Naval Anti-Aircraft Guns and Gunnery. Norman Friedman
The first ships with an anti-aircraft computer were the ‘Tribal’ class destroyers which were conceived to contribute to fleet air defence. They were given the fuse-keeping clock (FKC), ‘clock’ indicating a simple computer, the rest of the designation emphasising the need to set fuses. The FKC was first tested on board the sloop Fleetwood. This FKC was associated with a bare minimum system. The associated Rangefinder Director (not in the same series as the HA directors of the HACS) was sometimes described as a HA/LA director. The UK series rangefinder was modified to transmit director setting and training as well as range. It carried a control officer’s glasses with the angle of presentation graticule. Typically it was occupied by a control officer, a director layer, a director trainer and a range-taker; in contrast to the HACS director, there was no separate phone man.40
The other elements were an optical deflection calculator and a separate fuse-keeping clock which maintained future range so that the fuse could be read without the complication of a plot. Inputs were target course (via angle of presentation) and speed, plus initial range. As in a HACS, calculated range was transmitted back to the rangefinder as feedback. Change of range in time of flight was produced and added to rotate a dial to future range; the dial was marked with fuse curves, from which fuse numbers could be read. The first FKC used an inverted form of the HACS deflection calculator, in which the operator looked through a lens at the ellipse inside. The production version had the same kind of optical deflection calculator as a HACS.
Fuse-Keeping Clock Mk II, as shown in the 1945 Gunnery Pocket Book. (Photograph by Richard S Pekelney, Historic Naval Ships Association, courtesy of Mr Pekelney)
In effect an FKC was an HACT without its plot, with the same kind of feedback (range to the rangefinder, target course to the A/P binoculars used by the control officer). Early FKCs also had power follow-up for sight angle, which was an additional form of feedback: the FKC set the control officer’s binoculars at the generated sight angle, which in effect corresponded to the calculated range (for an aircraft flying level). Angle of sight was needed to convert angle of presentation, as seen by the control officer, into target course. An important difference from the HACS was that the same range operator tuned both rangefinder range and rate, a combination which turned out to be difficult in initial trials on board Fleetwood. Compared to an HACP, an FKC required eight rather than nine personnel.41
In a destroyer, the FKC worked in conjunction with the standard surface fire-control computer, the AFCC. In a smaller ship, it worked with the less sophisticated Admiralty Fire Control Box (AFCB), which provided the surface fire-control function. At the outbreak of war FKCs were on board large escorts (Bittern class and later sloops, e.g. Black Swan class) controlling 4in HA guns; on board ‘Tribal’ class destroyers; on board ‘J’ and ‘K’ class destroyers, and on board smaller escorts (Hazard class and ‘V&W’ class converted to anti-aircraft escorts). All had gyro roll correctors in their dual-purpose (anti-air and surface) TS (the British term for a fire-control centre). At this time the FKC was planned for the ‘L’ and ‘M’ class destroyers with their higher-elevation (50°) main batteries.
The success of the FKC, compared to the HACT, showed that the new kind of fire control based on computation really worked. No range plot was needed. This success is evident in the wide use of FKCs outside the destroyer series, in ships which might otherwise have had HACTs. The fleet carriers Indefatigable and Implacable had Mk III*, as did the monitors Roberts, Abercrombie and Erebus, many depot and repair ships, fighter direction ships and other major auxiliary ships (such as Prince Robert). The carrier/repair ship Unicorn had Mk II*. The cruiser Ontario had Mk V, and it was planned for the Tiger class. The fast minelayers Manxman and Apollo had Mk II**.
There was yet another series of dual-purpose directors for destroyers, intended to save space on crowded bridges by consolidating the surface director control tower (DCT) and the associated rangefinder director.42 Because destroyers were expected to have excellent surface fire capability, these units were described as LA/HA director control towers. The first was installed on the ‘J’ class (and its ‘K’ and ‘N’ class repeat versions). It proved unsuccessful; in 1941 the DCT in these ships was reduced to surface capability and the rangefinder converted into a three-man rangefinder director, a seat being added for the control officer (as well as a windshield to protect personnel). The other personnel were the layer and trainer. The associated computer was FKC Mk II. In these ships the 12ft rangefinder (UK series) had range limits of 2000 to 20,000 yds. The ‘L’ and ‘M’ class had another such director. These destroyers required additional HA capability because their power-operated gun mounts could elevate to 50°, the highest elevation of any British destroyer main battery to date.43 Because this Mk IVTP was unsuccessful against aircraft, a new ‘K’ type DCT (see below) was developed for the later war emergency destroyers. It formed the basis of yet another director series, and was designated ‘K’ Mk I. The K designation was taken from the ‘K’ sight used on the director.
Later destroyers had either the US-supplied Mk 37 or the wartime Mk VI director described in a later chapter. The six Vickers-built Havant class destroyers (taken over from Brazil) had pure surface fire-control systems. Their sole anti-aircraft control system was an auxiliary barrage director.
Barrage Fire
Barrage fire, largely by non-high-angle guns, turned out to be extremely important to the Royal Navy during the Second World War. The idea can be traced back at least as far as the 1919 Naval Anti-Aircraft Committee. In February 1920 its president Captain Usborne suggested using barrage fire by low-angle (secondary battery) guns against torpedo bombers and remote-controlled boats. Experiments having shown that no existing fuse could be relied upon to burst 6in shell making grazing hits at ranges inside 3000 yds, so the ship would rely entirely on the splashes the guns could create. Torpedo bombers were sometimes damaged by the splashes created by their torpedoes. CMB (motor torpedo boat) officers considered their boats (and, by extension, remote-controlled boats) ‘most probably highly vulnerable to a splash falling on board them and swamping them’. Since there was no real knowledge of the size of a splash, or how much of it was water and how much light spray which could not harm an aircraft or boat, the committee asked for experiments. Its report suggested firing shrapnel from heavy low-angle guns. This technique was tested by Tiger in 1926. It seemed that this type of fire would be effective against torpedo bombers.
Trials of 5.5in and 6in secondary guns at long range and low sight angle began with firing by Hood in 1928. A special control system called H.X was installed in Renown, Repulse and Curacoa for trials. At about the same time battleship secondary batteries fired barrage and splash trials against torpedo bombers. It turned out that shells burst too low above the water (at about 8ft altitude). Time-fused HE was likely to be more effective than a splash barrage. The control technique used a fixed fuse setting and a corresponding range set in the director sight. The director was laid on the incoming aircraft rather than on the horizon (as in a splash barrage), and the ship fired rapid salvos of time-fused HE shells. This seems to have been the beginning of a technique that was very effective in the Second World War. The more elaborate H.X system was dropped because it entailed too much complication in ships which already had multiple fire-control systems. Results were not so good as to justify continued ammunition expenditure.
The barrage trials were satisfactory enough that the 1930 issue of Progress in Naval Gunnery announced as policy that the secondary batteries of battleships and the main batteries of cruisers would continue to be used against aircraft, but only at close range. The necessary ammunition and practice facilities would be provided. The possibility of providing efficient high-angle control and anti-aircraft armament for primarily low-angle guns in future ships would be reconsidered. Battleship main batteries would not be involved (this was made explicit in 1933).
In 1932 the Mediterranean Fleet tried barrage fire by anti-aircraft batteries, also an important Second World War tactic (as a way of overcoming the limitations of HACS). There were three alternative techniques: (a) a stationary barrage (at 1500 yds) through which an