A twin 4.5in BD mounting: front view, and rear view with the cupola removed (the structure visible is all below decks).
Unfortunately no dual-purpose gun design was immediately ready for the projected (but abortive) new capital ships. The secondary battery choice made at the time carried over to the new Nelsons, which were effectively smaller versions of the aborted ships. They therefore had the new 4.7in high-angle gun plus 6in secondary guns intended to deal with attacking destroyers. After that the British ordnance industry was fully occupied developing 8in and then 6in cruiser mountings, so the battleship sketch designs developed in the late 1920s (for the expected new construction when the Washington Treaty ‘holiday’ on battleship building ended) also had a combination of 6in secondaries and 4.7in high-angle guns. However, they did have the latter in new cylindrical between-decks mountings. After successful tests on board the battleship Resolution, a production version was produced for the battlecruiser Repulse, then being modernised.47 The 1931 Naval Anti-Aircraft Committee supported this project.
By 1932, three different HA mountings were being designed, all using guns with the same ballistics as existing ones: an improved between-decks (BD) 4in mounting for Repulse; a twin 4.7in BD mounting for Nelson and Rodney; and a twin 4in weather deck (WD) mounting for fifteen battleships, Hood and possibly 8in cruisers. The latter was an admission that the BD mounting required so much below-decks structure that installation required total reconstruction of a ship. Twin mounting offered maximum firepower in given deck space: the Mk 19 twin 4in mounting had much the deck footprint of the existing single 4in gun.48 It fitted in well with the coming anti-aircraft rearmament programme, one of whose early goals was to double anti-aircraft firepower at minimum cost in time and material. Power loading had to be given up. That in turn limited any fixed round to about 63lbs, the heaviest which could be hand-loaded and rammed uphill with the gun nearly vertical (anything more would require separate shell and cartridge, and power ramming). However, the resulting mounting had considerable inertia, hence was difficult to manoeuvre quickly enough to match the movement of a fast target. Ultimately it required full power operation (other than loading). In addition to modernised ships, Mk 19 armed new cruisers, some new destroyers, and other units. Sea trials were conducted on board the sloop Fleetwood in 1937. It took about a year to solve the teething problems.49
The wartime British naval shipbuilding programme had to cope with delays in gun mounting production. They affected the 5.25in BD mounting used on board the King George V class battleships and the Dido class cruisers. To overcome that delay, two Didos were armed with the Mk III UD (hence much simpler) twin 4.5in mounting. The mountings involved had been manufactured to arm ‘D’ class cruisers, which would have been converted into anti-aircraft cruisers had the war not intervened. Scylla is shown in 1942.
The twin 4.5in Mk III as shown in the manual. (By courtesy of John Lambert)
Was the 4.7in powerful enough? In 1933 tests were conducted, bursting HE shells in flight against air targets to gauge effectiveness. The existing 4.7in (new design) and 4in (new design) were compared to a new 5.1in shell (70lb shell, 2500ft/sec) and to the US 5in/25 (70lb shell). If the new 4.7in was set at 1.00, the rating of the 5.1in was 1.27. Against that the US gun was rated at 0.79, and the new 4in at 0.5 – which made a twin 4in equivalent to a single 4.7in. Against that, the 5.1in could not be fully exploited because its fixed ammunition was too heavy (108lbs); trials in a destroyer had shown that it was awkward for one man to handle. To get the most out of the 5.1in, a still heavier round would be needed (a heavier shell or higher muzzle velocity). It was unlikely to be better than the 4.7in. Fleet experience showed that the rates of fire with existing designs were 13 rnds/min for the 4.7in and 20 for the 4in, not counting delays due to fuse-setting dead time.
On this basis an experimental 4.7in BD mounting was being designed in 1933 for planned trials in Nelson in 1936. If they succeeded, two single 4.7in would be replaced by 4.7in BD twins in both Nelson and Rodney. Meanwhile 4.7in designs with 55lb or 60lb shells were being considered. The 4in BD mounting tested in Resolution had failed to meet requirements, in that its loading rate was only eight rnds/gun/minute. There had been no time to fix the problem before producing two more mountings for Repulse, although they might be modified once installed. Two hand-loaded BD mountings were to be installed in Renown during her long refit (she was completely rebuilt, with different guns, instead). The prototype hand-loaded 4in WD mounting would be fitted in Iron Duke in 1934 for trials. Tests in Iron Duke (September 1934) were successful. A companion single Mk XX was planned for ships which could not accommodate the twin mounting. The prototype twin 4in ran sea trials on board the sloop Fleetwood in 1936.
By this time the British were beginning to mobilise. They had to produce as much as possible within stringent industrial and financial limits – much of the vast industry which had supplied so many ships and guns and shells before and during the First World War was gone. Progress in Naval Gunnery 1934 (describing developments in 1933) pointed out that, due to considerations of weight, space, expense, ammunition stowage and ammunition supply (production), capital ships and cruisers would have to retain the existing 4in guns – four twins in each of fifteen capital ships and two twins in each 8in cruiser, in each case a twin replacing an existing single mounting.
The most powerful of DNO’s BD mountings was the 5.25in, shown here on board the battleship King George V. Note that the two guns elevate independently. (Alan C Green via State Library of Victoria)
Internal arrangement of a twin 5.25in mounting. (Photograph by Richard S Pekelney, Historic Naval Ships Association, courtesy of Mr Pekelney).
In 1934 DNO decided to reduce the 4.7in gun to 4.5in, firing a 55lb shell. It was now known as the QF 4.5in Mk I, and was to be produced in both single and twin BD versions, the latter for certain capital ships. Trials were ordered, and it was decided not to mount the new gun in the Nelsons, presumably in hopes that they would receive more complete reconstruction (as described in their Covers). The single 4.5in gun was abandoned as there was no current requirement for it. The prototype BD twin was installed in Iron Duke in the summer of 1936. Trials showed that it could fire 11 rnds/gun/min.
The 5.25in gun had enough of a punch to make it a viable surface weapon, and it could be fired fast enough at high angles (using power loading) to give it effective anti-aircraft capacity. Using a single calibre made a small general-purpose cruiser viable. The result was the Dido class, which was not considered an anti-aircraft cruiser (the Royal Navy considered the 4.5in a better pure anti-aircraft weapon). This was much the same logic which led the US Navy to adopt a dual-purpose 6in gun for its planned prewar 8000-ton light cruiser (the tonnage of which was limited by treaty). The Royal Navy considered arming its own 8000-tonner with 5.25in guns, but turned back to single-purpose 6in guns and 4in anti-aircraft guns in its Fiji class. Euryalus is shown in 1941.
There was still interest in something more powerful. In 1935 a new 5.25in gun firing an 80lb shell with a separate cartridge was proposed. Preliminary trials were successful, and DNO reported that if further trials confirmed that, this gun would supersede the 4.5in. Trials having confirmed that the gun could fire separate AA ammunition, a twin mounting was designed, and in 1936 it was scheduled for trials on board Iron Duke late in 1937 or early in 1938. Both it and the 4.5in were used as dual-purpose capital ship secondary weapons.
Meanwhile the