Pennants and Flag Superiors and Classes and Types
This subject seems designed to cause a headache! By 1914 Royal Navy ships were identified by Pennants, flags flown to identify themselves, this comprised a letter which was termed the "Flag Superior" since it flew uppermost, followed by a number. Smaller ships had their Pennant painted on the hull.
Unfortunately Pennant numbers kept changing, perhaps to confuse the enemy or more likely because administrators love swapping filing systems around.
Flag Superiors do not correspond to Class letters, except by coincidence. Destroyers in particular from the First World War tended to be classed by letter for each "run" of building, the letter was incremented in much the same way as car registration plates are. Hence from 1903 to 1905 a series of destroyers were built named after rivers and were Class "E", hence they would be termed either as River Class or E Class, but their Flag Superior would be D, H or P. With the outbreak of WWI all destroyers became "F" but later "G" was added as an option and then "H" and "D"
During WW2 "K" was introduced for Corvette, other letters included "U" for Sloop.
Post war the system was overhauled and Flag Superiors standardised so that each represented a single Class of ship:
A
Auxilliary
B Battleship
C Cruiser
D Destroyer
F Frigate
M Minesweeper
N Minelayer
R Aircraft Carrier
S Submarine
H Hydrographic vessel
L Amphibious Warfare
P Fast Patrol Boat
This system came into force in June 1947, prior to that the "F" classification was only seen on Destroyers, Frigates would be U or K.
Richard
Further info on
http://www.middle-watch.co.uk/Classes.htm