The 1931 Naval Anti-Aircraft Gunnery Committee proposed a variety of anti-aircraft battery improvements, but money was too short. Even so, much effort went into providing the fleet with the new high-angle control system. By 1933 the British government accepted that war was on the horizon, and efforts to provide sufficient fleet anti-aircraft firepower accelerated with the Mediterranean (Abyssinian Crisis) war scare in 1935–6. By that time the services were submitting proposals to the Defence Requirements Committee.50 The navy divided its proposal into two classes, of which Class I was fleet requirements and Class II requirements for trade protection. Class I was counted as making up for known deficiencies in the main fleet, such as those which had been identified by the 1931 Naval Anti-Aircraft Gunnery Committee.51 There were two stages. First came major fleet units (capital ships, carriers, and cruisers). Second came existing sloops and minesweepers, which were needed at the least to ensure that the fleet’s bases remained usable.
Class II, which was much more expensive, went beyond the deficiencies of the existing fleet to deal with the threat of air attack against trade. Thus it included conversion of existing old cruisers and destroyers for trade protection and building up a reserve of weapons and other equipment which would arm auxiliaries in wartime. The sub-committee on Defence Policy and Requirements approved the Class I programme in April 1936. It approved the Class II proposal at its 40th meeting (24 June 1937) subject to further Treasury approval.52
The twin 4in Mk XIX became the standard Royal Navy medium anti-aircraft gun of the Second World War. This mounting was photographed on board a Canadian ‘Tribal’ class destroyer after the war. It carries the radar of a US-supplied Mk 63 fire-control system. The rear view shows a pair of fuse-setting machines, one on each side. Some wartime mountings lacked them. The mount was simple because it was not powered (except in RPC form) and because it had no integral ammunition hoist: ammunition was passed from a fixed hoist into the back of the mount. (MarComm Museum)
A twin 4in Mk XIX in action, showing the loaders. (MarComm Museum)
It did not help that, unlike the US Navy, the Royal Navy never standardised on its medium-calibre guns or mounts. Thus there were multiple types of 4.5in capital ship mountings; 4.7in (50- and 62pdr) destroyer mountings, single and twin; and 5.25in battleship mountings. Unfortunately this variety persisted as the British began to rearm in earnest in 1937. Not only was production of existing designs slow, but design work was delayed. The problems encountered by the King George V class battleships (when a new twin 14in mounting was suddenly required) are well known, but the destroyer programme encountered serious delays in the supply of twin 4.7in mountings, both that in the ‘Tribals’ and the entirely new between-decks type in the ‘L’ class.
Twin Mk XVI guns on a Mk XIX mounting, from the 1945 Gunnery Pocket Book. The fuse-setting machine on the left has been omitted for clarity, as has the divider between the guns. (Photograph by Richard S Pekelney, Historic Naval Ships Association, courtesy of Mr Pekelney)
Close-Range Weapons
Smaller guns were also needed. In an 11 February 1920 memo, the Naval Anti-Aircraft Committee recommended development of a multiple pom-pom to deal with low-fliers attacking with torpedoes and explosive boats controlled by aircraft.53 The mounting should be director-controlled. The committee proposed that Chatham Dockyard cut down the mounting of the standard 11in anti-submarine howitzer to take six 2pdr pom-poms spaced 3ft apart, the original training gear being retained but modified for director control. Guns should be arranged so that their lines of fire could be made to diverge to fill a desired area with projectiles (the committee thought the natural inaccuracy of the gun, which should be tested at ranges up to 3000 yds, might suffice to give the desired coverage). The Director of Naval Construction calculated that an Inconstant class cruiser could take one mounting in place of one set of torpedo tubes, with little modification. The cruiser’s director could be set up to control the gun mounting.
Trials compared 2pdrs and 3pdrs to decide which was the smallest (for maximum rate of fire) sufficient to defeat a torpedo bomber with one hit. Either was enough if it hit the engine, fuselage, or wings inboard of the outer struts; neither would suffice if it only hit the outer wings. The 3pdr offered a slightly greater chance that a fragment would hit the aircrew even if a direct hit on the outer wing failed to crash the bomber.54 That was not enough to disqualify the 2pdr.
Maximum acceptable weight for the multiple mounting was set at that of the 3in HA gun which then armed destroyers: 2 tons 12 cwt.55 Rate of fire was to be not less than 60 rounds per minute per barrel.
The initial pom-pom director was a simple dummy gun, the virtue of which was that it removed the aimer from the noise and vibration of the mounting. This is the Mk II version. The straps allowed the single operator to turn the director with his body. The cartwheel sight made it possible to estimate deflection. Mk II can be compared to the slightly later US Mk 44, which was also a very simple non-computing director. Director control would be exerted by the usual follow-the-pointer technique, the mounting being moved by a continuously-running electric motor which the crew could clutch in and out. The mounting should elevate and train at 15°/sec, with a maximum elevation of 45° (as yet there was no vertical-diving threat) and a maximum depression of 15°. As usual, both Vickers and Armstrong (Elswick) were asked to design mountings. Armstrong’s, which was designed for continuous rather than burst fire, was rejected as too complex, and a mock-up of the Vickers mounting was examined at Vickers in July 1923. Rate of fire was better than required (90 rounds per gun per minute) and maximum elevation was 80° rather than 45°. Rate of fire was 90 rounds per barrel per minute, compared to the required 60. The prototype mounting passed its shore trials in 1927 and it was successfully tested on board Tiger in 1928.
The most important British automatic weapon developed before the Second World War was the multiple pom-pom. This octuple one was on board Shropshire. The photograph is supposed to date from 1942–4, but note that the gun mounting is entirely unshielded, and that it is unmanned while it is being loaded. These guns were power-operated, which is why Prince of Wales suffered so badly as soon as she lost power. (State Library of Victoria)
The maximum allowed weight allowed for too few barrels. Ultimately a 15-ton eight-barrel mounting was designed for capital ships. The barrels were mounted so that they could diverge to spread their fire.56 Guns had to fired in sequence to avoid jarring the mounting. Initially they were fired in symmetrical pairs, but in later mountings they could be fired singly. The rate of fire was limited to limit the rate of erosion due to bore heating (the 2pdr was considered good for only 3000 rounds). Also, a much higher rate of fire would have required complete redesign of the gun.57 Guns were belt-fed from 150-round loading trays, which could be reloaded to maintain a rate of fire of 28 rounds per minute. Thus the gun could fire at 90 rnds/min for 2 minutes 25 seconds or at 108 rnds/min for 1 minute 50 seconds. Any increase in tray capacity would enlarge the mounting.
The quadruple pom-pom was designed specifically for destroyers and cruisers; this one is aboard