Amagi-class battlecruiser

Class overview
Operators	Imperial Japanese Navy
Preceded by	Kongō-class
Built	1920–1922
Planned	4
Completed	0, though one was converted into an aircraft carrier
Cancelled	4
Lost	1
Scrapped	3
General characteristics
Type	Battlecruiser
Displacement	Standard: 41,217 tons Full load: 47,000 tons
Length	826 ft 1 in (251.8 m)
Beam	101 ft (30.8 m)
Draft	31 ft (9.5 m)
Propulsion	4-shaft Gihon turbines, 19 Kampon boilers, 131,200 shp
Speed	30 knots
Range	8,000 nmi @ 14 knots
Complement	unknown
Armament	10 x 16 in (406 mm) L/45 guns 16 x 5.5 in (140 mm) L/50 guns 6 x 4.7 in (120 mm) L/45 AA guns 8 x 24 in (610 mm) TT
Armor	Main belt: 10 in (254 mm) Barbettes: 11–9 in (280–229 mm) Conning tower: 14–3 in (356–76 mm) Torpedo bulkhead: 2.875 in (73 mm) Deck: 3.875 in (98 mm)

The Amagi-class was a class of battlecruisers planned for the Imperial Japanese Navy as part of the so-called "Eight-Eight" fleet. The class was to have been composed of four ships: Amagi, Akagi, Atago, and Takao. The Amagi design was essentially an enlarged version of the Kaga-class battleship, but with a thinner armored belt and deck and a modified secondary battery arrangement.

However, the limitations imposed by the 1922 Washington Naval Treaty meant that the class had to be canceled. The treaty did allow for hulls under construction to be converted into aircraft carriers. Amagi and Akagi were both intended for conversion, but an earthquake damaged the hull of Amagi so extensively that Amagi was scrapped and the incomplete battleship Kaga was converted instead. Akagi served with distinction during World War II as part of the Kido Butai before being sunk at the Battle of Midway.

Design

Dimensions and machinery

The ships had a planned displacement of 41,217 tons and 47,000 tons at a full load. They would have been 820 ft 3 in (250 m) long at the waterline, and 826 ft 1 in (251.8 m) overall. The ships would have had a beam of 101 ft (30.8 m) and a draft of 31 ft (9.5 m) and would have used 4 propeller shafts, powered by Gihon turbines. The turbines would have been powered by 19 Kampon water-tube boilers, 11 of which would have been oil fired, while the other 8 would have mixed oil and coal for fuel. This system would have provided 131,200 shp for a top speed of 30 knots. The fuel stores would have amounted to 3,900 tons of oil and 2,500 tons of coal; at a speed of 14 kts, this would have enabled a maximum range of 8,000 nautical miles.

Armament

The ships of the class were planned to be equipped with a main battery of ten 16 in (406 mm) L/45 guns in five twin turrets, although an L/50 gun tested in 1920 might have been implemented instead. The guns fired 2,205 lbs. (1,000 kg) armor-piercing projectiles with a propellant charge weighing 494 lbs. (224 kg) at 2,592 fps (790 mps), at a rate of fire between 1.5 to 2.5 rounds per minute. Each gun had 90 rounds, and had an approximate barrel life of 250-300 shots. The turrets would have been arranged along the centerline: two superfiring turrets fore, and three in line aft of the superstructure. The gun turrets weighed 1,004 tons (1,020 mt), and allowed for depression down to -5 degrees and elevation to 30 degrees.

The secondary battery would have consisted of sixteen 5.5 in (140 mm) L/50 guns mounted in casemates along the center of the ship. These guns fired 83.8 lbs. (38.0 kg) projectiles and used between 22.8–24.2 lbs. (10.33–10.97 kg) of propellant at a muzzle velocity of 2,789–2,805 fps (850–855 mps). The guns had a maximum elevation of 25 degrees, which enabled a maximum range of 19,140 yards (17,500 m). Four—later increased to six—4.7 in (120 mm) L/45 anti-aircraft guns were to have been mounted amidships, along with eight 24 in (610 mm) above-water torpedo tubes.

Armor

The ships would have been protected by a main belt of armor 10 in (254 mm) thick, sloped at 12 degrees, and would have had a torpedo bulkhead 2.875 in (73 mm) thick. The main battery barbettes would have had between 9–11 in (229–280 mm) of armor plating, and the conning tower would have had armor ranging in thickness from 3 in (76 mm) to a maximum of 14 in (356 mm). The decks would have been 3.875 in (98 mm) thick.

Background

Experiences in the Russo-Japanese War convinced naval war planners that more fast capital ships were needed, so on 4 April 1907, the Imperial Defence Council approved an "Eight-Eight" policy. This plan originally called for a fleet of eight battleships and eight armored cruisers that would all be under ten years old (later changed to eight battlecruisers and reduced to eight years old). However, the advent of the dreadnought crippled this plan at the beginning; given Japan's economy and the enormous strain that had been put on it during the Russo-Japanese War (Japan emerged from the war victorious, but bankrupt), the launch of HMS Dreadnought was a "disaster" for Japan.

In 1907, Japan was nominally halfway to the eight-eight with two newly-delivered pre-dreadnought battleships (the Katori-class) in the fleet and two more pre-dreadnoughts (the Satsuma-class) and four armored cruisers authorized or under construction. In addition, three more battleships and four armored cruisers had been authorized, though not funded. However, naval technology was changing; armored cruisers were seemingly obsolete in the face of the new battlecruisers being laid down by Great Britain and Germany. The IJN recognized this, and so they proposed in 1909 that two battlecruisers be ordered from British plans, with one to be built in Great Britain and one to be built at home. These two ships became the Kongō-class.

In 1910, there was still authorization for one battleship and four armored cruisers. This battleship, the battleship version of the Kongo-class battlecruisers, became Japan's first super-dreadnought Fuso. With these ships, Japan appeared to be getting closer to the eight-eight goal; however, these new ships represented a "new level of naval strength" for the IJN, and they made all previous Japanese capital ships obsolete. This meant that any naval planner aiming for an eight-eight fleet would have to call for seven more battleships and four more battlecruisers when Japan was trying to weather a world economic depression.

After proposals from the IJN in 1911 and 1912 for massive shipbuilding programs, the Cabinet compromised down to a "four-four" plan; under this, three new battleships and no new battlecruisers were authorized. However, the Navy did not agree, and instead called for an "eight-four" fleet, while the Imperial Defence Council called for the original eight-eight. The Cabinet relented, and by July 1914, it was decided to go first for an eight-four fleet, followed by the eight-eight fleet. The eight-four plan was presented to the Diet in 1915; it planned to have the eight battleships and four battlecruisers by 1923 with the building of two Nagato-class and two Tosa-class battleships. The problem with this was the old plan that aimed for the ships to be under eight years old—by the time these ships were completed, Fuso and the first two Kongo ships would be past their replacement age.

Regardless, the plan was finally voted on in 1917 and approved, along with money for two battlecruisers which turned into the Amagi-class. In late 1917, the Navy proposed to expand the eight-four plan and add two more battlecruisers; this was approved, and the other two Amagi-class ships were ordered. However, having four 16-in battleships and four 16-in battlecruisers on order in addition to the operating cost of all of the other ships of the Navy put an enormous strain on Japan. At this time, Japan was spending about a third of the national budget on the Navy, and the massive size and scale of the building program was rapidly driving up the cost of naval construction and armament costs.

Construction, cancellation, and conversion

Akagi after her launch in April 1925; she had already been converted to an aircraft carrier

Akagi was the first ship of the class to be laid down, as she was on 6 December 1920 at the naval yard in Kure. Amagi followed ten days later at the Yokosuka naval yard. The projected completion dates for the first pair of ships were December and November of 1923, respectively. Atago was laid down in Kobe at the Kawasaki shipyard on 22 November 1921, and was projected to be finished in December 1924. Takao, the fourth and final ship of the class, was laid down at the Mitsubishi shipyard in Nagasaki on 19 December 1921, and was also projected to be completed in December 1924.

However, the Washington D.C. naval arms limitation treaty, signed in February 1922, greatly reduced the tonnage allowed for capital ships in the signatory nations. The treaty also instituted a moratorium on new warship construction; battlecruisers canceled under this included one class each from Japan, the United States, and Great Britain: this class, the Lexington-class and the G3 class, respectively. The treaty did allow for battleship and battlecruiser hulls currently under construction to be converted into aircraft carriers, but only if these new carriers were kept under a 27,000 ton limit. Considering that the Amagi-class were designed to displace 47,000 tons at full load in their battlecruiser configuration, this would have been a rather difficult displacement to obtain. However, the Americans also had the same problem when designing a conversion of their Lexington-class, so an exception, spearheaded by the United States' Assistant Secretary of the Navy Theodore Roosevelt Jr., was added to the treaty that gave the five signatories of the treaty the option of converting no more than two capital ships that were under construction to 33,000 ton aircraft carriers. As a result of this, the United States and Japan quickly reordered two ships each; Japan initially chose Amagi and Akagi, the two most complete ships of the class, for conversion. Atago and Takao were canceled on 31 July 1924, and broken up for scrap that year.

Akagi on trials in 1927; the ship originally had two short take-off decks and a main landing deck

However, the 1923 Great Kantō earthquake in Tokyo caused significant damage to the hull of the Amagi. The structure was too heavily damaged to be usable, and conversion work was abandoned. Amagi was stricken from the navy list and sold for scrapping, which began on 14 April 1924. Instead, the incomplete battleship Kaga, on which work had stopped on 5 February 1922, was reordered as a carrier to replace Amagi. Akagi was converted into an aircraft carrier, and was completed in March 1927. After undergoing a "massive" reconstruction in 1935–38, she conducted operations off China until taking part in the attack on Pearl Harbor on 7 December 1941 as Vice Admiral Chūichi Nagumo's flagship. Akagi was then sent to the Indian Ocean, where she took part in carrier raids, until she became the flagship of the First Carrier Striking Force, which was then sent to take part in the Battle of Midway. However, Akagi, along with the other three carriers of the force, were sunk in the battle.

References

• Friedman, Norman (1983). U.S. Aircraft Carriers: An Illustrated Design History. Annapolis, Maryland: Naval Institute Press. ISBN 0870-2-1739-9. (Google Books link)
• Gardiner, Robert; Gray, Randal, eds. (1984). Conway's All the World's Fighting Ships: 1906-1921. Annapolis: Naval Institute Press. ISBN 0870219073.

External links

• Haze Gray & Underway page on Japanese battlecruisers, dreadnoughts and semi-dreadnoughts