British 4"/45 (10.2 cm) BL Marks IX and X

British
4"/45 (10.2 cm) BL Marks IX and X
Updated 19 July 2006

Admiral Fisher had always objected to heavy secondary batteries on capital ships, so, at his insistence, the 4" (10.2 cm) caliber was chosen for the secondary weapons on the battlecruisers of the Repulse and Courageous classes. Initially, either the 4" (10.2 cm) QF Mark V or the 4" (10.2 cm) BL Mark VIII were to have been installed, but the first weapon was difficult to arrange for director firing and the second had a low rate of fire. The best features of both, the Mark V body and the Mark VIII breech mechanism, were combined to create the Mark IX.

The battlecruisers used both single and triple mountings, with the latter having the guns individually sleeved, an unusual feature for a secondary weapon. This mounting required a very large crew of 32 and, as it lacked power assist, proved to be quite cumbersome.

During World War II, this weapon was used in single mountings on many corvettes as well as on numerous smaller ships.

Mark IX was of wire wound construction with tapered inner A tube and jacket. Mark IX* differed in having no inner A tube. Mark IX** had no inner A tube, a B tube and overlapping short jacket and an old-style wire-winding method. All had Welin breech blocks with Vickers mechanisms. Some 2,382 were built. Actual bore length was 44.35 calibers.

The Mark X was originally built for the Norwegian Nidaros class coastal defense ships which were taken over at the start of World War I. These were of Elswick Pattern T and were partly wire wound with coned breech block. Fifteen guns were built and were used only on DAMS.

The data that follows is specifically for the Mark IX, but the ballistics for the Mark X were similar.

Amidships Detail of HMS Renown showing two of the triple mountings
Note that the gun barrels could be independently elevated in these mountings

One of the forward triple mountings on HMS Renown
In the lower left corner is one of the single 4"/45 (10.2 cm) BL Mark IX guns

HMS Dianthus
Typical 4"/45 (10.2 cm) Mark IX on Flower Class Corvette
IWM Photograph

4"/45 (10.2 cm) Mark IX on Monitor HMS Sir John Moore
Note the shaded light on top of the shield, which was useful during night firings

Gun Characteristics .

Designation	4"/45 (10.2 cm) BL Mark IX and Mark X
Ship Class Used On	Mark IX World War I Renown and Courageous classes Inflexible as rearmed World War II Flower Corvettes, Bathurst Minesweepers and a variety of smaller ships Mark X DAMS
Date Of Design	About 1913
Date In Service	1916
Gun Weight	Without Breech Mechanism: 4,620 lbs. (2,096 kg)
Gun Length oa	184.6 in (4.689 m)
Bore Length	177.4 in (4.506 m)
Rifling Length	149.4 in (3.795 m)
Grooves	(32) 0.037 in deep x 0.270 (0.94 x 6.86 mm)
Lands	0.1227 in (3.117 mm)
Twist	Uniform RH 1 in 30
Chamber Volume	470.3 in³ (7.707 dm³)
Rate Of Fire	10 - 12 rounds per minute

Ammunition .

Type	Bag
Projectile Types and Weights	HE 1920 - 22 lbs. (10 kg) HE 1940 - 31 lbs. (14.1 kg) SAP - N/A
Bursting Charge	N/A
Projectile Length	N/A
Propellant Charge	World War I: 7.7 lbs. (3.5 kg) MD16 World War II: 7.9 lbs. (3.59 kg) SC103 or 9.39 lbs. (4.3 kg) NF/S164-048
Muzzle Velocity	2,625 fps (800 mps)
Working Pressure	18.5 tons/in² (2,910 kg/cm²)
Approximate Barrel Life	3,600 rounds
Ammunition stowage per gun	N/A
Notes: 1) SAP was 3crh. 2) Outfits included up to 100 star shells per ship.

Range .

Elevation	With 22 lbs. (10 kg) HE Shell
Range @ 30 degrees	13,500 yards (12,344 m)
Elevation	With 31 lbs. (14.1 kg) HE Shell
Range @ 30 degrees	13,840 yards (12,660 m)

Mount / Turret Data .

Designation	Triple Mount Renown (5) and Courageous (6): T.I. Mark I Single Mount Renown (2): CPI Smaller ships (1): CPI
Weight	T.I. Mark I (less shield): 17.475 tons (17.755 mt) CPI: 4.721 tons (4.797 mt)
Elevation	-10 / +30 degrees
Elevation Rate	Manually operated, only
Train	360 degrees
Train Rate	Manually operated, only
Gun recoil	N/A
Note: As mentioned above, the triple mount was a clumsy design. Quoting from "The Loss of Repulse and Prince of Wales" by A.E. Jacobs: "The triple mountings were always extremely difficult to train, and the general procedure when moving through a large arc was for the two trainers at the 'normal' and 'director' training wheels to be assisted by the remainder of the crew pushing on the breeches or muzzles."

Data from
"Big Gun Monitors: The History of the Design, Construction and Operation of the Royal Navy's Monitors" by Ian Buxton
"Naval Weapons of World War Two" and "British Naval Guns 1880-1945 No 17" article in "Warship Volume X" both by John Campbell
"British Battleships of World War Two" by Alan Raven and John Roberts