In my opinion, this was Britain's best destroyer gun of World War II for surface action, although its low maximum elevation and speeds made it only marginally useful for anti-aircraft defense. Unfortunately, the cost of these destroyers was so much greater than the previous designs that only a few were built and newer destroyers reverted to the cheaper Mark IX gun.
The guns were in separate sleeves, allowing them to be elevated individually. The mountings had hydraulic training, ramming and shell hoists and were quite complicated, which resulted in long production delays and caused four of the L class destroyers to be completed with four twin 4" (10.2 cm) Mark XIX mounts in place of the 4.7" (12 cm) mounts.
In 1958 four "M" class destroyers were sold to Turkey and were extensively refitted. One of the modifications was to add power elevation, thus correcting one of the major deficiencies of this mounting.
Constructed of autofretted loose barrel, jacket to 85 inches (216 cm) from the muzzle, removable breech ring and sealing collar. Used a manually operated breech mechanism with semi-automatic breech opening. A total of 87 guns were manufactured.
Actual bore diameter of all British 4.7" guns was 4.724" (12 cm).
4.7"/50 (12 cm) Mark XI in Mark XX mounting
leaving the gun shop
Detailed Sketch of 4.7 in/50 (12 cm) Mark XX Mounting
See Mark XX Mounting
|Designation||4.7"/50 (12 cm) QF Mark XI|
|Ship Class Used On||L and M Classes|
|Date Of Design||about 1938|
|Date In Service||1940|
|Gun Weight||3.351 tons (3,405 kg)|
|Gun Length oa||247.7 in (6.292 m)|
|Bore Length||236.2 in (6.000 m)|
|Rifling Length||204.9 in (5.204 m)|
|Grooves||(38) 0.0365 in deep x 0.270 (0.927 x 6.86 mm)|
|Lands||0.1205 in (3.061 mm)|
|Twist||Uniform RH 1 in 30|
|Chamber Volume||670 in3 (10.98 dm3)|
|Rate Of Fire
|6 - 10 rounds per minute|
|Note: The firing cycle was nominally 6 seconds but large angles of train made loading more difficult and increased the cycle time.|
|Projectile Types and Weights||HE - 62 lbs. (28.12 kg)
SAP - 62 lbs. (28.12 kg)
|Propellant Charge||12.81 lbs. SC or 15.38 lbs. NF/S (5.81
or 7.0 kg)
Cartridge - 32.25 lbs. (14.63 kg)
|Muzzle Velocity||2,538 fps (774 mps)|
|Working Pressure||20.5 tons/in2 (3,230 kg/cm2)|
|Approximate Barrel Life||800 rounds|
|Ammunition stowage per gun||250 rounds|
1) This was the only weapon of this caliber to fire projectiles of modern design with 5/10crh heads and heavier weight.
2) Outfits included 150 SAP, 100 HE, 25 LA practice and 16 HA practice per gun plus 50 star shells for B mounting. Sixty special anti-submarine HE shells with RDX/TNT fillers were supplied to each ship by 1945 and the star shell allotment was increased to 150 - 200 rounds per ship.
|Elevation||With 62 lbs. (28.12 kg) HE Shell|
|Range @ 45 degrees||21,240 yards (19,420 m)|
|11,500 yards (10,500 m)||
|Note: Data from "British Battleships of World War Two." Assumes a perpendicular impact.|
L (3) and M (3): CP Mark XX
|Weight||Gunhouse including guns: 84,217
lbs. (38,200 kg)
Including mount base and hoists: 128,089 lbs. (58,100 kg)
|Elevation||-10 / +50 degrees|
(see Note 2)
|Manually operated, only|
|Train||about +150 / -150 degrees|
|Train Rate||10 degrees per second|
|Gun recoil||26.5 in (67.3 cm)|
1) Intended to save costs, an unusual feature of this mounting was that only the gunhouse rotated, the ammunition supply hoists did not rotate and were part of the fixed structure. This design necessitated that the ammunition and powder hoists needed to come up at the center of rotation, which explains why the guns are so widely separated in this mounting. Since the hoists did not rotate with the mounting, large angles of train caused difficulties with transferring ammunition to the guns and thus slowed down the ROF. "Destroyer Weapons of World War 2" notes that a true turret mounting including a rotating stalk would have almost certainly been rejected by the Exchequer for cost reasons. However, given the actual cost growth experienced during design and construction, one must wonder if the final cost of such a true turret mounting would have been much if any different from the final design. It almost certainly would have meant a smaller gunhouse and lower mounting weight.
2) The widely separated gun barrels meant that the guns had to be individually sleeved and thus dual elevation mechanisms were needed. As built, these were manually operated, as the extra weight involved ruled out power drives. Power elevation was fitted to the four "M" class destroyers sold to Turkey in 1958. I lack the elevation rate for these modified mountings.
3) The mountings were each powered by a 45 bhp (peak 102 bhp) electric motor and oil hydraulic pump on the fixed structure. Training was by a single hydraulic motor with two worm and pinion drives. Run out was powered by compressed air. Each gun had its own shell hoist and charge hoist. Shells and cartridges were placed by hand on the tilting tray of the fuze setter and then slid to the loading tray. The loading tray contained both the hydraulic and the manual rammers and was rotated by hand. Shell and cartridge were rammed together at any angle of elevation.
4) The gun axes were 96 in (244 cm) apart. How astonishingly wide this is may be appreciated by noting that the gun axes for the 15"/42 (38.1 cm) Mark II twin mounting used on HMS Hood were only two inches (5.1 cm) wider. The gun axes for the concurrent 4.7"/45 (12 cm) Mark CPXIX twin mounting used on other British destroyers were considerably narrower at 38 in (96.5 cm).
05 March 2007 - Benchmark
30 January 2009 - Added link, construction information and ammunition stowage and outfit quantities
09 September 2011 - Added mounting notes
24 November 2012 - Corrected typographical errors