In the Royal Navy and other navies of Europe and the Commonwealth of Nations, ships are identified by pennant number (an internationalisation of pendant number, which it was called before 1948). Historically, naval ships flew a flag that identified a flotilla or type of vessel. For example, the Royal Navy used a red burgee for torpedo boats and a pennant with an H for torpedo boat destroyers. Adding a number to the type-identifying flag uniquely identified each ship.
In the current system, a letter prefix, called a flag superior, identifies the type of ship, and numerical suffix, called a flag inferior, uniquely identifies an individual ship. Not all pennant numbers have a flag superior.
The Royal Navy first used pendants to distinguish its ships in 1661 with a proclamation that all of his majesty's ships must fly a union pendant. This distinction was further strengthened by a proclamation in 1674 which forbade merchant vessels from flying any pendants.
The system of numbering pendants was adopted prior to the First World War to distinguish between ships with the same or similar names, to reduce the size and improve the security of communications, and to assist recognition when ships of the same class are together. Traditionally, a pendant number was reported with a full stop "." between the flag superior or inferior and the number, although this practice has gradually been dropped, and inter-war photos after about 1924 tend not to have the full stop painted on the hull. The system was used throughout the navies of the British Empire so that a ship could be transferred from one navy to another without changing its pendant number.
Pennant numbers were originally allocated by individual naval stations and when a ship changed station it would be allocated a new number. The Admiralty took the situation in hand and first compiled a "Naval Pendant List" in 1910, with ships grouped under the distinguishing flag of their type. In addition, ships of the 2nd and 3rd (i.e. reserve) fleets had a second flag superior distinguishing from which naval depot they were manned; "C" for Chatham, "D" for Devonport, "N" for Nore and "P" for Portsmouth. Destroyers were initially allocated the flag superior "H", but as this covered only one hundred possible combinations from H00 to H99 the letters "G" and "D" were also allocated. When ships were sunk, their pendant numbers were reissued to new ships.
The flag superior for whole ship classes has often been changed while the numbers stayed the same. For example, in 1940, the Royal Navy swapped the letters "I" and "D" around (e.g. D18 became I18 and I18 became D18) and in 1948, "K", "L" and "U" all became "F"; where there was a conflict, a 2 was added to the front of the pendant number.
During the 1970s, the service stopped painting pennant numbers on submarines on the grounds that, with the arrival of nuclear boats, they spent too little time on the surface, although submarines do continue to be issued numbers.
HMS Lancaster was initially allocated the pennant number F232, until it was realised that in the Royal Navy, form number 232 is the official report for ships that have run aground; sailors being superstitious, it was quickly changed to F229.
Pendant number 13 was not allocated.
Pendant numbers 13 were not allocated to flag superiors. The letters J and K were used with three number combinations due to the number of vessels.
Flag inferiors were applied to submarines. Royal Navy submarines of the "H" and "L", and some transferred American vessels, were not issued names, only numbers. In these cases, the pendant number was simply the hull number inverted (i.e. L24 was issued pendant "24L"). Pre-war photos show the pendants painted correctly, with the flag inferior, but wartime photos show that the numbers tend to be painted "backwards", in that the inferior was painted on as a superior. For obvious reasons, the inferior "U" was not used so as not to confuse friendly ships with German U-boats. For similar reasons "V" was not used. Pendant numbers 00–10, 13, and those ending in a zero were not allocated to flag inferiors.
After the Second World War, in 1948, the Royal Navy adopted a rationalised "pennant" number system where the flag superior indicated the basic type of ship as follows. "F" and "A" use two or three digits, "L" and "P" up to four. Again, pennant 13 is not used (for instance the current Ocean (L12) is followed by Albion (L14)).
From 1925, flotilla leaders were issued with but did not paint on pendant numbers. Instead, a broad band 4 feet (1.2 m) deep was painted round their fore-funnel. Divisional leaders wore a pendant number and had a narrower 2 feet (0.61 m) deep band on the fore-funnel, painted 3 feet (0.91 m) from the top. The Mediterranean Fleet wore black leader bands and the Atlantic – later Home Fleet wore white bands. The flotillas wore combinations of bands on their after funnel to identify them. From 1925 the following bands were worn;
When single funnelled destroyers entered the fleet with the J class in 1939 and with an expansion in the number of flotillas, the system was changed accordingly. Single funnelled ships wore a 3 feet (0.91 m) deep band as a flotilla leader. As a divisional leader they had a 2 feet (0.61 m) wide vertical band the same colour as, and extending 6 feet (1.8 m) below, the upper flotilla band. Leaders bands were white for Home Fleet, red for Mediterranean Fleet, and the system of flotilla bands changed to;
Flotilla bands were used throughout the war although war-losses, operational requirements, and new construction broke up the homogeneity of the destroyer flotillas. Vessels were deployed as and when they were needed or available, and were often incorporated into mixed "escort groups" containing a range of vessel types such as sloops, corvettes, frigates and escort carriers. A few of the escort groups adopted funnel bands; others (like the B7 escort group) wore letters on their funnels.
Post-war Flotillas were no longer identified by bands, but by large cast metal numbers bolted to the funnels. Flotilla leaders continued to display a large band at the top of the funnel and half leaders would carry a thin black band around the funnel.
Aircraft carriers and vessels operating aircraft have a deck code painted on the flight deck to aid identification by aircraft attempting to land. This is in a position clearly visible on the approach path. The Royal Navy uses a single letter (typically the first letter of the ship's name) for aircraft carriers and large vessels operating aircraft, and pairs of letters (usually letters from the ship's name) for smaller vessels. The United States Navy, with its larger fleet, uses the numeric part of the hull classification number (a system analogous to pennant numbers). Deck codes used by contemporary major British naval warships include:
Several European NATO and Commonwealth navies agreed to introduce a pennant number system based on that of the Royal Navy. The system guarantees that, amongst those navies and other navies that later joined, all pennant numbers are unique. The United States does not participate in this system; its ships are identified by unique hull classification symbols.
The NATO pennant number system added the Y (for yard) symbol for tugboats, floating cranes, docks and the like.
The Royal Navy uses a single letter (typically the first letter of the ship's name) for aircraft carriers and large vessels operating aircraft, and pairs of letters (usually, letters from the ship's name) for smaller vessel.
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