Heavy Cruiser Aoba

Details of the bridge structure. The main gun Type 94 director and the accompanying 6 meter Type 14 rangefinder are mounted at the top. Visualisation 3D: Waldemar Góralski

The increasing importance of carrier aviation in naval warfare forced the brass of Imperial Japanese Navy to look for more sophisticated AA defense systems for their fleet. One of the new weapons was a 12 cm AA gun that was to replace the obsolete 8 cm model. After the weapon successfully completed a series of trials in 1927, it was introduced as standard anti-aircraft gun on Japanese warships. The gun was designed by engineer C. Hada and Cmdr. M. Oyamada was put in charge of its production at Kure and Mururoan. Engineer N. Sawamura oversaw the production of gun turrets at Kure, Yokosuka and Sasebo. The guns were designed to fire cartridge ammunition. The weapon’s specifications are presented in Table 4.

Table 4. 12 cm gun specifications
No.    Parameter    Data
 1    Designation    Type 10
 2    Caliber of bore    12 cm
 3    Actual caliber    12 cm (4,72 in)
 4    Length in calibers    45 cal
 5    Weight with breech    2,900 kg (6,400 lb)
 6    Length of barrel    540 cm (212.5 in)
 7    Length overall    560.4 cm (221 in)
 8    Barrel design    built-up – model IX5
 9    Breech mechanism    horizontal sliding
10    Number of grooves    34 (1.45 mm x 6.688 mm)
11    Length of lead section    464.93 cm (188 in)
12    Length of breech     65.6 cm (25.8 in)
13    Chamber volume    10.774 l
14    Powder charge    powder charge in the cartridge
15    Muzzle velocity     825 m/sec (common shell)
16    Bore pressure (max)     26.4 – 26.5 kg/mm2
17    Powder gas pressure    5.3 kg/mm2
18    AP shell weight     20.4 kg (45 lb)
19    Incendiary shell weight     16.4 kg (35.8 lb)
20    Shrapnel shell weight    5.55 kg (17 lb)
21    Fuze weight    0.02 kg
22    Maximum range    16,000 m
23    Maximum firing height    10,000 m
24    Barrel life    700 – 1,000 rounds

C. 25 mm anti-aircraft artillery
After the reconstruction completed on September 30, 1940 both cruisers were outfitted with standard 25 mm anti-aircraft batteries which consisted of four twin Type 96 25 mm cannons. Each gun had an ammunition supply of 2,600 rounds plus a 100 round reserve. The air cooled guns were top fed from 15 round magazines and had a theoretical rate of fire of 220 rounds per minute. Fire control was provided via Type 95 directors. The available types of ammunition included HE, tracer, incendiary and AP rounds. As a rule one tracer round was loaded per 4 – 5 common rounds to facilitate fire control. Eight ammunition carriers supplied cartridges to ammunition boxes. The full description of the weapon is presented in Table 5.

Bow installation of the 25 mm cannons and the close-ups showing a twin and triple gun arrangement.  Visualisation 3D: Waldemar Góralski

Table 5. 25 mm anti-aircraft gun specifications
No.    Parameter    Data
 1    Designation    96 shiki 25 mm Kiju 1 gata
 2    Caliber of bore    25 mm
 3    Actual caliber    25 mm (±0.04 mm)
 4    Length in calibers    60 cal
 5    Weight with breech    115 kg
 6    Length of barrel    150 cm
 7    Length overall    240 cm
 8    Barrel design    monobloc
 9    Breech mechanism    monobloc
10    Number of grooves    12 (0.25 mm × 3.58 mm)
11    Length of lead section    135 cm
12    Length of breech     24.23 cm (9.54 in)
13    Chamber volume    1.190 l
14    Powder charge    powder charge in the cartridge
15    Muzzle velocity     900 m/s (250 gram shell)
16    Bore pressure (max)     27 kg/mm2
17    Powder gas pressure    ?? [?]
18    AP shell weight     0.567 kg
19    Common shell weight     0.243–0.262 kg
20    Shrapnel shell weight    0.102 kg (105–110 g) *
21    Fuze weight    ?? [?]
22    Maximum range    7,500 m
23    Maximum firing height    5,500 m (5,250 m) *
24    Barrel life    3,000–15,000 rounds
* Depending on the source of data

Complementing the cruiser’s AA weapons fit were two Lewis 7.7 mm machine guns mounted on single mounts.

D. Torpedo launchers
Aoba’s torpedo armament featured six sets of twin Type 12 (1923) 61 cm torpedo tubes. The tubes were of special design known as “side door opening and closing type”. The experimental launchers measured 9.11 m in length and weighed 7.050 tons. Two twin torpedo tubes were installed between gun turret No. 2 and the bridge structure, the remaining four were located amidships. Loading the torpedoes into the bottom section of the launchers was performed using special hoists. In peacetime the ship carried 12 torpedoes loaded into the tubes. In combat conditions the torpedo load could be doubled. The situation changed after the cruiser’s reconstruction in 1940: new 61 cm torpedo launchers were grouped in two quadruple sets. The torpedo load was increased to 16 Type 90 weapons, later replaced by Type 93 torpedoes.

VII. Fire control systems

Japanese Imperial Navy warships featured two fire control systems. The first one – Kosha Sochi – was based on the Type 94 director and the Kosha Shagekiban artillery calculator. The system was used to direct anti-aircraft fire against high flying airborne threats. The other system – Hosen Shiki Sochi – was designed for anti-aircraft fire control against low flying aircraft. The systems were installed separately for weapons of different calibers. In the case of Aoba class cruisers fire control was performed using an integrated system. Main artillery was controlled using Type 94 director (Holiban 94) mounted at the top section of the bridge and a Type 92 artillery computer (Shagekiban 92). Standard Type 94 (Kosha Sochi 94) fire control system was used to direct 12 cm HA artillery fire. While working on the Myoko design, Cmdr. Fujimoto discovered that a pagoda-type bridge structure would be ideal for use on cruisers as it offered more stability for fire control systems. In theory it was a viable solution, although it had some serious drawbacks. Among other things, the design put the gun directors closer to the waterline. To remedy the situation Aoba’s bridge structure had to raised in comparison to Furutaka and Kako designs. […]

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