The trunnion-height problem also applied to small-calibre guns like this 20mm Oerlikon. Even at a moderate elevation, the gunner has to squat back uncomfortably, leaning back into the strap supporting him, because he has to look directly at the target through his sight. This Mk 4 Oerlikon mounting incorporated a column which could be raised or lowered so that the gunner did not have to squat too far (its operator is on the left of the gun; the gunner on the right reloaded when the ammunition drum was empty). Note the Mk 14 gyro gunsight atop the gun, a standard feature when this photograph was released in January 1944.
At Scapa Flow in 1918, Bellerophon displays the standard battleship anti-aircraft battery of two 4in HA guns. She also had a 6pdr. Most battleships had 3in guns (excepts were the 15in battleships and Centurion). The standard 4in gun was a Mk V on a HA Mk III mounting. This gun was also mounted in ‘D’ and ‘E’ class cruisers (the ‘C’ series had 3in guns). Many light cruisers carried the gun on a 60° HA mounting. In 1918 some cruisers were assigned 3in rather than 4in anti-aircraft guns because these weapons were also used against submarines, which might suddenly be encountered on the surface; the 3in was considered much handier, and it could be mounted in more convenient places. By late 1917 the 4in Mk V was favoured for anti-aircraft fire because its ceiling (30,000ft) was considered essential if British warships were to beat off future air attacks. Plans called for mounting one gun each on board capital ships, cruisers and the carrier Argus, a total of eighty-three guns, none of which had yet been delivered (109 had been ordered). Deliveries began in December 1917.
A few navies, particularly the Royal Navy, became interested in anti-aircraft weapons even before 1914, just as naval aircraft were being developed.1 It was by far the most advanced in the world before and during the First World War. Wartime thinking and experience shaped inter-war British efforts, including the development of guns and fire-control systems in the 1920s, on which systems in service at the outbreak of the Second World War were based. British experience also helped shape the thinking of allied navies, probably particularly the US Navy (given its participation in the Grand Fleet) and the French and Italian navies. Moreover, British firms which developed fire-control devices based on that wartime experience (and on post-war British work) seem to have provided the basis for Japanese and probably for post-war Italian, Dutch and German fire-control systems.
It was soon obvious that few aircraft would be shot down by heavy guns. However, anti-aircraft fire could dissuade a pilot or force him to take evasive action which would ruin his aim, or at the least force him to fly so high that he could not hit a ship. It turned out that pilots were often almost completely unaware of bursts below or behind them. Shells should be set to burst ahead of the target aircraft, not least to ensure that the pilot realised he was under attack.
The other aspect of wartime naval anti-aircraft fire was the need to deal with scouting aircraft and Zeppelins. They had to be destroyed, or at the least driven off. During the First World War it seemed that shipboard fire could do that, but later it was clear that only fleet fighters could deal with scouts or snoopers.
The Royal Navy
Fire Control
In surface gunnery, which was difficult enough, range and deflection (leading the target in bearing) were not tightly bound together. It was possible to imagine tracking a target in bearing, finding the rate at which bearing changed, and providing enough deflection to allow for the time the shell would take to get there. In fact the bearing rate kept changing, but often not fast enough to complicate matters unduly. An aircraft seemed to move both horizontally and vertically, even if it was flying straight and level. A gunner always had to allow for two separate deflections. Instead of simply elevating for range, he had to add a tangent elevation (super-elevation in US parlance) to the vertical deflection, and the tangent elevation depended on the vertical deflection. The earliest anti-aircraft sights allowed for vertical deflection and tangent elevation; typically deflections allowed for the 20 seconds or so of flight of a 3in or 4in shell. It seems unlikely that anyone tried to add an allowance for a climbing or diving target, although everyone accepted that it was needed. The great development in sight design was to allow separate setting of vertical deflection and tangent elevation.
As in surface gunnery, it was unlikely that the first shell would hit, so much depended on spotting. Unfortunately there was no equivalent to the shell splash which showed that a shell fired at a surface target had missed (and, approximately, by how much and in what direction). Initially the British tried to solve the problem by adding tracer to their shells, but it turned out to be difficult for spotters to follow their flight (and also very difficult to tell whether a shell was near its target). Aircraft turned out to be so difficult to see under some conditions that the author of the Admiralty 1916 pamphlet on anti-aircraft fire considered that the control officer could not take his glasses off an air target, as he would be unable to reacquire it. Similarly, sight-setters could not move their eyes back and forth between air target and sight dials. By that time the British realised that the only practicable aid to spotting would be the burst of the shell. Fuses were set to burst at estimated target range.2
It turned out that observers at or near the gun could not be sure whether a burst was short or beyond the target. Was a shell bursting on its way up (i.e., at nearly the desired range) or on its way back down (i.e., fired at excessive range)? The control officer could only distinguish bursts as above, below, right or left. It was impossible to disentangle range (elevation) errors from errors in setting the length of fuse (fuse delay). A shell fired at too short a range might burst below a target. So might a shell fired at too great a range, whose fuse burst it after it passed over the target and dropped on the other side. Unfortunately British powder fuses functioned erratically – for example, the rate at which powder burned changed with altitude. Under some circumstances fuse problems might burst a shell as much as a thousand yards from the anticipated point. Thus development of a reliable mechanical time fuse became an important post-war project.3
Tests showed that neither shrapnel nor common shell (powder-filled) did sufficient damage. The Royal Navy therefore chose time- (and impact-) fused high-explosive (HE) shells, sometimes carrying tracers.4 Such HE shell did not become available until some time after mid-1917.
Surface guns had their range dials marked with curves so that gunners could instantly translate ranges into elevation angles. The natural extension was a fuse curve relating fuse action to gun elevation: not to range, but to what was visible: the sight angle at which the fuse would burst the shell. The shell would follow the trajectory given by tangent elevation plus vertical deflection, and at a timed point along that trajectory it would burst. Conversely, for any fuse setting a curve could be drawn showing the range and altitude at which the fuse would burst at various gun elevation angles. The object was to ensure that the shell would explode reliably at the desired sight angle, just as previously the object had been to ensure that the shell would arrive at the desired range along the chosen sight angle.
By 1917 the British were using elevation dials marked with fuse curves (range data were retained on the edge of the dial).5 By this time it was clear that the most difficult task for an experienced anti-aircraft officer was to choose the right fuse setting. It should be based on either target range or height and on the angle of sight to the target when the first round was intended to burst. No one could set fuses by eye, based on spots. Often what looked good from the gun was clearly wrong when viewed from a distance. An observer to one side could often see that not a single round burst within a thousand feet of the target. Typically the fuse setting was chosen based on a target speed-altitude curve drawn on a card.6 Since the process of fuse setting was relatively cumbersome, this choice had to come first, and it could not quickly be changed.
Once the fuse setting had been chosen, the elevation sight was set on the curve associated with it. As the angle of sight changed, a pointer automatically moved