Naval Anti-Aircraft Guns and Gunnery. Norman Friedman
The 4in Mk V gun on its HA mounting, as shown in its 1918 handbook. The recuperator above the breech returned the gun to its firing position after it recoiled. Note the loading tray behind the breech, which was displaced to the left of the gun. The blank disk next to the gun is the fuse disc, used by the crew to associate a fuse setting with an elevation angle.
However, by that time it was increasingly understood that reconnaissance by Zeppelins had to be countered. It seemed that ships armed with more powerful anti-aircraft guns could provide what might now be called area defence and was then called offensive anti-aircraft warfare. Forty-four 4in guns (4in BL VII and 4in QF IV) were placed on modified 3in HA mountings (maximum elevation 60°) on board capital ships and light cruisers.12 Some battleships had one 4in in place of a 3in gun. At about the same time some destroyers were given ‘trap-door’ 4in mountings, the idea being to tilt the platform under the gun to increase its elevation. The idea proved unsuccessful and was abandoned.
As the Zeppelin problem worsened, a more powerful gun was developed: a 6in on an HA mounting. During 1916 some were mounted on board monitors and China gunboats on the British south-east coast, and others went into the land positions. This gun never proved satisfactory. It was difficult to load at high elevations and it was unwieldy. In effect the 6in experience showed that there was a practical upper limit to gun size – hence reach – unless radical changes such as power loading and power handling were made. Attention shifted back to smaller calibres.
At the end of 1916 work began on a new mounting for the 4in QF Mk V gun (HA III: 80° elevation). The first mountings were supplied in February 1918; it was the standard post-war capital ship HA weapon. Work on two other HA mountings began early in 1917: a 3in HA III for submarines and torpedo craft; and a 12pdr HA VIII (12pdr 12 cwt gun) for trawlers, minesweepers and other small craft. Meanwhile a broad policy for destroyers was adopted in mid-1916. The large new ones (‘V’ class and similar ships) were each to have one 3in HA II. The smaller modern destroyers (the ‘M’ to ‘R’ classes) were each to have one 2pdr pom-pom. Older ships would have one 6pdr on HA IV mounting or one 12pdr 8 cwt on a converted mounting or a Vickers 3pdr.
In 1918 the use of a larger calibre was being considered, its size limited by the weight of QF ammunition which could be handled without power. On this basis a 4.7in gun with an 80lb round (projectile and case) was chosen, and a few were ordered after the Armistice for tests. This gun armed the post-war Nelson class battleships. On the other hand, it could be argued that lighter guns such as the 3in and 4in had other roles. In 1918 there was considerable interest in dealing with suddenly surfacing submarines, and it was argued that 4in guns were not handy enough for that role. On that basis cruisers were armed with 3in rather than 4in HA guns. In 1919 most capital ships had two 3in HA guns, but the latest were scheduled to receive 4in instead.
Once war broke out, it was clear that much larger numbers of anti-aircraft guns were needed for lesser ships and craft. Existing mountings for Hotchkiss 3pdr and 6pdr guns were converted until a satisfactory new mounting could be supplied. These guns armed auxiliary patrol trawlers and similar ships. Maximum 3pdr Hotchkiss range was about 4000 yds, and maximum 6pdr range about 5000 yds. The higher-velocity Vickers 3pdr (which had replaced 12pdrs as an anti-torpedo boat gun about 1903) was considered a more effective anti-aircraft gun due to its high velocity and high rate of fire, though it was too light a gun; effective range was 5000 yds.
The existing Vickers 3pdr semi-automatic (i.e., single-shot) gun was mounted on a converted anti-aircraft mounting. A design for a special HA mounting was adopted shortly after war broke out (large numbers were supplied early in 1915). Maximum effective pom-pom range, as estimated in 1916, was about 4500 yds.
The next larger gun, the 12pdr (3in), was also adapted for anti-aircraft fire, and special HA mountings became available in 1915. At the time this gun was used largely by ships in the Nore, which it was hoped could shoot down Zeppelins en route to bomb London. As the preferred 3in 20 cwt gun became available in sufficient numbers, converted 12pdrs were shifted to auxiliary ships such as trawlers.
Vickers’ drawing of the version of the 4in HA gun it offered for export soon after the First World War, from its 1923 catalogue.
There was also interest in converting existing light machine cannon (pom-poms, both 1½pdr and 2pdr). A few 1½pdr pom-poms were mounted temporarily in ships of the Grand Fleet and in light cruisers at southern bases (Dover and Harwich); they were replaced by 2pdrs (delivery began in June 1915). The 2pdrs were issued on an urgent basis, without training personnel. It was therefore no surprise that they proved unreliable in early service. They were unreliable; it turned out that they had been made without using the necessary gauges and jigs, and so were not standardised. The pom-pom was recoil-operated, and guns had been assembled with too little recoil length. The cartridge cases also proved too weak. The gun seems not to have been entirely reliable until early 1918.
Issue to the fleet having been suspended for a time, it was resumed in November 1917. By this time the Germans had introduced remote-controlled explosive motor boats off the Belgian coast (the British called them distance-controlled boats [DCBs]). In a sense they were analogous to aircraft, and the obvious counter was the sort of automatic gun otherwise intended to deal with aircraft: the 2pdr pom-pom. Plans called for three guns in each capital ship, two in each light cruiser, two in each destroyer leader, and one in each destroyer. Supply was not yet complete at the Armistice. By the end of 1919, a total of 932 of these guns had been supplied.13
The pom-pom thus became and remained the key British automatic anti-aircraft weapon; between the wars it was mounted both singly and multiply. It was a cannon version of the first really successful machine gun, the recoil-operated Maxim, invented in 1884 by the American Hiram Maxim, and licensed to Vickers (it was standard in the British army by 1891). The name ‘pom-pom’ came from the sound of firing. In back of the bolt of the gun was a toggle joint, which could bend like an elbow when unlocked. Conversely, by straightening out it propelled the bolt forward and locked it onto the barrel. The Luger pistol used a similar toggle, whose joint formed the distinctive crosswise cylinder atop it. When the gun fired, barrel and bolt recoiled together. The toggle, which had been straight (to hold the bolt in place), was unlocked so that it could bend up to allow the bolt to move back from the barrel. As bolt and barrel moved back, the bolt was unlocked from the barrel. The barrel hit a stop and the bolt kept going back, the toggle bending and the spring compressing. By moving back, the bolt also tripped the ejector and also stripped the next round from the belt, which was above the barrel. The fresh round was pulled into a feed block directly behind the belt. As the spring moved the bolt back, the toggle pulled down the feed block into line with the barrel. Then the toggle straightened, pushing the bolt and the new round forward for another shot. This technique is called short recoil, because the length over which the barrel itself recoils is limited (typically ¾in for a Maxim rifle-calibre machine gun). Compared to the original Maxim, the Vickers gun (the pom-pom) differed in that the toggle opened above rather than below the barrel. The standard US Browning machine gun, which the US Navy used before and during the Second World War, also used a recoil-powered short-recoil action, but of a different type, without a toggle.
The standard First World War 3in anti-aircraft gun was Mk I (3in 20 cwt) on a HA Mk II mounting. It was tested in the autumn of 1911 as the future anti-aircraft gun for both army and navy (the navy considered its performance inadequate, but bought it anyway). In Mk II or IIA mountings it armed the following ships: all dreadnoughts except Audacious, all battlecruisers, Lord Nelson class, first five King Edward VIIs, Albion, Prince George, Courageous class, Furious, Vindictive, Minotaur class, Achilles, Cochrane, Duke of Edinburgh, Danae, Dauntless, Dragon, Carlisle, Ceres class, Caledon class, Centaur class, Cleopatra, Conquest, Arethusa class (except Galatea), Chester, Birkenhead and Birmingham, Chatham, Falmouth, and Bristol classes, all scouts (except Pathfinder),