Flying bombs launched from aircraft flew even lower than those launched from ramps, the majority speeding over the guns line of fire at 1,000 feet (many flew even lower, some crashing into the sea before reaching land or sand dunes on reaching land). To avoid detection the German bombers adopted a tactic which became known as ‘low-high-low’ – they flew only a few hundred feet when over the sea and would then climb to 2,000 feet or so to launch their bombs before dropping back down low over the sea for the return flight. To counter the low height at which the bombs flew at, a number of gun layouts were tried for the Diver Strip:
HAA –sites varied between two guns to eight guns but four was by far the most common. A HAA site deployed on a four gun basis manning four 3.7” Mk. IIC guns, radar AA No. 3, Mk. V Predictors or No.10 Mk 1 and 30KVA generators was typical.
‘Welter’ – deployed on a two gun basis manning two 3.7” Mk.IIC guns and RO3 predictor and two 30 KVA generators.
'Intermediate' – the specifications appear to have varied between AA Brigades but the following are the specifications that seem to have been mostly used in Suffolk:
Type A –‘Welter’ (i.e. two 3.7” Mk. IIC guns with Predictor RO3), eight 40mm guns with two No. 10 Predictors and Radar AA No.3 Mk V and three 40mm guns with three No. 3 Predictors.
Type B – ‘Welter’ (i.e. two 3.7 Mk. IIC guns with Predictor RO3) and eight 40mm guns with two AA No. 3 Mk II Predictors.
Type C – four 40mm guns, each controlled by Predictor No.3 , deployed in normal four-gun static troops
‘Welter’ guns referred to the combination of two 3.7” HAA guns controlled by LAA predictors equipped with radar range-finding apparatus, and manned by LAA personnel. It was an attempt to deal with the exceptionally low heights at which the flying bombs approached as it was considered that two 3.7” guns firing Bonzo ammunition and controlled by a LAA Predictor would be more effective than four 40mm guns controlled by the same equipment.
‘Intermediate’ initially referred to the practice of grouping four 40mm guns together (instead of the usual single siting of 40mm guns in a LAA role) with a modified predictor to allow aircraft to be engaged up to 12,000 ft with 12 second tracer. As can be seen from above, other types of Intermediate layouts were specified for Diver. In fact a forth type of Intermediate site seems to have been added later to Diver:
Type D – eight 40mm guns, controlled by 2 AA No. 3 Mk II Predictors and AGLT Mk I radar.
AGLT Mk I was Air Ministry radar equipment for laying and controlling fire from an aircraft turret. It was an attempt to deal with the low height of flying bombs at 500 ft and below by allowing some unseen fire by LAA guns (the RO3 only acted as range finders).
To further confuse matters, one War Diary refers to a "Bantam" deployment - 3 x 40mm guns with a No.3 Predictor!
The general principle of siting HAA guns during Diver was in a line, 30 yds apart, at right angles to the primary bearing – this was 45˚ for all sites in Suffolk. The introduction of Bonzo restricted the arc in which guns could engage flying bombs, and in Suffolk the nature of the coast meant that guns could generally only fire to the right of the primary bearing. As a result it was specified that guns were to be placed at right angles to a line bisecting the Bonzo engagement arc. The low height that many bombs flew at prevented many engagements by HAA batteries due to the 6˚ Q.E. stop on the 3.7” gun. To overcome this problem, two guns of each four gun site were modified with 5˚ Q.E. stops. To prevent accidents with the guns fitted with the 5˚ stops, the layout of the four guns was modified to an inverted bow, with the two guns with 5˚ stops on the outside, to take them further away from the general site of activity, and the two guns with 6˚ stops on the inner side.
Left: Layout of a four 3.7" gun site. The
guns are laid out at right angles to a
line bisecting the Bonzo engagement arc
at 30 yds spacing. The two outer guns,
fitted with 5 deg QE stops are offset by
30 deg to the inner guns.
Right: Layout of a Welter site. The guns
are laid out at right angles to a line
bisecting the Bonzo engagement arc at
25 yd spacing.
In both these examples, the Bonzo
engagement arc is 30 deg to 170 deg.
Little information is available in War Diaries on the layout of the Intermediate Sites, although AA Command did recommend for the Diver Gun Strip that eight 40mm gun sites should be laid out in a line parallel to the expected direction of attack, the spacing of the guns being 6 yds apart. It was felt this layout was the best to deal with the low approach of flying bombs. AA Command clearly stated that the layout of the site must be governed by the position of the radar, which had to be chosen first.
Field work on what could be the remains of a four 40mm gun site, Diver site SC17, suggests that such sites may have been laid out with the guns in a bow profile. It also suggests that static 40mm guns were mounted on ‘Pile’ platforms rather than the usual fixed wooden cruciform platform.
Above: Left - layout of an eight 40mm gun site recommended by AA Command for the Diver Gun Strip.
Right - Top: Layout of an eight 40mm gun site in use in the Kentish and Coastal Gun Belts, which may have also been used for the Diver Gun Strip. Bottom - possible
layout of a four 40mm gun site based on field work on Diver Site SC17.
The deployment of LAA, Intermediate and Welter sites was short lived with HQ AA Command deciding to withdraw these equipments from the Diver Deployment at the end of October. This decision was taken as these sites had not proved as effective as hoped. This withdrawal was expected to be completed by the end of November. Construction work was halted on these sites resulting in some being abandoned immediately, the guns redeploying temporarily to other sites. By December only 36 LAA guns remained, representing permanent defences and eight guns with the one remaining Welter Site.
The decision to abandon the Welter sites meant that a revision of the HAA layout was required, with the provision of additional HAA sites (to be formed by conversion of 28 out the 29 Welter sites into fourteen four gun HAA sites). These new sites were to be laid out with guns in a straight line 30 yrds apart, the line of the guns being offset by 20˚ west to the northern limit of the Bonzo engagement arc. Thus if all guns were fitted with a bearing stop of 20˚ either side of the line of the guns, all four guns could open fire when the target entered the Bonzo engagement arc. Work was also under way to look at the possibility of fitting guns with QE stops of 2˚ instead of 5˚ or 6˚ stops but this was never actually done.
Right - layout for a four 3.7" gun site for the Diver Gun Strip,
specified from October onwards.