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

In order to follow the development of the scout cruiser design we now have to go back to June 1922 when Hiraga was promoted to the rank of rear admiral and sent abroad the following year. The oversees assignment was nothing else than an attempt to get rid of Hiraga who at that time was in conflict with members of the Navy General Staff. When Kikuo Fujimoto took over as the chief designer, Hiraga’s absence was used in an unprecedented way to introduce changes to his design. Gunreibu exerted pressure on Fujimoto to include twin 20 cm gun turrets in the design of the four 7,500 ton cruisers. Since the construction of the first two ships – Furutaka and Kako had already reached an advance stage, it was too late to incorporate those changes into the design. However, the remaining two ships could be modified, so Fujimoto complied with the Navy General Staff demand and altered the Kinugasa and Aoba designs to include the twin gun turrets. The ships were outfitted with three twin-gun armored turrets, two in the forward section of the ship and one aft. The anti-aircraft artillery now included four 12 cm L/45 guns in place of the original 8 cm weapons. The guns were placed amidships, on each side of the ship. The plans also included aircraft facilities complete with a 27 m catapult. A 10 m catapult section was to be placed on top of turret No. 4 and the remaining 17 m section was mounted behind a smoke stack and in front of turret No. 4. The catapult design was later changed in the Aoba and Kinugasa plans: the original unit had several drawbacks that needed to be eliminated, so it was replaced by a 19 m catapult. As a consequence of this modification the aft section of the bridge and the smoke stack on both vessels were redesigned. Finally the catapult was placed aft of the stern superstructure in front of turret No. 3. Both vessels were to carry a single Nakajima E4N2 seaplane.
Both ships were designed as cruisers with the normal displacement exceeding 7,000 tons and were officially classified as first class cruisers (itto junyokan), although the “A Class” classification (ko-kyű junyokan) was also commonly used. Aoba’s construction as No. 4 cruiser was officially authorized in March 1923. The order for the ship’s construction was placed in June 1923 and the cruiser was laid down on February 4, 1924. On September 25, 1926 Aoba was launched off slipway No. 2 at Nagasaki Shipbuilding and Engineering Co. The ship took her name from mount Aoba in Miyagi Prefecture. The official naming ceremony took place on September 18, 1923.
II. Hull characteristics
The hulls of the cruisers had a high length-to-beam ratio (11.72 m). The ships’ high bow line and freeboard along with the slim hull lines were expected to produce high top speeds in the Pacific waters. The design requirements of the cruisers demanded a high degree of flooding resistance which was to be achieved through the use of armored citadel and a sophisticated arrangement of watertight compartments below the waterline. The hull measured 185.166 m in length and had a maximum beam of 16.506 m. It was internally subdivided by transverse and longitudinal bulkheads into watertight compartments. To maintain internal rigidity of the hull structure extensive riveting was used in its construction with only a limited use of welding. The bulkheads were irregularly spaced throughout the hull and the exact distances between them is shown in Table 1.

Aoba’s amidships section. A quadruple revolving torpedo launcher protrudes from a hatch below the flight deck.  Visualisation 3D: Waldemar Góralski

Most of the hull structure was manufactured from HT steel, except main armor belt, middle deck and smoke stack armor plating above the middle deck, which were all made of NVNC steel. Individual structural members, including shell plating, frames, longitudinals, bottom plating, decks and bulkheads were made of HT steel of varying thickness – from 14 to 27.5 mm. The steel plates were mostly joined by rivets. Welding was limited to a minimum and only in certain parts of the hull to maintain the overall rigidity of the ship.
The hull was divided into the following compartments: the bow section housed the crew quarters and the officer and NCO mess areas, corridors and magazines. Below them, on the lower deck level, were ammunition magazines No. 1 and 2 serving main gun turrets No. 1 and 2 and 12 cm AA gun positions. Behind them, in an armored citadel, there were 7 boiler room compartments and four main engine rooms. Ammunition magazines No. 3 and 4 were located in the after section of the hull. Storage magazines and the steering gear rooms could be found in the same area, with additional crew quarters located above them. The exact dimension and displacement data is shown in Tables 1 and 2.

Table 1. Aoba class cruiser – dimensions
No.    Dimension    Value in meters
 1    Length (pp)    176.784
 2    Length (oa)    185.166
 3    Length (WL)    183.497
 4    Beam (maximum)    16.506
 5    Beam (WL)    15.48
 6    Freeboard (bow)     7.310
 7    Freeboard (amidships)     4.351
 8    Freeboard (stern)     3.348

Table 2. Displacement data*