Design for the Tosa class
|Operators:||Imperial Japanese Navy|
|Preceded by:||Nagato class|
|Succeeded by:||Kii class|
|Completed:||1; converted to an aircraft carrier (Kaga)|
|Displacement:||39,900 long tons (40,500 t) standard/44,200 long tons (44,900 t) full load|
|Length:||760 ft (230 m)|
|Beam:||100 ft (30 m)|
|Draught:||31 ft (9.4 m)|
|Propulsion:||4-shaft turbines, 12 Kampon boilers, 91,000 shaft horsepower (68,000 kW)|
|Speed:||26.5 knots (30.5 mph; 49.1 km/h)|
|Armament:||10x 16.1in (406mm)/45cal rifles
20x 5.5in (140mm)/50cal
4x 3in (76mm) AA
8x24in (610mm) torpedo tube
|Armour:||11in (280mm) belt, 4in (101mm) deck, 9-12in (228-305mm) barbettes, 14in (355mm) CT|
The Tosa class (加賀型戦艦 Kagagata Senkan )[A 1] battleships, known as Design A-127 during early planning, was a Imperial Japanese Navy dreadnought class planned as part of the "Eight-Eight" fleet. The ships were a larger version of the preceding Nagato-class, and mounted an additional dual 16 in (406 mm) gun turret. The ships were also larger, in both length and beam, and drew an additional 10 inches (25.4 cm) at full load. The design for the class also served as a basis for the contemporary Amagi class battlecruisers.
The Tosa class consisted of Tosa and Kaga. Both were canceled according to the terms of the Washington Naval Treaty before either could be completed. Tosa was sunk as a gunnery target in the Bungo Channel, while the incomplete hull of Kaga was converted into an aircraft carrier.[A 2] After having been converted into an aircraft carrier, Kaga took part in the attack on Pearl Harbor, before eventually being sunk at the Battle of Midway in 1942.
The ships had a planned displacement of 39,900 long tons (40,500 t), and 44,200 long tons (44,900 t) at a full load. They would have been 715 feet (218 m) long at the waterline, and 760 feet (230 m) overall; they would have had a beam of 100 feet (30 m) and a draft of 30 feet 10 inches (9.40 m). Although a turbo-electric drive was considered for these ships after the United States announced that the plant was a great success in the battleship USS New Mexico (the Japanese estimated that a 70,000 shaft horsepower (52,000 kW) turbo-electric plant could be installed in the Tosa class, which would have given the ships a speed of 25.25 knots (29.06 mph; 46.76 km/h), a 2,500 nautical miles (2,900 mi; 4,600 km) range at full speed, and a 7,800 nautical miles (9,000 mi; 14,400 km) range at 14 knots (16 mph; 26 km/h), it was rejected, and more conventional Curtis turbines were chosen. These turbines would have used 4 propeller shafts, and would have been powered by 12 Kampon water-tube boilers, 8 of which would have been oil fired, while the other 4 would have mixed oil and coal for fuel. This system would have provided 91,000 shaft horsepower (68,000 kW) for a top speed of 26.5 knots (30.5 mph; 49.1 km/h). The fuel stores would have amounted to 3,600 tons of oil and 1,800 tons of coal; at a speed of 14 knots, this would have enabled a maximum range of 6,500 nautical miles (7,500 mi; 12,000 km).
The ships of the class were planned to be equipped with a main battery of ten 16-inch (406 mm) L/45[A 3] guns in five twin turrets. The guns fired 2,205-pound (1,000 kg) armor-piercing projectiles with a propellant charge weighing 494 pounds (224 kg) at 2,592 feet per second (790 m/s), 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 to 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 long tons (1,020 t), and allowed for depression down to −5° and elevation to 30°.
The secondary battery would have consisted of twenty 5.5-inch (140 mm) L/50 guns mounted in casemates along the center of the ship. These guns fired 83.8-pound (38.0 kg) projectiles and used between 22.8 and 24.2 pounds (10.3 and 11.0 kg) of propellant at a muzzle velocity between 2,789 and 2,805 ft/s (850 and 855 m/s). The guns had a maximum elevation of 25°, which enabled a maximum range of 19,140 yards (10.87 mi; 17.50 km). Four—later increased to six—4.7-inch (120 mm) L/45 anti-aircraft guns were to have been mounted amidships, along with eight 24-inch (610 mm) above-water torpedo tubes.
The ships would have been protected by a main belt of armor 11 inches (280 mm) thick, sloped at 15°. The belt armor was designed to be able to defeat 16-inch (410 mm) shells from a distance of 12,000–20,000 meters (13,000–22,000 yd). The main battery turrets and barbettes would have had between 9 and 12 inches (230 and 300 mm) of armor plating, and the conning tower would have had armor 14 inches (360 mm) thick. The decks would have been 4 inches (100 mm) thick. The Tosa-class battleships would have had a torpedo bulkhead 3 inches (76 mm) thick, which connected at the top to a 1.5-inch (38 mm) splinter deck beneath the main deck.
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 Imperial Japanese Navy 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 ship 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 ships were laid down in 1920, with Tosa being laid down on 16 February 1920 at the Mitsubishi shipyard in Nagasaki, and Kaga following on 19 July 1920 at the Kawasaki shipyard in Kobe. Both were launched less than two years after they were laid down; Tosa on 18 December 1921 and Kaga on 17 November 1921. Even though Tosa was laid down first, she was scheduled to be completed in March 1923 while Kaga's planned completion date was 25 December 1922. However, the 1922 Washington Naval Treaty intervened; this mandated the cancellation of all naval ships under construction, including the two Tosa class battleships; construction stopped on both ships on 5 February 1922. After being stricken on 1 April 1924, Tosa was expended as a target in the Bungo Channels on 9 February 1925; she was used as practice for naval gunfire and to test her armor scheme against mines and torpedoes. Accounts of these tests mention that shells struck the ship below the waterline, which could explain later IJN interest in internal armor; the results from all of these tests was used to design the protection of the Yamato class battleships.
It was originally planned that Kaga would be scrapped. However, Amagi, one of the two Amagi class battlecruisers which were being converted to aircraft carriers, was severely damaged in the Great Kantō earthquake in 1923 and rendered unusable. This meant that her conversion under the terms of the treaty[A 4] was impossible, so Kaga was reordered as a carrier in her stead; the conversion took 5 years (1923–28).
As originally completed, Kaga seemed to combine the concept of an aircraft carrier with a cruiser; she mounted a cruiser-like ten 8-inch (200 mm)/50 caliber guns (two twin turrets mounted on each side of the middle flight deck and six mounted singly in casemates) and had a belt that was 11 in (280 mm) thick. The carrier officially displaced about 29,600 t, although the actual figure was likely closer to 30,000 t; she was capable of steaming for 8,000 nautical miles (15,000 km) when going 14 knots (26 km/h), though she was able to go at a maximum of 27.5 knots (50.9 km/h) if needed. Similar to the former Amagi class carrier Akagi, Kaga was fitted with two flying-off decks "stepped down" from a flight deck that extended two-thirds of the ship; in theory, this allowed planes to take off directly from the hangars while other planes landed on the top. However, because operations proved dangerous and were impossible to conduct in rough weather, a 1935 reconstruction removed the lower two decks and extended the top flight deck to the bow.
After the reconstruction, Kaga officially displaced 38,200 t (37,600 LT; 42,100 ST) standard; better boilers allowed her to obtain a top speed of around 28 knots (52 km/h), and the aircraft capacity was raised to 90. Interestingly, the ten 8-inch guns, although now all mounted singly in casemates, were retained. After supporting operations in China, Kaga took part in the attack on Pearl Harbor on 7 December 1941. She covered amphibious assaults for the first three months of 1942, although she hit a reef in February. After receiving repairs for that damage in April, she sailed to support the landings on Midway; however, the IJN was surprised by the appearance of three U.S. carriers, beginning the Battle of Midway. Due in part to Admiral Isoroku Yamamoto's plan, in which ships were dispersed and not close enough to mutually support one another, Kaga, along with the other three carriers present, were sunk by U.S. aircraft from USS Enterprise, Hornet and Yorktown.