Hull Classification Symbols, United States

A numbering system for US Navy ships was initially set up in 1895 which was fairly simple, with ships being designated with their full, unabbreviated ship type followed by a number which was assigned in ascending order, unlike the random pennant numbers that the Royal Navy began adopting around this time. As such, the first modern battleship, the USS Indiana was known as Battleship No. 1 and the first destroyer USS Bainbridge was Destroyer No. 1. From 1907 ward, the ship types began to be abbrivated into single-letter symbols such as B-1 and D-1. For some ship types, a multi-letter symbol was used such as for Torpedo Boats (TB) and Armored Cruiser (ACR). Note that it is US practice to assign hull numbers in the order that the ships were authorized by Congress, rather than when they have been laid down or commissioned. It is also customary to name the ship's class on the first authorized ship (ex: the Colorado-class battleships were named as such, even though the USS Colorado was laid down in 1919 and commissioned in 1923, two years after its sister ship, the USS Maryland). This practice has persisted to this day.

The massive expansion of the US Navy during World War I resulted in a huge number of new ship types entering service which rendered the old classification system obsolete. On July 17th 1920, a new hull classification system was approved which has formed the basis of all hull symbols since. The new system used a two-letter hull symbol which had a base designation for the ship type (first letter) and a suffix for the sub-type (second-letter), followed by a hull number. The new system largely borrowed the designations used under the old system, but given that single-letter hull symbols were no longer allowed, meant that main ship types like battleships and destroyers used the base designation letter twice; this became the origin of the now-famous designations of BB (battleship), DD (destroyer), and SS (submarine). As for numbering sequence, most major combat ship types of the same base designation remained within their number sequence; for example, heavy (CA) and light cruisers (CL) all followed the same cruiser sequence. In contrast, amphibious, patrol, auxiliary, mine craft, and yard craft typically had numbering sequences for each sub-type given that hundreds of these smaller ships are frequently built. Except for certain exceptions described in the next section, the numbering sequence remains uninterrupted to this day. And unlike Royal Navy/NATO practice, no hull number is ever recycled.

Cruisers, which by then already came in various sub-types, were representative of this new system, with former armored cruisers being designated CA and scout cruisers CS. Notably, a new type of ship that became prominent in the 1920s, the aircraft carrier, was given the CV designation as they were orginally considered aircraft-carrying cruisers; the V being standard US Navy designation for "heavier-than-air" craft. As an even wider variety of ships came into service, three-letter and even four-letter hull symbols also came into use soon thereafter, particularly when certain sub-types (like CVs) became main ship types in their own right. Confusingly, the USN also frequently changed prefixes even in short spaces of time, paricularly in the case of new ship types whose role was not firmly established when they were first conceived during wartime. A fine example of this were escort carriers, the first of which were given the designation AVG (Auxiliary Aircraft Ferry) in 1941, then switched to ACV (Auxiliary Aircraft Carrier) in 1942, and finally the definitive CVE (Aircraft Carrier, Escort) in 1943. An illustrative ship during this whole process was USS Card which went through the following designations throughout its nearly 30-year US Navy and Military Sea Transportation Service history: AVG-11 (when ordered), ACV-11 (when built), CVE-11 (1943), CVHE-11 (reclassified as helicopter escort carrier in 1955), T-CVU-11 (utility aircraft carrier in MSTS service from 1958), and T-AKV-40 (cargo ship and aircraft ferry from 1959).

Although hull symbols have changed since (particularly after the 1975 reclassificaton explained below), the base designation prefixes have remained the same since 1920, and in most cases the sub-type prefixes as well. Note that sub-type suffixes generally referred to more than one type of ship, which means that it is not always possible to determine a ship type on the basis of the hull symbol unless one knows the meaning of the hull symbol beforehand. Indeed there are so many exceptions and oddities in the system that it is impossible to list them all. It was also the case that base designations and suffixes were used interchangeably, such as with submarine oilers that were given AOSS and SSO designations at different times. In some cases the same ship type used two differnet hull symbols, such as World War II-era submarine chasers which were assigned PC or SC depending on size; the latter symbol being particularly odd since S is the submarine base designation and SC was also used for cruiser submarines during the pre-war period.

It was common for hull symbols to include more than one sub-type suffix though there has never been a hull symbol with more than four letters. A small number of suffixes are two-letter, such as AC (air cushion), and CM (countermeasures). In some cases, more specific roles for a particular ship type were specified through the use of parenthesis. This was very common with amphibious vessels which were often converted to other uses such as the LST(H) tank landing ship, evacuation (hospital) or the LSM(R) medium landing ship, rocket. Another oddity was the use of an undercase c for certain costal ships such as the AMb harbor and AMc coastal minesweeper. And in the sole case of a hull symbol named for a particular class of ships, the World War I-era Eagle Boats were designated PE.

The following tables list all base designations and the most common sub-type suffixes since 1920 for major combat ship types. Note that there are also two special prefixes in use: US Coast Guard ships before 1965 used US Navy hull symbols with a W prefix, thus a USCG cutter (patrol gunboat) would have a WPG hull symbol. Ships in service with the Military Sea Transportation Service (later Military Sealift Command) would have a T- prefix to their standard US Navy hull symbol (see case of the USS Card above).

The US Navy's hull classification system has been flexible enough to allow the introduction of new hull symbols and the disuse or retirement of others over time. However, a major reclassification took place on June 30th 1975 in response to the controversy over the US's perceived "cruiser gap" with the rapidly expanding Soviet Navy. After World War II, the US Navy reintroduced the pre-war concept of the destroyer leader (DL) and applied this term to large task force escorts that were of intermediate size between existing gun/missile cruisers and destroyers. After 1955 these ships were reclassifed as frigates, which went against the standard use of the term as an anti-submarine escort that was smaller than a destroyer; at the time the US Navy continued the World War II tradition of referring to these ships as destroyer escorts (DE). The result was that by 1975 (when most of the old gun cruisers had been decommissioned) the US Navy only had a handful of ships that were officially classified as "cruisers". In contrast, the Soviet Navy, which tended to build smaller ships of the same type as their US counterparts, had over three times the number.

Naval authorities responded to the political and media "cruiser gap" panic by implementing a major reclassification of the Navy's surface ships. Most of the the former destroyer leaders/frigates (Leahy-, Belknap-, and California-classes) were reclassified as cruisers bringing the total number of such ships to 27 (from just 6) in an instant. This revealed a clear advantage over the Soviet Navy's 19. The only exception was the smaller Farragut-class which were reclassified as guided missile destroyers. At the same time, the destroyer escorts were finally reclassifed as frigates, bringing this in line with practice among other NATO navies. There were an number of other changes such as the removal of the A (attack) suffix for carriers leaving them at CV (conventional) or CVN (nuclear powered). Since 1975 the cruiser definition has been used entirely as a function of capability rather than hull size. This is evidenced by the fact that the Ticonderoga-class is designated as such despite using the same hull as the Spruance-class destroyers and having been originally been designated guided missile destroyers when first ordered.

Although there has not been another reclassification on the scale of 1975, the hull classification system continues bringing up new oddities. For starters, the 1975 changes interrupted the new numbering sequences that had been established in the post-war period where guided missile cruiser, destroyers, and destroyer leaders had reverted to 1 in order to differentiate them from the gun platforms (which continued using the 1920 sequence). The DLG/DLGNs that converted to CG/CGNs required only a change in hull symbol but retained their numbers given that the last assigned cruiser number was CG-14 and the lowest destroyer leader number was DLG-16 (thus, CG-15 was skipped). This was not possible with the Farragut-class ships which were asigned DLG-6 to DLG-15, numbers that had already been taken by existing DDGs. Upon reclassification, their numbers changed to DDG-37 to DDG-46. The Ticonderoga-class was meant to continue the sequence at DDG-47 but upon reclassification as cruisers in 1980 these became CG-47 ownard, resulting in the skipping of numbers 44-46 since the previous Virginia-class cruisers ended in the (unbuilt) CGN-43.

Even more arbitrary numbering followed. The Zumwalt-class began with DDG-1000, which would appear to revert to the 1920 destroyer sequence which ended at DD-997 (the last Spruance-class ship), thus skipping two numbers. Another arbitrary sequence change was the Seawolf-class submarines which began with SSN-21 ("21st century"), the first submarine to break the 1920 sequence. It also arguably violates the no recycling rule since an SS-21 already existed. The subsequent Virginia-class boats, however, reverted to the 1920 sequence beginnig with SSN-774. Earlier, the first hunter-killer submarines were briefly designated SSK-1 to SSK-3 but later switched to the 1920 sequence.

A further post-1975 quirk is the introduction of hull symbols that do not follow any base designation/ship-type suffix and instead are acronyms of their ship type. The most obvious example of this practice are the littoral combat ships of the Freedom- and Independence-classes which are designtaed LCS. Their numbering sequence is also unique, with each class getting only odd and even numbers respectively. This could create gaps in the event that more ships of one class are built (construction of both is ongoing). Acronym hull symbols are more common among support craft such as the Advance Base Section Dock (ABSD) or Maritime Prepositioning Force Utility Boat (MPFUB).

The following is a comprehensive list of all hull symbols used since 1920. Ship types in bold are those currently in use as of 2016 (SECNAV Instruction 5030.8C).

Until World War II, the US Coast Guard did not use a hull classification system for its larger, oceangoing ships which have traditionally been known in the service as "cutters" regardless of their size and armament (an exception was the lease of 25 US Navy destroyers during the prohibition era in the 1920s-30s that were given the CG hull symbol). This changed in February 1942 when it adopted the US Navy's hull classification system, albeit with a W prefix indicating that the vessel was in USCG use. On May 1st, 1965 the USCG adopted its own unique hull classification system which maintained the tradition of a W prefix but which otherwise referred to a ship's endurance at sea. The USCG hull designation system is largely based on acronyms and new ship descriptions frequently result in new hull symbols even when the ship's role is indentical to an earlier type. So for example, the new "national (or maritime) security cutters" of the Legend-class (WMSC) are indentical in role to the "high endurance cutters" of the Hamilton-class (WHEC) that they are replacing. Not all new cutter types get their own hull symbol: the modern Sentinel-class "fast response cutters" retain the WPB hull symbol of normal patrol boats.