text: tosa - class battleship the tosa - class battleships ( , tosa - gata ) were two dreadnoughts ordered as part of the " eight - eight " fleet for the imperial japanese navy ( ijn ) during the early 1920s . the ships were larger versions of the preceding nagato class , and carried an additional 41 - centimeter ( 16 . 1 in ) twin - gun turret . the design for the class served as a basis for the amagi - class battlecruisers . both ships were launched in late 1921 , but the first ship , tosa , was cancelled in accordance with the terms of the washington naval treaty before it could be completed , and was used in experiments testing the effectiveness of its armor scheme before being scuttled in the bungo channel . the hull of the second ship , kaga , was converted into an aircraft carrier of the same name . the carrier supported japanese troops in china during the second sino - japanese war of the late 1930s , and took part in the attack on pearl harbor on 7 december 1941 and the invasion of rabaul in the southwest pacific in january 1942 . the following month her aircraft participated in a combined carrier airstrike on darwin , australia , during the dutch east indies campaign . she was sunk during the battle of midway in 1942 . background the ijn believed that a modern battle fleet of eight battleships and eight armored cruisers was necessary for the defense of japan ; the government ratified that idea in 1907 . this policy was the genesis of the eight - eight fleet program , the development of a cohesive battle line of sixteen capital ships less than eight years old . advances in naval technology represented by the british battleship hms dreadnought and the battlecruiser hms invincible forced the ijn to reevaluate on several occasions which ships it considered " modern " and , in 1911 , it restarted the program with orders for the fusō - class dreadnoughts and the kongō - class battlecruisers . by 1915 , the ijn was halfway to its goal and wanted to order four more dreadnoughts , but the diet rejected the plan and authorized only the dreadnought nagato and two battlecruisers in the 1916 budget . later that year american president woodrow wilson announced plans for ten additional battleships and six battlecruisers , and the diet authorized three more dreadnoughts in response the following year : a second nagato - class battleship — mutsu — and two to a modified design , tosa and kaga . design and description the ijn began reevaluating the nagato design in light of lessons learned from the battle of jutland in may 1916 , experiments evaluating armor protection , and newly acquired information on the protective schemes of british and american capital ships . these lessons highlighted the need for better protection of the main gun turrets and magazines , as well as thicker deck armor to protect against plunging fire . existing methods of defense against mines and torpedoes had also proved to be inadequate and needed improvement . eleven new designs were rejected between october and early 1917 before captain yuzuru hiraga , superintendent of shipbuilding and the naval architect in charge of the fundamental design of the ships of the eight - eight fleet , presented a heavily modified version of the nagato design , a - 125 , to be built in lieu of the second ship of the class , mutsu , on 12 june 1917 , well before she was actually laid down . hiraga 's design for the ship reflected the latest combat experience as well as incorporating advances in boiler technology . it added an extra twin main - gun turret , using space and weight made available by the reduction of the number of boilers from 21 to 12 while the power remained the same . he reduced the secondary armament from 20 guns to 16 ; they were moved up a deck to improve their arcs of fire and their ability to shoot during heavy weather . to increase the ship 's protection he proposed to angle the belt armor outwards to improve its resistance to horizontal fire , and to thicken the lower deck armor and the torpedo bulkhead . hiraga also planned to add anti - torpedo bulges to improve the ship 's underwater protection . he estimated that his ship would displace as much as nagato , although it would cost about a million yen more . these changes would have considerably delayed the ship 's completion and were rejected by the navy ministry . the rejected design formed the basis for a much larger 39 , 000 - metric - ton ( 38 , 000 - long - ton ) battleship , designated as a - 127 , with nearly twice as much armor weight as the . it was designed to achieve the same speed as the older ships , to allow them to maneuver together as a tactical formation . this design was accepted on 27 march 1918 and became the tosa class . the tosa - class ships had a planned displacement of 39 , 900 t ( 39 , 300 long tons ) , and 44 , 200 t ( 43 , 500 long tons ) at a full load . they would have been 231 . 65 meters ( 760 . 0 ft ) long at the waterline , and 234 . 09 meters ( 768 . 0 ft ) overall ; the ships would have had a beam of 30 . 48 meters ( 100 . 0 ft ) and a draft of 9 . 39 meters ( 30 . 8 ft ) . the tosa class would have had a metacentric height of 1 . 292 meters ( 4 ft 2 . 9 in ) at normal load . a turbo - electric propulsion system was considered for these ships after the united states announced that the system was a great success in the battleship uss new mexico , and 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 ( 46 . 76 km / h ; 29 . 06 mph ) , a 2 , 500 - nautical - mile ( 4 , 600 km ; 2 , 900 mi ) range at full speed , and a 7 , 800 - nautical - mile ( 14 , 400 km ; 9 , 000 mi ) range at 14 knots ( 26 km / h ; 16 mph ) , but this system was rejected . more conventional curtis geared steam turbines were chosen , powered by 12 kampon water - tube boilers , eight of which would have used fuel oil and four of which would have used a mixture of oil and coal . this system would have provided 91 , 000 shaft horsepower ( 68 , 000 kw ) to four propeller shafts for a top speed of 26 . 5 knots ( 49 . 1 km / h ; 30 . 5 mph ) . the fuel stores would have amounted to 3 , 600 long tons ( 3 , 700 t ) of oil and 1 , 800 long tons ( 1 , 800 t ) of coal ; at a speed of 14 knots , this would have enabled a maximum range of 6 , 500 nautical miles ( 12 , 000 km ; 7 , 500 mi ) . armament the tosa - class ships were intended to be armed with a main battery of ten 45 - caliber 41 - centimeter guns in five twin turrets , four of which were superfiring fore and aft . numbered one through five from front to rear , the hydraulically powered turrets had an elevation range of − 2 to + 35 degrees . the rate of fire for the guns was around two rounds per minute . the ships were designed to carry 90 rounds per gun , although space was available for 110 . the guns used type 91 armor - piercing , capped shells . each of these shells weighed 1 , 020 kilograms ( 2 , 250 lb ) and had a muzzle velocity of 780 meters per second ( 2 , 600 ft / s ) . also available was a 936 - kilogram ( 2 , 064 lb ) high - explosive shell that had a muzzle velocity of 805 meters per second ( 2 , 640 ft / s ) . the ships ' secondary armament of twenty 50 - caliber 3rd year type 14 - centimeter guns would have been mounted in casemates , 12 on the upper sides of the hull and eight in the superstructure . the 3rd year type guns had a maximum range of 19 , 750 meters ( 21 , 600 yd ) at an elevation of + 35 degrees . each gun could fire a 38 - kilogram ( 84 lb ) high - explosive projectile at a rate up to 10 rounds per minute and was provided with 120 rounds . anti - aircraft defense was provided by four 40 - caliber 3rd year type 8 - centimeter aa guns in single mounts . the 3 - inch ( 76 mm ) high - angle guns had a maximum elevation of + 75 degrees , and a rate of fire of 13 to 20 rounds per minute . they fired a 6 kg ( 13 lb ) projectile with a muzzle velocity of 680 m / s ( 2 , 200 ft / s ) to a maximum height of 7 , 500 meters ( 24 , 600 ft ) . the guns were normally supplied with 250 rounds each , although space was available for a total of 400 rounds per gun . these 3rd year type guns were intended to be replaced by four 45 - caliber 12 - centimeter ( 4 . 7 in ) anti - aircraft guns . the were intended to mount eight 61 - centimeter ( 24 in ) torpedo tubes , four above water and four below . the former were to be provided with two torpedoes each and the latter with three each . armor the ships ' armor protection was designed to break up 16 - inch ( 406 mm ) shells from a distance of 15 , 000 – 20 , 000 meters ( 16 , 000 – 22 , 000 yd ) and the primary armor plates were backed up by splinter bulkheads intended to contain any shell fragments . they would have been protected by a waterline main belt of vickers cemented armor that sloped outwards 15 degrees at the top . amidships it would have been 280 mm ( 11 in ) thick and 254 mm ( 10 in ) thick fore and aft . approximately 1 . 83 meters ( 6 ft 0 in ) of the armor belt was below the waterline . the side armor was closed off at its ends by bulkheads 229 – 254 mm ( 9 – 10 in ) thick . the main battery turrets and the portions of the barbettes above the main deck would have had between 229 and 305 mm ( 9 . 0 and 12 . 0 in ) of armor plating , and the conning tower walls would have had armor 254 and 356 mm ( 10 . 0 and 14 . 0 in ) thick and a roof of 178 mm ( 7 . 0 in ) armor plates . the communications tube below the conning tower would have had walls 76 – 127 mm ( 3 . 0 – 5 . 0 in ) thick . the middle deck was the primary armored deck and was connected to the top of the armor belt . it would have consisted of a 63 . 5 mm ( 2 . 5 in ) plate of new vickers non - cemented armor on top of a 37 mm ( 1 . 5 in ) plate of high - tensile steel ( hts ) above the engine and boiler rooms . above the magazines , the thickness of the hts plate would have increased to 63 mm . the lower deck would have consisted of two 19 mm ( 0 . 75 in ) plates of hts . for the first time in a japanese ship , the would have had the lower portion of the single funnel protected by 229 mm of armor . in addition , the funnel openings in the lower deck would have been protected by armor gratings . the ships would have had an internal torpedo bulge to provide protection against underwater explosions . this was backed by a torpedo bulkhead also made up of three 25 mm ( 0 . 98 in ) layers of hts and angled outwards to meet the base of the waterline belt . it connected to a 12 . 7 – 32 mm ( 0 . 50 – 1 . 26 in ) splinter bulkhead on the lower deck behind the waterline belt . behind the torpedo bulge and the splinter bulkhead was another splinter bulkhead 12 . 7 – 19 mm thick . ships cancellation and fates construction of both ships began in 1920 , but the 1922 washington naval treaty intervened , mandating the cancellation of all capital ships being built . work stopped on the two tosa - class battleships on 5 february 1922 . after being stricken on 1 april 1924 , tosa 's incomplete hull was used to test her armor scheme against long - range naval gunfire , aerial bombs , mines , and torpedoes . two of the shells fired at her fell short , but deeply penetrated her hull through the thin armor of the torpedo bulge below the waterline armor belt . this sparked an interest in optimizing underwater performance of japanese shells that culminated in production of the type 91 armor - piercing shell . conversely , the ijn took measures to defend against shells of this type when reconstructing its existing battleships during the 1930s , as well as in the designs of the yamato - class battleships and the heavy cruisers of the mogami and tone classes . tosa 's torpedo defense system proved able to defeat 200 kg ( 440 lb ) torpedo warheads , but not larger 350 kg ( 770 lb ) ones . after the conclusion of the tests , the ship was scuttled by opening her kingston valves on 9 february 1925 in 650 m ( 2 , 130 ft ) of water in the bungo channel after the demolition charges failed to detonate . the battlecruiser amagi , which was being converted to an aircraft carrier under the terms of the treaty , was wrecked in the great kantō earthquake in 1923 and rendered unusable . as a result , kaga , which was originally slated to be scrapped under the terms of the washington naval treaty ( chapter i , article ix ) , was converted in amagi 's stead . no work took place until 1925 as new plans were drafted and earthquake damage to the yokosuka naval arsenal was repaired . although the ship was commissioned on 31 march 1928 , she did not join the combined fleet ( kantai ) until 30 november 1929 . much like the converted amagi - class battlecruiser 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 . as aircraft became heavier during the 1930s , they required longer distances to get airborne and the lower flight decks became useless . kaga 's 1935 reconstruction removed the lower two decks and extended the top flight deck to the bow . as completed , the ship had two main hangar decks and a third auxiliary hangar with a total capacity of 60 aircraft . kaga was provided with a heavy gun armament in case she was surprised by enemy cruisers and forced to give battle , but her large and vulnerable flight deck , hangars , and other features made her more of a target in any surface action than a fighting warship . carrier doctrine was still evolving at this time and the impracticability of carriers engaging in gun duels had not yet been realized . the ship was armed with ten 20 cm / 50 3rd year type guns : one twin - gun turret on each side of the middle flight deck and six in casemates aft . kaga 's waterline armored belt was reduced from 280 to 152 mm ( 11 . 0 to 6 . 0 in ) during her reconstruction and her deck armor was also reduced from 102 to 38 mm ( 4 . 0 to 1 . 5 in ) . the carrier displaced 26 , 900 long tons ( 27 , 300 t ) at standard load , and 33 , 693 long tons ( 34 , 234 t ) at full load , nearly 6 , 000 long tons ( 6 , 100 t ) less than her designed displacement as a battleship . this reduction in her displacement increased her speed to 27 . 5 knots ( 50 . 9 km / h ; 31 . 6 mph ) and gave her a range of 8 , 000 nautical miles ( 15 , 000 km ; 9 , 200 mi ) at 14 knots ( 26 km / h ; 16 mph ) . in 1933 – 35 kaga was rebuilt to increase her top speed , improve her exhaust systems , and adapt her flight decks to more modern , heavier aircraft . after the reconstruction , the ship displaced 38 , 200 long tons ( 38 , 800 t ) at standard load , better boilers gave her a top speed of 28 . 3 knots ( 52 . 4 km / h ; 32 . 6 mph ) , and additional fuel storage increased her range to 10 , 000 nautical miles ( 19 , 000 km ; 12 , 000 mi ) at 15 knots ( 28 km / h ; 17 mph ) and raised her aircraft capacity to 90 . the ten 20 cm ( 7 . 9 in ) guns , although now all mounted singly in casemates , were retained . kaga 's aircraft first supported japanese troops in china during the shanghai incident of 1932 and participated in the second sino - japanese war in the late 1930s . with five other fleet carriers , she took part in the pearl harbor raid in december 1941 and the invasion of rabaul in the southwest pacific in january 1942 . the following month her aircraft participated in a combined carrier airstrike on darwin , australia , helping secure the conquest of the dutch east indies by japanese forces . she missed the indian ocean raid in april as she had to return to japan for repairs after hitting a reef in february . following repairs , kaga rejoined the 1st air fleet for the attack on midway atoll in june 1942 . the ijn was surprised by the appearance of three american carriers and , partly due to admiral isoroku yamamoto 's plan in which ships were too dispersed to support each other , kaga , along with the other three carriers present , was sunk by aircraft from uss enterprise , hornet and yorktown on 4 june .