patrol and interdiction . Mobile logistics type . Mobile logistics ships have the capability to provide direct material support to other deployed units operating far from home ports . Auxiliary type . An auxiliary ship is designed to operate in any number of roles supporting combatant ships and other naval operations . Support ships . Support ships are not designed to participate in combat , and are generally not armed . For ships with civilian crews ( owned by and / or operated for Military Sealift Command and the Maritime Administration ) , the prefix T - is placed at the front of the hull classification . Support type . Support ships are designed to operate in the open ocean in a variety of sea states to provide general support to either combatant forces or shore based establishments . They include smaller auxiliaries which , by the nature of their duties , leave inshore waters . Service type craft . Service craft are navy - subordinated craft ( including non - self - propelled ) designed to provide general support to either combatant forces or shore - based establishments . The suffix " N " refers to non - self - propelled variants . United States Coast Guard craft . Prior to 1965 , U . S . Coast Guard ships used the same designation as naval ships , but preceded by a " W " to indicate Coast Guard subordination . Temporary designations . United States Navy Designations ( Temporary ) are a form of U . S . Navy ship designation , intended for temporary identification use . Such designations usually occur during periods of sudden mobilization , such as that which occurred prior to , and during , World War II or the Korean War , when it was determined that a sudden temporary need arose for a ship for which there was no official Navy designation . During World War II , for example , a number of commercial vessels were requisitioned , or acquired , by the U . S . Navy to meet the sudden requirements of war . A yacht acquired by the U . S . Navy during the start of World War II might seem desirable to the Navy whose use for the vessel might not be fully developed or explored at the time of acquisition . On the other hand , a U . S . Navy vessel , such as the yacht in the example above , already in commission or service , might be desired , or found useful , for another need or purpose for which there is no official designation . Numerous other U . S . Navy vessels were launched with a temporary , or nominal , designation , such as YMS or PC , since it could not be determined , at time of construction , what they should be used for . Many of these were vessels in the 150 to 200 feet length class with powerful engines , whose function could be that of a minesweeper , patrol craft , submarine chaser , seaplane tender , tugboat , or other . Once their destiny , or capability , was found or determined , such vessels were reclassified with their actual designation .