4.7 inch 40 caliber QF Mark VIII BD
The 4.7"/40 QF Mark VIII BD mount is a never-were mounting proposed by the Royal Navy in the late 1920's which used the 4.7"/40 Mark VIII gun.
From Norman Friedman's The British Battleship 1906-1946:
By 1928 DNO was working on a new type of high-angle mounting, a BD cylinder often called a 'thimble'. Compared to an open mounting, it was blast-proof, hence could be mounted much closer to the main battery turrets. The crew was also better protected. Unlike an open mounting, it depended entirely on director control: there were no on-mounting sights. The 'thimble' first appeared in sketches of new battleship designs (see the next chapter), carrying the 4.7in high-angle gun wich armed the Nelsons. The new mounting, and the 4.7in high-angle gun already on board the Nelsons, figured in plans for anti-aircraft imrpvement. In April 1931 plans called for two 4in high-angle mountings on each side. The Anti-Aircraft Gunnery Committee, which was then meeting, wanted 4.7in mountings, preferably three each side. Preliminary examination of drawings showed that the ships could accomodate the 4.7in BD mounting, but only two on a side. A few days later DNC offered five twins for the Nelsons (one on the centerline) and six for Hood and the Renowns. The final report of the Naval Anti-Aircraft Gunnery Committee of 1931 added a requirement for eight (four twins) on the 15in battleships. For 4in guns, the alternative was a weather (upper) deck (UD) twin mounting with an open-backed shield, which was far easier to install. Its prototype was due in 1934 and (as of 1931) work on the first experimental twin 4.7in BD mounting was to begin in 1933. All ships should have two HACS, so they could engage two targets. DTSD was more conservative. Repulse was already being fitted with two twin 4in BD and four single 4in and Renown should be similarly fitted (but not until this mounting had been tested in Repulse). Hood should have two twin and four single 4in, all UD rather than BD mountings rather [sic]. The prototype twin 4in weather deck mounting was expected in 1934. The Nelsons should have two twin 4.7in BD and four singles. The 15in battleships should have two twin weather deck mountings and two single 4in (see below). DTSD's plan was approved in principle on 24 October 1932.
The prototype 4in Mk XVII 'thimble' was tested on board HMS Resolution in February and March 1931. These BD mountings were less than succesful. The guns were hand-loaded in accordance with Royal Navy policy that anti-aircraft guns should be entirely hand-operated. In cramped conditions, that was difficult; the prototype mounting in Resolution managed only eight rounds per minute (presumably per gun per minute). At the same time the Mediterranean Fleet was managing twenty rounds per gun from its single 4in high-angle weapons. It was no surprise, then, that when Repulse was taken in hand at Portsmouth (28 October 1938-9 March 1939) partly to provide accommodations for the King and Queen to visit Canada (and the United States), they were replaced by single 4in.
- Battleship Design 16A (proposed)
The drawing only shows the above-deck elements, which would be the only visible parts in a Shipbucket drawing. Deck penetration is considerable and ammounts in practical terms to a full-deck height space. The lower version has metal safety rails around the mount as seen on 4.5"/45 (11.4 cm) QF Marks I and III mounts used on some Royal Navy capital ships.
[http://navweaps.com/Weapons/WNBR_45-45_mk1.php 4.5"/45 (11.4 cm) QF Marks I, III and IV (Marks 2, 3, 4 and 5)]