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

No.    Displacement    As designed    As completed
 1    Standard    7,100 T    8,300 T
 2    Normal    7,500 T    8,840 T
 3    2/3 trial    8,910 t    9,820 t
 4    Full load    -    10,583 t
 5    Light    -    7,814 t
*Data gathered on September 19, 1927


III. Armor protection

The armor above the waterline was supposed to protect the cruiser against 20 cm (8”) shells, although it never loved up to the expectations. The limitation imposed on the vessel’s displacement necessitated the use of thinner armor plates and reduced their use to only the most critical elements of the ship. The cruiser’s main armor belt was 79.88 m long and 4.12 wide. It ran from frame No. 105 to 241. The belt was made of 76 mm NVNC steel segments. The armor was attached to longitudinal No. 7 and was sloped at a 9 degree angle in relation to the middle deck. It protruded above the design waterline by 3.277 m. The middle deck in the engine room area was manufactured from 35 mm NVNC steel plates. The smoke stack uptakes were protected by 38 mm NVNC steel plates reaching up to about 1.28 m above the middle deck level. Additional upper deck armor protection was provided in the form of HT plates of varying thickness – from 19 to 48 mm. The walls of forward and aft magazines were covered with 51 mm NVNC plates and the floors and roofs were made of 35 mm NVNC plates. The conning tower lacked armor protection, only the steering room was covered with 12.7 – 25 mm steel plates. Armor protection below the waterline was limited to the fuel bulges just below the main armor belt. That design feature ruled out the use of thicker armor.

IV. Propulsion and machinery

Aoba’s powerplant consisted of four Mitsubishi-Parsons turbine assemblies delivering a total of 102,000 SHP. A single turbine set consisted of an impulse high pressure turbine rated at 12,500 SHP and a 13,000 SHP low pressure turbine connected via a gear reduction mechanism. The reduction gear reduced the high pressure turbine speed (3,000 rpm) and the low pressure turbine speed (2,000 rpm) to 360 rpm for each shaft and propeller arrangement. An astern turbine was incorporated in each low pressure turbine and had a rated power of 7,000 SHP (for a total power of 28,000 SHP). The ship’s powerplant was divided into four separate assemblies, each driving a shaft-propeller combination. Under combat conditions each assembly could operate independently from the other ones. The steam, pressurized to 18.3 kg/ cm2 and superheated to 156°C was delivered by 12 Kanpon “RO GO” boilers. Ten of those were oil-firing units, while two used oil or coal as fuel. The boilers were mounted in seven boiler rooms: to medium oil-firing boilers were installed in boiler room No.1, followed by 8 large boilers in four boiler rooms and a single boiler room housing two small alternative fuel units. Auxiliary powerplant consisted of four generators: two 90 kW and two 135 kW units, delivering a total of 450 kW of power. The generators were used to provide 225 V electrical power for the ship’s systems. The steering gear was powered by vertical steam engines, which powered hydraulic pumps used in the hydro-electric drive mechanisms. Aoba class cruisers carried 1,400 tons of oil and 400 tons of coal, which gave them the endurance of 7,000 nm at 14 knots.

V. Bridge structure

Details of the bridge superstructure as seen from the bow. Small turrets of the look-out posts are visible on each side with medium artillery Type 94 (94 Shiki) gun directors just behind them. Slightly above are the turrets housing 1.5 meter Type 14 navigation rangefinders. Visualisation 3D: Waldemar Góralski

Lessons learned from operational use of smaller vessels such as light cruisers were applied in the design of the bridge structures of the scout cruisers. The bridge had to accommodate all vital systems dedicated to steering and fire control, as well as defense against the surface, submarine and air threats. Aoba’s bridge was divided into six levels. The top level housed a firing control platform (shageki shiki sho) with the main Type 4 Hoiban gun director. Below the firing control platform was the main target survey platform (sokuteki sho). Just in front of it and on each side there were three Sokutekiban Type 13 target speed and course calculators. The compass bridge (rashin kankyo) was located just below. When the ship was first completed the compass bridge area was unprotected and a roof was added later in the ship’s service. Two 3.5 meter rangefinders were mounted on the tower next to the compass bridge. The two lowest levels of the bridge included an upper bridge (jobu kanko) and a lower bridge (kabu kankyo). Among the compartments located within the bridge structure were storage areas for sea maps and navigation equipment, command post, captain’s mess and various magazines for day and night observation equipment used on the ship.

VI. Armament