Hull Classifications - United States Navy

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US Navy

Having previously rambled on Ship Prefixes, I thought that running through hull classifications would be a good follow-on.

The services use hull classification symbols (also called hull codes or hull numbers) to identify their ship types and each individual ship within each type. 

I’ll be discussing the United States Navy in this particular article, then follow it up with another article covering the United States Coast Guard, National Oceanic and Atmospheric Administration (NOAA), United States Army and United States Air Force.

United States Navy

The Navy began to assign unique Naval Registry Identification Numbers to its ships in the 1890’s. The system was simple - 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), the armored cruiser Pennsylvania was USS Pennsylvania (Armored Cruiser No. 4), and so on.

Beginning in 1907 (the turn of the century seems to bring in many changes to classifications and nomenclature in the branches of the military), some ships also were referred to alternatively by single-letter or three-letter code – using the previous examples, 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), and USS Pennsylvania (Armored Cruiser No. 4) could be referred to as USS Pennsylvania (ACR-4).  This was not replacing the previous system, but rather these codes coexisted and were used interchangeably with the older system - until the modern system was instituted on 17 July 1920.

One of the driving factors for making the change was because of what happened during World War I – in order to meet the requirements of the war, the Navy acquired large numbers (several thousand) of privately owned and commercial ships and craft (basically anything from a 40-foot motor boat or 112-foot motor yacht [even a houseboat, believe it or not] up to merchant freighters and passenger liners) for use as patrol vessels, mine warfare vessels, and various other purposes, some of them with identical names. So, in an effort to keep track of them, the Navy began a registry and assigned unique identifying numbers to them, classifying these craft not by the kind of ship, but rather by what the assignment of the ship was to be.  Those craft 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 references identify the ID as Inland Defense). 

The SP/ID numbering sequence was unified and continuous, with no SP number repeated in the ID series.

This was a good thing, in that some of these ships and craft changed from an SP to an ID number (or vice-versa) during their careers, without their unique numbers themselves changing – depending on their assignment.  However, there were approximately 200 vessels that were acquired that never received an ID or SP number (some due to being acquired at the end of the war and no longer needed after the Armistice).

The Navy up and did away with section patrol numbers, "identification numbers", and the other numbering systems described above and instituted its modern hull classification system on 17 July 1920.  In the new system, all hull classification symbols are at least two letters and allows for a ship to have a unique designation.  For basic types of ships the symbol is the first letter of the type name, doubled  - with the exception of aircraft carriers being designated as “CV”.

So, the basics were / are…

Battleships – BB

Destroyers – DD

Frigates – FF

Submarines – SS

And, as mentioned, the exception to the “basic craft” rule was the Aircraft Carrier being CV.

Some craft, such as Cruisers listed below, had no “basic type” (CC is actually a Command Ship), but rather had several types –

CA - Heavy Cruiser

CB - Large Cruiser

CG – Guided Missile Cruiser

CL – Light Cruiser

CS – Scout Cruiser

One thing that remains the same across all classes – if the final letter of the hull classification is “N”, that indicates a Nuclear Powered Craft.

Up until the 1975 reclassification, there could be many variations within a ship type – in the Destroyer category, there were DD, DDE, DDG, DDK, DDR, DE, DEG, DER, DL, DLG and DLN (and I think I may have missed one or two…)

So, concentrating mostly on the first letter, these days one can generally determine the kind of ship it is:

A – Auxiliary (ships designed as support to combatant ships and other naval operations – these days, most are operating under Military Sealift Command)

B - Battleship

C – Cruiser (unless it is a CV, indicating Aircraft Carrier)

D – Destroyer

F – Frigate

J – Joint Service Craft (operated by the Navy and another branch of service)

L – Amphibious warfare – (ships having organic capability for amphibious assault)

M – Mine Warfare

P – Patrol

S – Submarine

Y – Yard Craft

And finally, IX is a “catch-all” category for Unclassified Miscellaneous Ship / Unit

There are some exceptions to this list, discussed a little later in the article.

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, the aircraft carrier Valley Forge saw several reclassifications through it’s career – beginning as a basic aircraft carrier (CV-45), then becoming an attack carrier (CVA-45), then an anti-submarine carrier (CVS-45), and finally being converted into an amphibious assault ship (LPH-8 for Landing Platform, Helicopter).

Names are generally not repeated with currently active craft, though names have been reused after a specific craft has been decommissioned or renamed.  Referring to “Valley Forge”, the first ship of that name was CV-37 (and was later renamed Princeton), the second ship was CV-45, and the third ship was CG-50.  Some names do come close, though – CVN-71 Theodore Roosevelt, and DDG-80 Roosevelt.

Hull numbers are assigned by classification. Duplication between, but not within, classifications is permitted. Meaning that the hull number of “1” could be assigned in any of the classes: CV-1 was USS Langley and BB-1 was USS Indiana.

As with any system, the symbols have changed a number of times – and as well, how a type of ship is defined.  For example, my first ship was USS Downes FF-1070, indicating it was a Frigate – but when it was originally commissioned, it was designated as a Destroyer Escort, DE-1070.  The hull number change was not based on any physical change to the ship or major repurposing, but rather the way the Navy defined a “Frigate”.  And DE’s weren’t the only ships reclassified – in 1975, the Navy’s reclassification of cruisers, frigates, and ocean escorts brought U.S. Navy classifications into line with other nations' classifications and eliminated several classes.

Some exceptions and anomalies in the system

Veering from the general list above …

Under “D”, there is Deep Submergence Vehicles – DSV & DSVR.

Under “F”, there is / was a testing ship designated as FSF-1 for Fast Sea Frame, an experimental littoral combat ship under development from 2005-2008. 

Under “L”, the littoral combat ship is designated as “LCS”  [note:  to be reclassified according to USNI Article SNA: Modified Littoral Combat Ship Class Name to be ‘Fast Frigate’

Under “S”, there is SSC, for Ship to Shore Connector, intended as the replacement for the existing fleet of Landing Craft, Air Cushion (LCAC) vehicles.  There is also a testing ship designated as SDTS for Self Defense Test Ship(, a refurbished ship, operated by remote control, which is designed to support self-defense engineering, testing, and evaluation), and there was also SES for Surface Effect Ship.

As well, there have been one or two very unique hull numbers – the specific example that comes to mind is the G-class submarine G-1 (named Seal when laid down for construction) which held the hull number of SS-19½ (and the Dictionary of American Naval Fighting Ships (DANFS) entry doesn’t explain why “½”). And – even though the system was designed not to repeat hull numbers - in 1920, G-1 was re-designated as SS-20 even though that hull classification symbol and number had already been used – the avoidance of confusion was that the original SS-20 F-1 had sunk in a collision with F-3 in 1917, so there was no overlap in time of service for that hull number. 

Another exception to the system is USS Constitution – in honor her unique historical status, her former classification of IX-21 was removed and she was reclassified as having no hull number, effective 1 September 1975.

Military Sealift Command

If a Navy ship's hull classification symbol begins with "T-", then it is part of the Military Sealift Command, and 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) with an all-military crew.  Otherwise, the hull number classification follows standard Navy form.  For example, Hospital Ships are T-AH, Oilers are T-AO – again with some exceptions:  while operated by MSC, Submarine Tenders “AS” are still listed without the “T-“ lead.

Hull classification codes for Naval ships in active duty in the United States Navy are governed by Secretary of the Navy Instruction 5030.8A (SECNAVINST 5030.8A).  Ship types and classifications have come and gone over the years, and many of symbols are no longer in use - the Naval Vessel Register (NVR) maintains a list of Ship Classifications with current as well as Inactive Ship Classifications. 

The Navy’s “Our Ships” identifies current ship categories, with links to fact files (many links included in this article) of each type.

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