While Mk II was entering production, a more elaborate Mk IV, with a tachymetric training unit, was ordered for trials. Its core was the Automatic Control Officer’s Forward Area Sight (AUTOCOFAS), which used tachymetric measurements to set lateral deflection.61 AUTOCOFAS was fully (locally) gyro-stabilised. The director would automatically feed range, enemy movement, and own ship movement into the AUTOCOFAS calculator, so that the control officer could concentrate on keeping the gun on target. This director had Magslip transmission. Mk IV* was the initial version, a converted Mk II or III. In 1940 the first Mk IVs equipped the battleship King George V and the carrier Courageous.
Mk IV was still rather elaborate, with a four-man crew. Ultimately DNO wanted something simpler, based on a ‘disturbed’ line of sight. Layer and trainer would aim off, following the target. There would be no need for independent operators to obtain and supply deflection. That was how the later US Draper systems (such as Mk 51) worked, and it was how the late-war British Simple Tachymetric Director (see below) worked. DNO hoped to begin this development once the larger Tachymetric Control System Mk I was far enough advanced.
At about the same time that Mk IV was being designed, it was decided that the 2pdr shells should be self-destroying at 2500 yds. That would allow ships to engage aircraft attacking between columns or from the direction of screening ships, but it was much more important as a deterrent. It turned out that an attacking pilot might well ignore tracers climbing towards him, but he would see the self-destroying rounds as they exploded, much as he might see time-fused bursts from heavier guns.
Tracers were associated with hosepipe control. As it emphasised eye-shooting and the associated form of director control, the Royal Navy moved away from tracers; they would only confuse the control officer. Late in 1940 trials were run to see whether that was a good idea, and also to compare local with director control.62 It turned out that the combination of tracers and director control was best, except when tracers could not be observed, when fire had to be opened at very short range (so no delay was acceptable), when pointer-following errors were likely to be excessive (due to heavy rolling), and with high-velocity guns using Mk II directors. Tracer was relatively ineffective for locally-controlled guns because the gun layer and trainer needed so much skill to follow the target, estimate and superimpose aim-off (deflections), and at the same time observe tracer and apply corrections. It was also difficult or impossible for the gun crew to see the tracer through smoke.
Quadruple 0.5in machine guns on a Royal Australian Navy cruiser show the splinter shields added in wartime.(State Library of Victoria)
Poor results using director fire were mainly due to errors in following pointers controlled by the director. The solution was RPC, which was being fitted to cruisers and larger ships in conjunction with the new Mk IV director. Both the Metro-Vick-Elswick electro-hydraulic and the ‘Metadyne’ electric systems had recently passed their tests. New director rangefinders (FV 3 instead of FV 2) were being supplied, and a programme to fit Type 282 radars was also in hand. New ship designs would have their pom-pom directors as close as possible to the guns, to eliminate convergence errors.
A lightweight quadruple Mk VII mounting was developed for cruisers and destroyers. In 1932 it was expected to weigh 6¼ tons, still far too much for existing destroyers and surviving First World War cruisers, but acceptable for more modern cruisers. It also appeared on board ‘Tribals’ and some later destroyers, and some carriers. Development continued even though the 1931 Naval Anti-Aircraft Gunnery Committee considered the quadruple pom-pom useless (it wanted a new gun, which emerged as the 0.661in). The committee considered the octuple mounting just good enough to be worth retaining. Once rearmament began, the quadruple pom-pom was easier to produce in quantity than a new mounting, because it had such commonality with the octuple 2pdr already in production. About 1935 tests showed that the quadruple 2pdr had considerable potential against a new threat, the motor torpedo boat (MTB), which became much more prominent during the Ethiopian crisis in the Mediterranean (the Italians had large numbers of MTBs). That was why the ‘Hunt’ class destroyer escorts were ordered modified to accommodate this weapon (with unfortunate, albeit unforeseen, effects on their stability). As completed, Mk VII was overweight: A typical Mk VII* weighed 8.6 tons. By way of comparison, the single 4.7in CP Mk XVII in a ‘G’ class destroyer weighted 8.8 tons. The prototype quadruple 2pdr was tested aboard the cruiser Shropshire in 1934. This prototype was transferred to the destroyer Crusader for 1935–6 trials which confirmed that it was suited to the new ‘Tribal’ class.
The destroyer installation did not include a director, and without it the pom-pom was considered unlikely to get many hits at over 1000 yds range. During discussions leading up to the construction of the ‘Tribal’ class, it was pointed out that the value of the pom-pom might lie more in the enemy’s knowledge that it existed than in its chance of actually shooting down aircraft. Trials on board the ex-battleship Centurion suggested not more than 5 per cent hits, and the pom-pom would have only 10 seconds of firing time before a dive bomber released its bomb. A trial using a fixed position suggested a hitting rate as high as 16 per cent, but that would decline for a lively ship.
The 1919–21 Committee also wanted a shorter-range back-up, a replacement for the wartime 0.303in (rifle calibre) Lewis gun. It had to be more powerful, because future aircraft might well be armoured.63 The result was the quadruple 0.5in Vickers machine gun. Compared to the pom-pom, it had a much higher rate of fire (600 rounds per barrel per minute, or 2400 total, compared to 720 for the eight-barrel pom-pom), but the rounds were far less destructive and the effective range shorter. Effective 0.5in range, as evaluated in 1932, was about 1000 yds (out to 1800 yds time of flight was less than for the 2pdr, but bullets were not dense enough to do much damage). The gun was far inferior to the multiple pom-pom, but also far better than a single 2pdr. Many ships retained twin Lewis gun mounts, which were set up on a temporary basis.
The 1931 Naval Anti-Aircraft Gunnery Committee considered the 0.5in gun inadequate; it wanted a new gun, which would also replace the quadruple 2pdr. Nothing happened initially, but in 1934 DNO announced that ‘to meet improvements in aircraft and new methods of attack’ a new close-range weapon was being developed, capable of disabling an attacker at a mean range of 1200 yds in 6 seconds. He was interested in both an improved Mk M and an improved machine gun firing solid bullets. The latter had to offer reduced time of flight (a heavier bullet and/or higher muzzle velocity). The machine gun requirement led to a 1935 order for a Vickers prototype 0.661in gun (for use in a sextuple mounting). Muzzle velocity was 3125ft/sec. Time of flight to 2000 yds would be 60 per cent that of the 0.5in machine gun. Expected rate of fire was 300 rounds per gun per minute, each shell weighing 3oz.
The 0.661in grew to weigh so much more than the 0.5in machine gun that it was compared to the quadruple pom-pom instead. Thus the 0.661in fired 1800 rounds per minute, compared to about 400 for the quadruple pom-pom; on the other hand, each of its shells weighed less than 10 per cent as much, so the weight of fire was only about 45 per cent that of the pom-pom (though far more than that of the 0.5in). Moreover, the sights planned for the 0.661in were considered much inferior to that of the pom-pom. This mount came too late for the ‘Tribal’ and ‘J’ classes, but in 1936 DNO proposed that the next (‘L’) class have two such mounts in place of their pom-poms and 0.5in. Like other weapons of the period, the 0.661in continued to grow in weight. Originally weight was to have been 2.75 tons, and in various versions of the ‘L’ class design the assumed weight was 3.4 tons. However, at the mock-up stage in 1938 expected weight was 4 tons, nearly half as much as the pom-pom. The decisive argument against the 0.661in may have been its inadequacy against MTBs, given its light bullets and short effective range. In January 1938 the Assistant Chief of the Naval Staff said that he would prefer two pom-poms to one pom-pom and a 0.661in. The sextuple 0.661in was cancelled in 1938.
The improved Mk M was more fruitful. At the outset DNO wanted a larger pattern, which would normally mean more barrels, and/or a higher rate of fire. He also wanted to reduce