|
The Mark I and Mark III guns were used in twin mountings and were interchangeable with each other, the only difference being in details of the firing mechanism. Several ships actually carried both gun types, depending upon what was available when they were being built or modernized. The Mark IV was ballistically identical to the Mark I and Mark III guns but slightly modified in order to fit on the standard 4.7" (12 cm) CP XXII single mountings used on destroyers. The Mark II was an Army AAA weapon, generally similar to the naval guns but not mounted afloat. Following the failure of the 5.1"/50 (13 cm) QF Mark I, the 4.5" (11.4 cm) caliber was selected in the middle 1930s for a new DP weapon for carriers, as it was believed that this was the largest caliber that could be used for a fixed round. The complete round for this weapon weighed 85 lbs. (38.6 kg) versus 108 lbs. (49 kg) for the complete round of the 5.1" (13 cm) gun. However, the decision on the basis of weight was contradictory for two reasons. First, the slightly lighter 4.7" (12 cm) ammunition for destroyers had always been made in separate form in order to reduce the task of the ammunition handlers. Second, a fixed round had been designed in the 1920s for the 4.7"/40 (12 cm) Mark XII anti-aircraft gun used on the Nelson class battleships and this weighed 74 lbs. (33.6 kg) complete. However, during service evaluations, it was found that this weapon could not maintain a high rate of fire - a necessity for an AA weapon - as the heavy round rapidly wore out the gun crews. So, if by the early 1930s, it was being found that a 74 lbs. (33.6 kg) fixed round was too heavy to allow sustained firing, then it is odd that a few years later an 85 lbs. (38.6 kg) fixed round was deemed to be acceptable. It would appear that the active Navy and the Admiralty had a considerable lack of communications regarding practical experience with fixed ammunition. Twin mountings used on carriers and capital ships built before and during World War II fired fixed ammunition. As noted above, this fixed round proved to be too heavy in service use and there was a tendency for the projectiles to separate from the cartridge cases during normal handling. These problems resulted in a rapidly decreasing rate of fire during prolonged firing periods. The mountings for the post-war built carriers Ark Royal and Eagle were modified versions using separate ammunition. On 8 May 1941 while Renown was engaging Italian torpedo bombers, a fail-safe lockout system malfunctioned and her P3 mounting fired into the back of P2 mounting, killing six and wounding twenty-six. Angle iron frameworks were later erected around the mountings to prevent a repeat. The fail-safe gear was overhauled during a refit at Rosyth later that same year. In 1944, guns of 4.7" (12 cm) caliber, which had been used on nearly every British destroyer built since 1918, gave way to a new destroyer weapon, the 4.5" (11.4 cm) QF Mark IV gun. It had originally been planned that destroyers would also use fixed ammunition, but reports from the cruisers Scylla and Charybdis told of loading problems during rough weather due to the heavy weight of the ammunition. As these were 6,000 ton (6,100 mt) cruisers, it was an obvious conclusion that the much lighter destroyers would have even worse handling problems - a conclusion that the Royal Navy had been aware of for over a decade. To alleviate the problem, a new 4.5" (11.4 cm) gun designed to use separate ammunition was developed for arming destroyers. Compared to the standard 4.7" (12 cm) Mark IX, this new destroyer weapon had a higher rate of fire and used a heavier projectile with better ballistic properties, but it is questionable if these advantages outweighed the disadvantage of adding yet another mid-caliber weapon and its specialized munitions to the logistical system. It did have a post-war advantage in that this new caliber became the standard size for Royal Navy destroyers and continues in use to the present day. Nomenclature note: In the 1950s the British weapon designation system changed from being per the gun itself to being per the mounting. At the same time, arabic numerals replaced roman numerals. Some confusion was created under this new system because older weapons were redesignated, even though the weapons and mountings themselves did not change. Under this new system, the "4.5-in (11.4 cm) Mark I, Mark III and Mark IV guns" in the "Mark II, Mark III, Mark IV and Mark V mountings" were re-designated as "Mark 2" and "Mark 3" for the twin mounts used on carriers and as "Mark 4" (twin) and "Mark 5" (single) for the mounts used on destroyers. Asterisks used in the old designation system to denote changes were carried forwards to the new system, but new changes were given "Mod" numbers. This meant that the Royal Navy now had designations such as "4.5-in Mark 5* Mod 2" which meant that this particular mounting had gone through three modifications since it was first designed. As could be expected, these changes have led to much confusion as to what weapons were actually used on any particular ship. For this reason, at the top of this datapage, I show both the original per-the-gun designation and, in parenthesis, the per-the-mounting redesignations. Gun construction was of an autofretted loose barrel, jacket, removable breech ring and sealing collar. The breech mechanism used a horizontal sliding breech block with hand-operated closing and semi-automatic opening. Mark I had both percussion and electric firing while the Mark III was designed for all-electric firing. The Mark I was later modified to permit electric only firing. The Mark IV used electric firing and the breech mechanism was considerably lighter with more generous radii on some components. Five experimental, 46 Mark I, 524 Mark III and at least 199 Mark IV guns were manufactured. Guns needed to be dismounted in order to change the barrels. It should be noted that these weapons had a relatively short barrel life. All British 4.5" naval guns have an actual bore diameter of 4.45" (11.3 cm). |
4.5" Mark II BD mountings on HMS Renown
|
| .
|