Warships in the United States Navy were first designated and numbered in a system originating in 1895.  Under this system, ships were designated as "Battleship X", "Cruiser X", "Destroyer X", "Torpedo Boat X" and so forth where X was the series hull number as authorized by the US Congress.  These designations were usually abbreviated as "B-1", "C-1", "D-1", "TB-1," etc.  This system became cumbersome by 1920, as many new ship types had been developed during World War I that needed new categories assigned, especially in the Auxiliary ship area.  On 17 July 1920, Acting Secretary of the Navy Robert E. Coontz approved a standardized system of alpha-numeric symbols to identify ship types such that all ships were now designated with a two letter code and a hull number, with the first letter being the ship type and the second letter being the sub-type.  For example, the destroyer tender USS Melville, first commissioned as "Destroyer Tender No. 2" in 1915, was now re-designated as "AD-2" with the "A" standing for Auxiliary, the "D" for Destroyer (Tender) and the "2" meaning the second ship in that series.  Ship types that did not have a subclassification simply repeated the first letter.  So, Battleships became "BB-X" and Destroyers became "DD-X" with X being the same number as previously assigned.  Ships that changed classifications were given new hull numbers within their new designation series.

The designation "USS" standing for "United States Ship" was adopted in 1907.  Prior to that time, no designation was used in official documents.  New-construction ships not yet in commission are currently prefixed with "PCU" which stands for "Pre-Commissioning Unit."

It should be noted that in the United States Navy, unlike European Navies, the first ship in a class to be authorized by the US Congress is the designated class leader (class name ship), regardless of the order in which the ships of that class are laid down, launched or commissioned.  For example, contrary to many European texts, for the last class of "Standard" battleships, the battleship USS Colorado BB-45 (commissioned 30 August 1923) is the class leader under USN designation standards, not USS Maryland BB-46 (commissioned 21 July 1921).  These battleships are thus properly designated as being "USS Colorado BB-45 Class" and not as "USS Maryland BB-46 Class."

Please note that the listings below include many designations that are no longer in use by the current-day US Navy and that others were proposed designations not actually used or were intended for ships that were never built.  Designations highlighted in Blue Font are for those ships actually in commission or currently under construction as of the present time (2009).
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Aircraft Carriers
 

ACV	Aircraft Carrier, Auxiliary
AVG	Escort Carrier, Auxiliary
AVT	Aircraft Carrier, Training Ship
CV	Aircraft Carrier
CVA	Aircraft Carrier, Attack
CVAN	Aircraft Carrier, Attack, Nuclear Powered
CVB	Aircraft Carrier, Large
CVE	Aircraft Carrier, Escort
CVGH	Aircraft Carrier, Guided Missile Helicopter
CVH	Aircraft Carrier, Helicopter
CVHE	Aircraft Carrier, Escort, Helicopter
CVL	Aircraft Carrier, Light
CVN	Aircraft Carrier, Nuclear Powered
CVS	Prior to 1957:  Seaplane Carrier
CVS	After 1957:  Anti-Submarine/Support Carrier
CVV	Aircraft Carrier, Vertical Take Off And Landing

Notes:

The ex-collier USS Langley AC-3 was rebuilt and recommissioned as the first US aircraft carrier CV-1 on 20 March 1922.  The CVB designation was approved by the Secretary of the Navy on 10 June 1943 and the CVL designation was approved on 15 July 1943.  The CVS designation was established in 1953 with USS Bunker Hill (CVS-17) being the first so designated on 8 August 1953, although she was in reserve at the time and never did recommission.  The designation CVA replaced both CV and CVB on 1 October 1952.  CVL went out of use on 15 May 1959 when the last light carrier was decommissioned.  With the decommissioning of the last CVS in 1974, CV and CVN replaced CVA and CVAN on 30 June 1975 as carriers are now considered to be multi-mission capable rather than specialized.

Escort Aircraft Carriers were originally designated AVG (Escort Carrier, Auxiliary) on 31 March 1941, with the USS Long Beach AVG-1 being the first ship so commissioned on 2 June 1941.  This designation was changed on 20 August 1942 to ACV (Aircraft Carrier, Auxiliary), and then changed again on 15 July 1943 to CVE.  Escort Carriers built for the British Royal Navy were designated BAVG until they were transferred.  The CVE designation went out of use when the remaining escort carriers were reclassified AKV (Auxiliary, Aircraft Ferry) on 7 May 1959.

A common question is "what does the 'V' stand for in CV or CVA or CVS or CVE?"

[Thanks to C. Bossie who provided much of the following answer.]

The following is taken from "United States Naval Aviation 1910-1995, Appendix 16:  US Navy and Marine Corps Squadron Designations and Abbreviations":

On 17 July 1920, the Secretary of the Navy prescribed a standard nomenclature for types and classes of NAVAL VESSELs, including aircraft, in which lighter-than air craft were identified by the type "Z" and heavier-than air craft by the letter "V".  The reference also speculates that:  "The use of the "V" designation has been a question since the 1920s.  However, no conclusive evidence has been found to identify why the letter "V" was chosen.  It is generally believed the "V" was in reference to the French word volplane.  As a verb, the word means to glide or soar. As a noun, it described an aeronautical device sustained in the air by lifting devices (wings), as opposed to the bag of gas that the airships (denoted by "Z") used.  The same case may be regarding the use of "Z".  It is generally believed the "Z" was used in deference to Count Ferdinand von Zeppelin.  However, documentation has not been located to verify this assumption."

Battleships
 

B	Battleship (prior to 1920)
BB	Battleship (after 1920)
BBC	Battleship, Command Ship
BBG	Battleship, Missile
BBH	Battleship, Helicopter
M	Monitor (prior to 1920)
BM	Monitor (after 1920)

Notes:

The early pre-dreadnoughts Maine and Texas were commissioned as "Second Class Battleships" but apparently did not receive hull series numbers.  Maine was originally designated as Armored Cruiser #1 (ACR-1) but was reclassified during construction.

Many pre-dreadnoughts were colloquially known as "Coastal Defense Ships" as they were not designed nor intended to fight far from home.  Some of these were officially renamed as "Coast Battleship #X" (with "X" being their hull number) in March - April 1919 in order to free up their names for new construction.  Surviving pre-dreadnoughts were reclassified as "Battleships" and given the BB designation in the 17 July 1920 assignment, although many of these were then rapidly scrapped under the terms of the Washington Naval Limitation Treaty.

The "BBC" "BBG" and "BBH" designations were all for proposed conversions of Iowa class (BB-61) battleships.  The first two were developed during the 1950s while the last one was planned during the 1980s commissions.  None of these conversions ever took place.

Monitors were coastal defense ships of the 1890s, obsolete even when commissioned.
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Cruisers
 

ACR	Armored Cruiser (prior to 1921)
C	Cruiser (prior to 1920)
CA	Cruiser, First Line (1920 to 1921)
CA	Armored Cruiser (1921 to 1931)
CA	Heavy Cruiser - Cruiser armed with guns 8" (20.3 cm) or larger (after 1931)
CAG	Guided Missile Heavy Cruiser - Heavy cruiser converted to carry missiles
CB	Large Cruiser
CBC	Large Command Ship
CC	Battlecruiser (prior to 1961)
CC	Command Cruiser or Command Ship (after 1961)
CF	Flight-deck Cruiser
CG	Guided Missile Cruiser
CGH	Guided Missile Cruiser with an assigned Helicopter
CGN	Guided Missile Cruiser, Nuclear Powered
CH	Cruiser with an assigned Helicopter
CL	Light Cruiser
CLAA	Anti-Aircraft Light Cruiser
CLC	Command Light Cruiser
CLD	Light Cruiser, Dual-Purpose (AA and ASu)
CLG	Guided Missile Light Cruiser - Light cruiser converted to carry missiles
CLGN	Guided Missile Light Cruiser, Nuclear Powered
CLH	Helicopter Light Cruiser - Cruiser with an assigned Helicopter
CLK	Light Cruiser, ASW (submarine Killer)
CLV	Aviation Cruiser
CS	Scout Cruiser
CSG	Strike Missile Cruiser
CSGN	Strike Missile Cruiser, Nuclear Powered

Notes:

The history of cruiser classification is somewhat complicated.  Initially, there were three major classifications:  Protected Cruisers, Peace Cruisers, Third Class Cruisers and Unprotected Cruisers were all designated "C-X", Armored Cruisers were designated "ACR-X" and Scout Cruisers were designated "CS-X" where "X" was the hull series number.  There were also older and non-classified cruisers which had not been assigned hull numbers.  As part of the 20 July 1920 redesignation mentioned above, there were the following four major changes:

• The Scout cruisers of the Salem and Omaha classes were merged into a single new category, Light Cruisers, and were given the hull series numbers CL-1 to CL-13.
• Armored Cruisers formerly designated as "ACR-X" were redesignated as "CA-X" and used the same hull number as previously assigned.
• All un-numbered cruisers were now classified into one of the above categories and received hull series numbers accordingly.
• The newer Peace Cruisers were removed from the cruiser category entirely and were now reclassified as gunboats PG-27 through PG-34 and PG-36.

However, the Navy was not satisfied with this reclassification, and so there was a second reclassification on 8 August 1921.  On this date, Gunboats PG-28 to PG-34 and PG-36 were again classified as cruisers, this time as light cruisers CL-16 to CL-23, which left only USS Marblehead (PG-27, formerly C-11), still classified as a Gunboat.  Also on this date, USS Olympia (CA-15) was reclassified as a light cruiser (CL-15) and it was intended for the old cruiser USS Chicago (CA-14)  to be reclassified as a light cruiser (CL-14).  Thus, these ships changed classification, but retained their previous hull numbers.  Chicago was not actually redesignated and instead she was decommissioned in 1923 and became the uncommissioned barracks ship IX-5 at Pearl Harbor.

This classification system was used throughout the 1920s with the new 10,000 ton "Treaty" cruisers built during that time being commissioned into the CL hull number series, starting with USS Pensacola (CL-24).  Then, as a result of the provisions of the London Conference of 1930, cruisers were split into two categories, heavy and light.  The main differentiator was the size of the guns carried, with those cruisers having guns of 8" (20.3 cm) now being designated as "Heavy Cruisers" and those cruisers with smaller gun calibers being designated as "Light Cruisers."  On 1 July 1931, this new classification was officially adopted by the USN and the meaning of the "CA" designation was changed from "Armored Cruiser" to "Heavy Cruiser."  As a result of this reclassification, the "Treaty" cruisers USS Pensacola CL-24 through USS Portland CL-33 were redesignated as CA-24 through CA-33.  Thus, these ships retained their previous Light Cruiser hull series numbers, even though the last Armored Cruiser, USS Charleston, was CA-19.  From this time forward, all heavy and light cruisers built or planned between 1931 and 1949 used a single series of hull numbers, with the hull numbers interleaved between classes.  This interleaving created somewhat of a jumble in the sequence of hull numbers in the latter part of World War II, when the light cruisers of the Juneau (ii), Cleveland, Fargo and Worcester classes and the heavy cruisers of the Baltimore, Oregon City and Des Moines classes were all being built simultaneously.  The nuclear powered missile cruiser USS Long Beach, originally designated as CLGN-160, was the last ship to be numbered under this system, although she did not commission with this hull number, as noted below.

The Lexington class were the only battlecruisers ever laid down by the USN and were assigned hull series numbers CC-1 to CC-6.  When construction was suspended following the Washington Naval Limitation Treaty of 1922, the first two of these ships, USS Lexington CV-2 and USS Saratoga CV-3, were completed as Aircraft Carriers.  The Alaska class of the 1940s were designated as Large Cruisers and assigned hull series numbers CB-1 to CB-6.  The USN did not considered these ships to be battlecruisers.

In the late 1940s and 1950s, four new cruiser classification groups were created:

• Anti-aircraft Cruisers - On 18 March 1948 anti-aircraft cruisers of the USS Atlanta CL-51 class were redesignated as CLAA, but retained their previous cruiser series hull numbers.  An improved anti-aircraft cruiser class was cancelled in the 1940s and no other ships were ever given the CLAA classification.
• ASW Cruisers - A planned class of Submarine Hunter-Killer Cruisers were designated CLK and numbered in a separate series, but only one ship of this type, USS Norfolk CLK-1, was actually laid down and she was redesignated as destroyer leader DL-1 on 2 February 1951 prior to her commissioning on 4 March 1953.
• Missile Cruisers - Two Baltimore class (CA-68) Heavy Cruisers and six Cleveland class (CL-55) Light Cruisers were converted into Guided Missile Cruisers in the late 1950s and early 1960s.  These missile ships kept some of their large-caliber guns and started a new numeric series, but they retained their previous sub-type designations as either CAG-X (Heavy Missile Cruiser) or CLG-X (Light Missile Cruiser).  The first of these conversions was USS Boston (CA-69) which was redesignated as CAG-1 on 4 January 1952, but she was not actually in commission as a missile cruiser until 1 November 1955.  Three other Baltimore and Oregon City class heavy cruisers had all of their former gun armament and superstructures removed and were more heavily modified into all-missile cruisers.  These three cruisers formed a single class (Albany) and were designated as CG-10 to CG-12.  Two other planned conversions were cancelled and their hull numbers (13 and 14) were not reassigned.  The nuclear powered missile cruiser USS Long Beach, originally designated as CLGN-160 and later as CGN-160, finally commissioned as CGN-9 on 9 September 1961.
• Command Cruisers - Command Cruisers (later, Command Ships) had little armament but were equipped as fleet flagships with a great deal of command, control and communications facilities.  USS Northampton (ex-CA-125) was launched as CLC-1 on 27 January 1951 and later redesignated as CC-1 on 15 April 1961.  As noted above, prior to this time CC had been the designation for Battlecruiser.  The light carrier USS Wright (CVL-47) was designated as CC-2 on 15 March 1962 and then converted into a Command Ship, recommissioning on 11 May 1963.  It was planned to convert the Large Cruiser USS Hawaii (CB-3) into a large command cruiser and she was so redesignated as CBC-1 on 26 February 1952, but this conversion was later cancelled and she reverted back to her original designation on 9 October 1954.

The designations "Cruiser," "Destroyer" and "Frigate" were a source of some confusion in the USN during the 1970s.  Existing US carrier escorts were designated as Frigates (DLG and DLGN), but foreign warships of the size of US Frigates were generally classified as destroyers or cruisers.  The USN was also applying the term "Patrol Frigate" or "PF" to the much smaller USS Oliver Hazard Perry class of convoy escorts then being designed.  This usage was more in line with the "Frigate" designation as used in foreign navies, where it was usually given only to smaller warships of about the same size as the Perrys.  On 30 June 1975, this issue was partially resolved when the following reclassifications were made:

• Most DLG and DLGN Frigates were redesignated as Guided Missile Cruisers CG and CGN, with these warships retaining their original hull numbers (CG/CGN-16 through 40).  This reclassification resulted in hull number CG-15 not being assigned.  In addition, USS Arkansas, originally planned as DLGN-41, was commissioned on 18 October 1980 as CGN-41.  An unnamed Virginia class warship, CGN-42, was cancelled in 1976, then redesigned and but again cancelled in 1978, 1979 and finally in 1983.
• Due to their smaller size, the Farragut class missile frigates (DLG-6 through DLG-15) were reclassified as Guided Missile Destroyers and given new hull numbers in that series (DDG-37 through DDG-46).
• The remaining CLG cruisers that were still active or in reserve were redesignated as CG and retained their existing hull numbers.
• Existing Guided Missile Escorts (DEG-1 to DEG-6) were redesignated as Guided Missile Frigates (FFG-1 to FFG-6) and those Escorts (DE) still in commission were redesignated as Frigates (FF) and retained their existing hull numbers.  USS Oliver Hazard Perry herself was commissioned on 17 December 1977 as FFG-7.

Destroyers, Escorts and Frigates
 

BDE	Destroyer Escorts to be transferred to Britain during World War II
D	Destroyer (prior to 1921)
DD	Destroyer (after 1921)
DE	Destroyer Escort, Escort or "Ocean Escort"
DEG	Destroyer Escort, Guided Missile
DER	Destroyer Escort, Radar Picket
DDE	Destroyer converted to Fleet Escort
DDG	Guided Missile Destroyer
DDH	Destroyer with an assigned Helicopter
DDK	Destroyer, ASW (Submarine Killer)
DDR	Destroyer, Radar Picket
DL	Destroyer Leader (1920 to 1955)
DL	Frigate (after 1955)
DLG	Frigate, Guided Missile
DLGN	Frigate, Guided Missile, Nuclear Powered
EDD	Destroyer, Experimental Test Ship (before 2005)
EDD	Destroyer, Self-Defense Test Ship (after 2005)
EDDE	Experimental Escort Destroyer
FF	Frigate
FFG	Guided Missile Frigate
FFH	Frigate with an assigned Helicopter
FFT	Reserve/Training Frigate
PE	Patrol Escort
PF	Frigate or Patrol Frigate
PFG	Patrol Frigate, Guided Missile
PFR	Radar Picket Frigate
TB	Torpedo Boat

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Notes:

Following World War II, many Fletcher (DD-445), Allen M. Sumner (DD-692) and Gearing (DD-710) class destroyers were modified and given new, specialized missions as Fleet Escorts (DDE), radar pickets (DDR) and ASW submarine killers (DDK).  The ASW destroyers (DDK) were redesignated as Fleet Escort destroyers (DDE) on 4 March 1950.  All Fleet Escort destroyers (DDE) were reclassified as destroyers (DD) on 30 June 1962.  Most radar picket destroyers (DDR) were reclassified as destroyers (DD) during the 1960s.  One Fleet Escort destroyer USS Saufley (DDE-465) was reclassified as an Experimental Escort Destroyer (EDDE) on 1 January 1951 and then used as a test bed during the 1950s for sonar and ASW experiments.  She was reclassified as a general purpose destroyer (DD-465) on 1 July 1962.

USS Gyatt (DD-712) was modified to carry a twin Terrier launcher and she was designated as DDG-712 on 1 December 1956.  She was redesignated as DDG-1 on 23 May 1957.  Her missile emplacement was later removed and her designation reverted back to DD-712 on 1 October 1962.

The "DL" designation meaning "Destroyer Leader" was originally created as part of the 17 July 1920 designation system revision.  This designation was not assigned to any ship prior to 2 February 1951 when the ASW cruiser USS Norfolk CLK-1 was redesignated as DL-1 as described above.  In addition, the new fleet destroyers of the Mitscher class, which were originally designated in the DD series as DD-927 to DD-930, were redesignated as DL-2 to DL-5.  This designation change was meant to indicate that these warships were significantly larger than those destroyers built during World War II yet still smaller than cruisers.  The DL designation was changed to mean "Frigate" on 1 January 1955.

The follow-on Farragut class frigates were originally to be an all-gun design and the first three ships of this class were designated as DL-6 to DL-8.  The next three ships starting with USS Coontz were to a similar design but were given a Terrier missile launcher and were designated accordingly as DLG-1 to DLG-3.  However, the Navy subsequently decided to equip the Farragut class with Terrier missile launchers and their designation was then changed to become DLG-6 to DLG-8 on 14 November 1956.  USS Coontz and her sisters were renumbered in sequence starting with DLG-9 on that same date.  As a result of these changes, all DL and DLG warships are in the same hull number series.

The Mitscher class frigates Mitscher DL-2 and John McCain DL-3 were converted to carry a Tartar missile launcher during the 1960s and they were then redesignated as missile destroyers DDG-35 and DDG-36, respectively.  As noted above in the cruiser section, on 30 June 1975 most Frigates were redesignated as CG and CGN with the same hull number as previously, the exception being the Farragut DLG class which were redesignated as missile destroyers DDG-37 through DDG-46.

The designation "DE" was originally assigned to Destroyer Escorts of World War II.  It is sometimes stated that "DE" meant "Ocean Escorts" for those convoy escorts of the 1950s-1970s starting with the USS Dealey (DE-1006) class and ending with the USS Knox (DE-1052) class, but this does not appear to have ever been made a formal designation.  The DANFS entry for USS Knox herself describes her as "the prototype in a new class of destroyer escorts" while the Naval Vessel Register (NVR) currently lists all DE-designated ships as simply "Escort" including those built during World War II.  As stated above, on 30 June 1975 existing Guided Missile Escorts (DEG) were redesignated as Guided Missile Frigates (FFG) and those Escorts (DE) still in commission were redesignated as Frigates (FF).

A number of Edsall (DE-129) class destroyer escorts were converted to Radar Pickets (DER) during the 1950s.  They carried that designation during the rest of their careers, although none of them appear to have been used in that role after about 1960.

The "PF" designation was originally used for World War II frigates of the Asheville (PF-1) and Tacoma (PF-3) classes.  In the 1970s, it was proposed to designate what became the Oliver Hazard Perry (FFG-7) class as PF which would have then meant "Patrol Frigates."

During the late 1960s and early 1970s, a series of design studies were made for the "next generation" of ASW and guided missile carrier escorts.  These warship designs were given various interim designations, such as "DX" for the conventionally powered ASW destroyers, "DXG" for conventionally powered guided missile destroyers, "DXGN" for nuclear powered guided missile destroyers and "DG/Aegis" for an "austere" Aegis missile destroyer.  These proposals coalesced around what eventually became the conventionally powered Spruance (DD-963) ASW destroyers and Ticonderoga (CG-47) Aegis missile cruisers.  The nuclear powered version, which had grown considerably in size and was now known as a "Strike Cruiser" or CSGN and then later as the "CGN-42" class, was cancelled in the fall of 1978 although it was resurrected a few times until being cancelled once and for all in 1983.

A similar series of interim designations is currently in progress for new destroyer and cruiser designs, with the "DD-21" and "CG-21" designations having been replaced with the "DD(X)" and "CG(X)" designations.  The first destroyer in the DD(X) series will be christened as USS Zumwalt DDG-1000.  This means that the DDG hull series will now be in the DD series, rather than the DDG series, and it appears likely that the unused hull numbers DD-998 and DD-999 will not be assigned to any ship.

As expected, the delay and ballooning costs of the Zumwalt class destroyers has resulted in additional orders for Arleigh Burke DDGs and now it is expected that advanced versions of this class will continue in production for at least the next decade.

The DD designation went out of service when USS Cushing DD-995 was decommissioned on 21 September 2005.
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Submarines
 

AGSS	Auxilary/Experimental Submarine
APS	Auxiliary Cargo Submarine
NSSN	New Nuclear Powered Attack Submarine (temporary designation)
S	Submarine (Attack/Fleet) - prior to 1920
SS	Submarine (Attack/Fleet) - after 1920
SC	Cruiser Submarine
SF	Fleet Submarine
SM	Submarine, Minelayer
SSA	Auxilary/Experimental Submarine
SSAG	Auxilary/Experimental Submarine
SSB	Submarine, Ballistic Missile
SSBN	Submarine, Ballistic Missile, Nuclear Powered
SSG	Attack Submarine, Guided Missile
SSGN	Attack Submarine, Guided Missile, Nuclear Powered
SSK	Hunter-Killer Submarine
SSM	Midget Submarine
SSN	Attack Submarine, Nuclear Powered
SSP	Transport Submarine
SSR	Radar Picket Submarine
SSRN	Radar Picket Submarine, Nuclear Powered
SST	Submarine, Target/Training

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Notes:

Early submarines were given the designation "Submarine Torpedo Boats" (later shortened to just "Submarine") and most were given fish names.  On 17 November 1911 these names were discontinued and in their place was substituted an alphanumeric designation system representing their class and sequence.  For example, USS Bonita (S-15) was renamed as USS C-4 (S-15), with C-4 meaning that she was the fourth submarine of the third class of USN submarines.  Newer classes of submarines generally followed the letters of the alphabet, with "S" being the last letter used during the building spree generated by World War I.  Following the reclassification of 1920, all of the older submarines changed their hull series number designation from S to SS, but new submarines built during the mid-1920s to 1931 period were separated into different hull number series, depending upon their assigned role.  Fleet Submarines (SF) were intended to scout ahead of the battle fleet, Cruiser Submarines (SC) were larger boats with heavy gun armament while the single Minelayer Submarine (SM) had special tubes for launching mines.  All of these newer submarines were given "V" alphanumeric names, although most of them were to differing designs.

This rather confusing practice of giving submarines both an alphanumeric name and an alphanumeric designation was continued until 1931.  In that and the following year, all of the submarines built since the mid-1920s were given fish names and redesignated into the SS hull number series.  The older submarines built during and immediately after World War I continued with their alphanumeric names (mainly the O, R and S classes).  New submarines built since 1931 have continued to be designated into the SS numeric series, with a few exceptions, as detailed below.

In a reprise of the alphanumeric naming of the 1911 to 1931 period, Training and Hunter-Killer submarines of the 1950s originally had only an alpha-numeric designation instead of a hull number.  Training submarine SST-1 was commissioned as "T-1" and SST-2 was "T-2."  On 15 May 1956 these training submarines were given fish names but they retained their SST designations and hull numbers throughout their service lives.  In a similar fashion, the "Hunter-Killer" ASW submarines SSK-1, SSK-2 and SSK-3 of that period were also given only alphanumeric names, "K-X" with the X being the same number as in their hull designations.  On 15 December 1955, these three boats were given standard fish names and on 15 August 1959 they were renumbered into the SS series.

The modern Seawolf class submarines were designated as SSN-21 to SSN-23, with this apparently meaning "21st century" submarine.  This designation seems to have been an aberration, as the follow-on Virginia class submarines continue the previous series, starting at SSN-774.

Some former Ohio (SSBN-726) class ballistic missile submarines were converted during the early 2000s into Tomahawk-carrying guided missile submarines (SSGN).
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Amphibious Forces
 

AGC	Amphibious Fleet Flagship
LC(FF)	Landing Craft, Flotilla Flagship
LCAC	Air Cushion Landing Craft
LCC	Amphibious Command Ship
LCI	Infantry Landing Craft
LCI(G)	Infantry Landing Craft, Gun
LCI(L)	Infantry Landing Craft, Large
LCI(R)	Infantry Landing Craft, Rocket
LCM	Mechanized Landing Craft
LCM/ML	Minelaying Mechanized Landing Craft
LCP	Personnel Landing Craft
LCP(L)	Personnel Landing Craft, Large
LCP(N)	Personnel Landing Craft, Nested
LCP(R)	Personnel Landing Craft, Ramp
LCR	Landing Craft, Rubber or Riverine
LCS	Support Landing Craft (prior to 2004)
LCS(L)	Support Landing Craft, Large (Rocket)
LCT	Tank Landing Craft
LCU	Utility Landing Craft
LCV	Vehicle Landing Craft
LCVP	Vehicle and Personnel Landing Craft
LCW	Wing-in-Ground-Effect Landing Craft
LFR	Inshore Fire Support Ship
LHA	Amphibious Assault Ship, General Purpose
LHD	Amphibious Assault Ship, Multi-purpose
LKA	Amphibious Cargo Ship
LPD	Amphibious Transport Dock Ship
LPH	Amphibious Assault Helicopter Carrier
LPR	Amphibious Transport, Small
LSD	Dock Landing Ship
LSM	Medium Landing Ship
LSM(R)	Medium Landing Ship, Rocket
LSS	Landing Support Ship
LSSL	Small Landing Support Ship
LST	Tank Landing Ship
LST(H)	Tank Landing Ship, Evacuation (Hospital)
LST/ML	Minelaying Tank Landing Ship
LSU	Utility Landing Ship
LSV	Vehicle Landing Ship

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Notes:

Most of the larger "Landing Craft" of World War II were later reclassified as "Landing Ships."
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Littoral Warfare Ships
 

FSF	Fast Sea Frame
LCS	Littoral Combat Ship (after 2004)
PAC	Air Cushion Patrol Craft
PB	Patrol Boat
PBL	Amphibious Assault Patrol Boat
PBM	Riverine Monitor
PC	Coastal Patrol Craft/Submarine Chaser (173 ft)
PCC	Submarine Chaser (Control)
PCE	Submarine Chaser (Escort)
PCEC	Escort (Control)
PCE(R)	Escort (Rescue)
PCI	Fast Coastal Interceptor
PCS	Submarine Chaser (136 ft)
PCSC	Submarine Chaser (Control)
PG	Patrol Gunboat or Corvette
PGM	Missile Gunboat
PGH	Helicopter Gunboat
PH	Hydrofoil Gunboat
PHM	Missile Hydrofoil
PHT	Hydrofoil Torpedo Boat
PL	High Endurance Cutters
PM	Medium Endurance Cutters
PR	Radar Picket Conversion
PSOC	Special Operations Patrol Craft
PT	Patrol Torpedo Boat
PTC	Patrol Motor Boat Submarine Chaser
PW	Wing-in-Ground-Effect Patrol Craft
PY	Patrol, former Yacht
PYe	Coastal Patrol, former Yacht
SC	Submarine Chaser (110 ft)
SCC	Submarine Chaser (Control)

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Notes:

The "PG" designation was given both to patrol gunboats built in the US during the first half of the twentieth century and to corvettes transferred from the Royal Navy under reverse lend-lease in 1942.

The "LCS" designation was originally assigned during World War II to "Landing Craft, Support."  In 2004, the USN reassigned this designation to the "Littoral Combat Ship."
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Mine Warfare Ships
 

ACM	Auxiliary Minelayer
AM	Minesweeper
AMc	Coastal Minesweeper
AMc(U)	Coastal Minesweeper (underwater locator)
CM	Minelayer
CMc	Coastal Minelayer
DM	Prior to 1955 - Light Minelayer (converted destroyer)
DM	After 1955 - Destroyer Minelayer (converted destroyer)
DMS	High Speed Minesweeper (converted destroyer)
MCAC	Air Cushion Mine Countermeasures Craft
MCD	Mine Countermeasures Drone
MCM	Mine Countermeasures Ship
MCS	Mine Countermeasures Support Ship
MCT	Mine Countermeasures Conversion Trainer
MHC	Coastal Minehunter
MHI	Inshore Minehunter
ML	Minelayer
MMD	Fast Minelayer (ex-DM destoyers)
MSA	Auxiliary Minesweeper
MSAC	Air Cushion Minesweeper
MSC	Coastal Minesweeper
MSCO	Coastal Minesweeper, Old
MSD	Drone Minesweeper
MSF	Steel Hull Fleet Minesweeper
MSH	Harbor Minesweeper
MSI	Inshore Minesweeper
MSO	Ocean Minesweeper
MSR	River Minesweeper
MSS	Special Device Minesweeper
YMS	Motor Minesweepers

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Notes:

Those Benson and Gleaves class destroyers converted to minesweepers (DMS) during World War II were reclassified as destroyers (DD) during 1954/1955 and quickly thereafter decommissioned.

Converted Allen M. Sumner Light Minelayers (DM) were redesignated as Destroyer Minelayers (DM) on 7 February 1955 and redesignated as Fast Minelayers (MMD) on 1 January 1969.
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Auxiliary Ships
 

AB	Crane Ship (later changed to ACS)
AC	Collier (coaling ship)
ACS	Crane Ship
AD	Destroyer Tender
ADC	Dry Cargo Replenishment Ship
ADG	Degaussing Vessel
AE	Ammunition Ship
AF	Refrigerated Stores Ship
AFDB	Large Floating Drydock
AFDL	Small Floating Drydock
AFDM	Medium Floating Drydock
AFS	Combat Stores Ship
AG	Auxiliary, Generic/General (miscellaneous) use
AGB	Icebreaker
AGD	Seagoing Dredger
AGDE	Experimental Destroyer Escort
AGDS	Deep Submergence Support Ship
AGER	Experimental Research Ship
AGF	Fleet Flagship
AGFF	Experimental Frigate
AGH	Helicopter Support Ship
AGI	Intelligence Gathering Ship
AGL	Lighthouse Tender
AGM	Missile Range Instrumentation Ship
AGMR	Communications Major Relay Ship
AGOR	Oceanographic Research Ship
AGOS	Ocean Surveillance Ship
AGP	Patrol Craft Tender
AGR	Radar Picket Ship (converted cargo ship)
AGS	Surveying Ship
AGSc	Surveying Ship, Coastal
AH	Hospital Ship
AHR	Riverine Hospital Ship
AK	Cargo Ship
AKA	Attack Cargo Ship
AKE	Dry Cargo Ship
AKL	Light Cargo Ship
AKN	Net Cargo Ship
AKR	Roll-on/Roll-off Cargo Ship
AKS	General Stores-issue Ship
AKV	Aircraft Cargo Ship or Ferry
AN	Net Laying (Net Tender) Ship
ANL	Net Laying (Net Tender) Ship
AO	Fleet Oiler
AOE	Fast Combat Support Ship
AOG	Gasoline Tanker
AOL	Small Oiler
AOR	Replenishment Oiler
AORL	Small Replenishment Oiler
AOT	Transport Oiler
AP	Personnel Transport Ship
APA	Attack Personnel Transport Ship
APB	Barracks Ship
APc	Coastal Transport
APD	Troop Transport (High Speed)
APH	Troop Transport, Hospital
APL	Floating Barracks
APM	Mechanized Artillery Transport
APR	Rescue Transport
APV	Aircraft Transport
AR	Repair Ship
ARB	Repair Ship, Battle Damage
ARC	Cable Ship
ARD	Repair Dock
ARDM	Medium Repair Dock
ARL	Repair Ship, Landing Craft
ARS	Salvage Ship
ARS(D)	Salvage Ship, Lifting Vessel
ARS(T)	Salvage Ship, Tender
ARV	Aircraft Repair Ship
ARVA	Aircraft Airframe Repair Ship
ARVE	Aircraft Engine Repair Ship
AS	Submarine Tender
ASE	Submarine Ammunition Ship
ASR	Submarine Rescue Vessel
ATA	Auxiliary Ocean Tug
ATGB	Large Icebreaking Tug
ATF	Fleet Tug
ATR	Rescue Tug
ATS	Salvage Tug
AV	Seaplane Tender
AVB	Aviation Logistics Support Ship
AVC	Catapult Lighter
AVD	Aviation Tender (Converted Destroyer)
AVP	Small Seaplane Tender
AVS	Aviation Supply Ship
AW	Distilling Ship
AWK	Water Tanker
AX	Training Vessel (Current meaning) Auxiliary Tender, Large (Former meaning)
AXS	Sail Training Vessel
AY	VIP Yacht (Current meaning) Auxiliary Tender, Small (Former meaning)
AZ	Aerostat Tender

Note:

Attack Cargo (AKA) and Attack Personnel Transport (APA) ships differ from Cargo (AK) and Personnel Transport (AP) ships by being specially equipped to partake in amphibious landings.  The AKA and APA designations were replaced by LKA and LPA as of 1 January 1969.
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Plan Orange Conversions
 

XAV	Seaplane Carrier, converted
XAVP	Small Seaplane Carrier, converted
XCV	Aircraft Carrier, converted

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Notes:

During the 1920s and 1930s Plan Orange included mobilization and conversion of various merchantmen and liners into auxiliary warships.  The number of suitable ships fluctuated throughout this period as older ships went to the breakers and new ships were completed.  As of April 1939, ships available for conversions would have added five "fleet" carriers (none capable of more than 24 knots), two seaplane tenders and eight small seaplane tenders to the rolls.  These plans were all abandoned in November 1940, mainly because the conversions would have tied up too much shipyard capacity for little real gain.
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Support Craft
 

DSRV	Deep Submergence Rescue Vehicle
DSV	Deep Submergence Vehicle
IX	Unclassified Miscellaneous
IXSS	Unclassified Miscellaneous Submarine
NR	Nuclear Powered Research Submersible
SDV	Swimmer Delivery Vehicle
SLWT	Side Loading Warping Tug
TWR	Torpedo Retriever
WLB	Seagoing Buoy Tender
WLM	Coastal Buoy Tender
WLI	Inland Buoy Tender
WLIC	Inland Construction Tender
WLR	River Buoy Tender
X-1	Midget Submarine (research vessel)
YAG	Miscellaneous Service Craft
YC	Lighter
YCF	Car Float
YCV	Aircraft Transportation Lighter
YD	Floating Crane
YDT	Diving Tender
YF	Covered Lighter
YFB	Ferry
YFN	Covered Lighter
YFNB	Large Covered Lighter
YFND	Dry Dock Companion Craft
YFNX	Special Purpose Lighter
YFP	Power Barge
YFRT	Range Tender
YFU	Harbor Utility Craft
YGN	Garbage Barge
YHLC	Salvage Lift Craft, Heavy
YLC	Salvage Lift Craft
YM	Dredge
YMLC	Salvage Lift Craft, Medium
YMN	Dredge
YNG	Gate Craft
YO	Oil Barge
YOG	Gasoline Barge
YOGN	Gasoline Barge
YON	Oil Barge
YOS	Oil Storage Barge
YP	Seamanship Training Craft
YPD	Pile Driver
YPR	Parasail Training Craft
YR	Repair Barge
YRB	Repair and Berthing Barge
YRBM	Repair, Berthing, and Messing Barge
YRDH	Hull Repair Barge
YRDM	Machinery Repair Barge
YRM	Missile Retriever
YRR	Radiological Repair Barge
YRST	Salvage Craft Tender
YRT	Torpedo Retriever
YSR	Sludge Removal Barge
YTB	Large Tug
YTL	Small Tug
YTM	Medium Tug
YTT	Torpedo Trials Craft
YW	Water Barge
YWN	Water Barge

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As part of the 17 July 1920 redesignation system, a series of "old ship" or "second line" designations was created in order to distinguish between front line ships and those that were past their prime.  As far as I have been able to determine, it would appear that these were not officially assigned to any ship.  This may have been a result of these old ships being scrapped as a result of the downsizing of the fleet following World War I and the adoption of the Washington Naval Limitation Treaty.
 

OBM	Monitor, Second Line
OCC	Battle Cruiser, Second Line
OCA	Cruiser, Second Line
OCL	Light Cruiser, Second Line
OCM	Minelayer, Second Line
OCV	Aircraft Carrier, Second Line
ODD	Destroyer, Second Line
OSS	Submarine, Second Line
OSF	Fleet Submarine, Second Line

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USS Constitution

The two hundred year-old USS Constitution is unique in that she is the only ship in the USN that does not have a classification other than her name.  The Secretary of the Navy, J. William Middendorf, II, ordered her reclassification from "IX-21" to "None" effective as of 1 September 1975.  She is now listed on the Naval Vessel Register (NVR) of active in-commission ships only by her name.
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