Even at the beginning of the 1930s there was some concern about the adequacy of the British weapons. The problem was considered by the Naval Anti-aircraft Gunnery Committee, which produced a report in April 1932.
The committee went into great technical detail in calculating the effectiveness of different guns. They took into consideration such matters as the nature of the threats, the time during which aircraft could be brought under effective fire by different weapons, the range and rate of fire of the guns, the effectiveness of shells and fuzes, and appropriate methods of fire control. Their conclusions make interesting reading.
Three types of attack were considered: precision (i.e. level) bombing, torpedo bombing and close-range attack with bombs or machine guns. With remarkable prescience, the committee also identified a further potential risk:
The possibility of explosive aircraft being manoeuvred by human pilots to hit ship targets cannot however be ruled out. It is reported that, sooner than accept defeat, ramming other aircraft is a recognised principle among Japanese pilots.
Note: The Air Ministry regard this idea as exceptionally secret and would prefer that it be not generally promulgated.
It was clear that the committee considered such attacks to be potentially extremely difficult to deal with, a concern fully justified by the experience of a dozen years later.
In considering short-range defence, the committee was most concerned about torpedo bombers, calculating that any weapon system able to cope with them would be able to deal with other forms of short-range attack easily enough. Exercises between 1928 and 1931 had shown that the probability of a torpedo bomber hitting a ship was only 10% at 1,250 yards (1,140m) but rose to 30% at 1,000 yards (910m), 50% at 750 yards (690m) then increased very sharply to 85% at 600 yards (550m).
This led to a demand for a range of 2,500 yards (2,300m) from automatic AA guns, in order to achieve the aim of certain destruction of an aircraft with ten seconds of firing at a mean range of 2,000 yards (1,820m). It was estimated that an aircraft dropping a torpedo from 1,200 yards (1,100m) would already have been under fire from such weapons for seventeen seconds, which was assumed to be more than enough time to shoot it down.
The two existing weapons - the .5" machine gun and 2pdr - were considered in detail. There were strong indications that the committee was not much impressed with either, suspecting that the .5" was ineffective and that the 2pdr Pom-pom (yes, that was the title they gave it!) had too low a muzzle velocity. They were also concerned about the 2pdrs rate of fire. This was felt to be just about adequate in its eight-barrel form but the committee recommended that no further steps should be taken to develop the four-barrel version, which was all that smaller vessels could carry. They expressed a hope that both guns would be replaced by an intermediate calibre with a 760 m/s muzzle velocity and a total output of at least 1,250 rpm, and recommended a series of trials to determine the ideal calibre.
In fact, two experimental weapons emerged. One was the very powerful Vickers .661" heavy machine gun developed between 1935 and 1938, which was intended to be fitted in a six-barrel mounting. Development was cancelled in 1938 in favour of the
20mm Oerlikon S, which had the advantage of firing the explosive shells which were by then regarded as essential. The other, somewhat closer to the committees views, was the 35mm l!/2pdr Mk V, which fired a 0.68kg shell at 790 m/s. However, the eight-barrel mounting ended up weighing more than the 2pdr s, and the weapon was cancelled in
Later, a more powerful 'high-velocity' loading of the 2pdr was introduced which raised the muzzle velocity to 730 m/s, partly achieved by reducing the shell weight from 0.9kg to 0.76kg. This could only be fired in modified guns and the 'hot' high-pressure loading cannot have done much for the reliability. The rate of fire was also increased to 115 rpm.
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