Akanagi-class Cruiser

## Akanagi-gata jun'yōkan (赤椰型巡洋艦)

Akanagi in 1927

The *Akanagi*-class (赤椰型巡洋艦- *Akanagi-gata jun'yōkan*) was a class of two heavy cruisers laid down for the Imperial Japanese Navy in 1924, that served in World War II.

## Background

The *Furutaka* class of heavy cruisers (also called "A class" cruisers in the Imperial Japanese Navy) was originally planned to consist of 4 ships, carrying six single 20cm (8 in) guns. In 1924 the IJN Naval General Staff pushed to use the newly designed twin 20 cm (8 in) gun turrets on the *Furutaka*-class ships. However, as the construction of the *Furutakas* was already well-underway, a decision was instead made to complete two *Furutakas* and two larger ships of a newer class.

Under the recently implemented Washington Naval Treaty, cruisers of a maximum displacement of 10000 tons were allowed, which was significantly higher than the intended 8300 ton displacement of the modified *Furutakas*. The Imperial Japanese Navy, fearing being out-constructed by larger navies, thus requested for designs for maximum tonnage cruisers, intent on constructing a ship with individual superiority in firepower, protection and speed over any opponent.

## Design

Initial proposals called for cruisers carrying 8 20cm/50 3rd Year Type No.1 guns, in 4 twin turrets. In order to create the level of individual superiority required, it would be necessary to increase firepower to 10 guns. This would prove to be the heaviest armament of any heavy cruiser in the world at the time, a level of firepower that would be maintained on subsequent Japanese cruiser classes. Complimenting the main battery, Akanagi carried 8 single 12cm/45 10th Year Type guns, able to engage both surface and air targets. Anticipating close range night exchanges to occur, *Akanagi* was designed with a heavy torpedo armament. 12 61cm torpedoes were carried in 4 triple tubes, two on each side, along with 6 reloads.

*Akanagi* had heavy armor for the period too, with a 105mm main belt running the full length of the machinery and magazines. The main citadel deck was 40mm thick. Outside of the main belt, a small strake of 40mm armor ran the length of the ship's stern, protecting the steering gear, along with a 25mm deck. The hull void was shallow, at 0.6m deep, with a single 60mm structural holding bulkhead. The main battery was more lightly armored, as such was normal Japanese practice- 25mm gunhouses and 60mm barbettes. The conning tower was also only 25mm thick.

With a length of just over 204m at the waterline, a beam of 19.25m and a normal draft of 6.7m, *Akanagi* appeared larger than most other cruisers of the era. Initially, analysts were in disbelief that her capabilities and size fell within the 10000 ton limit- it later turned out *Akanagi* did indeed displace exactly 9999 tons standard. *Akanagis* had a flush deck hullform, with the amidships sloped upwards, providing additional longitudinal strength, and a wide transom stern. This high length-beam ratio, fine hullform, and stern construction was necessary for the high speed of 33.5 knots, achieved efficiently from 8 Kampon oil-fired boilers providing 22000 horsepower to each of 4 geared steam turbines powering her shafts.

Impressive on paper, the *Akanagi* was not free of flaws. She carried no aircraft as-built, and while stable and structurally sound, refits would eventually have adverse effects on the performance of the class as the original hullform was already heavily optimized. The low freeboard also meant the ships were wet at the ends at top speed. Future Japanese cruiser classes were generally over treaty limits and had more leeway to be worked with.

## Ships in Class

### Akanagi **(赤椰)**

The lead ship of the class, named for Mount Akanagi in the Kiso Mountains, was laid down at the Mitsubishi Nagasaki Shipyard in May 1924. She was launched in March 1926 and commissioned in October 1926.

### Tokachi **(十勝)**

The second ship of the class, named for Mount Tokachi in Hokkaido, was laid down at the Kōbe-Kawasaki Shipbuilding Yard in June 1924, launched in April 1926 and commissioned in November 1926.