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Mogami Class Light Cruiser

Mogami running trials off Sukumo Bay in 1935 - the welded hull suffered serious damage during the trials
Mogami running trials off Sukumo Bay in 1935 - the welded hull suffered serious damage during the trials
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Ship Builder Laid Down Launched Completed Fate
Mogami Kure Dky 27 Oct 31 14 Mar 34 28 Jul 35 Lost 25 Oct 1944
Mikuma Kure Dky 24 Dec 31 31 May 34 29 Aug 35 Lost 6 Jun 1942
Suzuya Yokosuka Dky 11 Dec 33 20 Nov 34 31 Oct 37 Lost 25 Oct 1944
Kumano Kawasaki, Kobe 5 Apr 34 15 Oct 36 31 Oct 37 Lost 25 Nov 1944

Displacement: 8,500 tons/8,636 tonnes (standard); 11,169 tons/11,347 tonnes (full load).
Length: 661ft 1 in/201.5m (oa); 620ft 1in/189m (pp); 646ft 4in/197m (wi).
Beam: 59ft 1in/18m; Draught: 18ft 1in/5.Sm (mean).
Machinery: 4-Shaft geared turbines; 10 except Kumano & Suzuya 8 boilers.
Performance: 152,000shp=37kts; Bunkerage: 2,163 tons oil fuel.
Range: 8,150nm at 14kts.
Protection: 1in to 4in (machinery) main belt, 5.5in magazines; 1.25in to 2.25in main deck; 3in to 4in barbettes; 1in turrets.
Guns: fifteen 6.1in (5x3); eight 5in DP (4x2); eight 25mm (4x2); four 13.2mm MGs.
Torpedoes: twelve 24in (4x3).
Aircraft: three, two catapults.
Complement: 850.

Design

After the signing of the London Naval Treaty in April 1930, Japan built up to its limit in 8in-gunned ships. As far as the 6in-gunned ships were concerned, Japan's position was little better, because existing tonnage amounted to 98,415 tons standard, leaving only 2,035 tons for new construction. However, as much of the existing tonnage was either over-aged or replaceable by 1934, Japan could legitimately lay down new ships in that year. It was decided that four ships of 8,500 tons would be built before 1936, and two of 8,450 tons later. The new ships would replace the 'Improved Takaos' cancelled as a result of the London Naval Treaty. Work on the design was started in 1930, the basic requirements being fifteen 15.5cm (6.1in) guns in triple turrets with HA capability, and twelve 24in torpedo tubes. It was intended from the outset that the 6in guns would be replaced by 8in at the earliest opportunity, and the ship was designed with this in mind. The armour was to protect the magazines against 8in shellfire and the machinery against 6in shells. These requirements were all but the same as those of the 8in cruisers and, not suprisingly, proved impossible to attain on 8,500 tons, even with the adoption of welding on a large scale. In comparison with the previous cruisers, the hull of this class was very lightly built, even dangerously so, plate and scantling thicknesses being considerably reduced to save weight. Even so, the displacement as designed in 1931 was 9,500 tons. The actual design showed many similarities to that of Takao, but the measures taken to reduce weight low down in the ship, combined with the excessive height of the bridge and superstructure, reduced stability to a critical point.

The protection system, however, had some variations from that of the A class cruisers and also some similarities. The main belt over the machinery spaces was 100mm thick at its upper edge and tapered down to 65mm at half its depth. From this point it tapered further down to 30mm at its lower edge, the whole belt being inclined inwards from top to bottom by 20°. Its upper edge joined the sloped 60mm deck armour, while the lower portion behind the bulge acted as the antitorpedo defence. Abreast the magazine spaces the side armour was 140mm thick at its upper edge, tapering down to 30mm at the lower and inclined in the same manner as abreast the machinery. Transverse armoured bulkheads closed off the machinery and magazine spaces. The horizontal armour was 30mm with the sides sloped at 20° and 60mm in thickness. Above the magazines the deck was 40mm thick. Barbette armour was 75mm-100mm thick. The total armour weight was 2,061 tons, and slightly more in the later pair because their side belt was longer.

The machinery was a four-shaft geared turbine layout of a higher power, 152,000shp, than the previous ships, for a maximum speed of 37kts. There was also a difference in its arrangement, in that the forward turbines drove the inboard shafts. The first pair of ships had ten boilers, eight large and two small, all with superheating and air pre-heaters. The two small boilers were in the forward boiler room, while the remainder each had their own space, two abreast, separated by the now standard longitudinal bulkhead, which was continued through the turbine rooms. The later pair had only eight large boilers, forward boiler room being eliminated but without loss of power.

The main armament was disposed in five triple turrets, three forward, two aft, with C turret super-firing on A and B. The main gun was the 15.5cm (6.1in) 60cal 3 Nendo Shiki adopted in 1934 an firing a 123lb shell, with a maximum range of 30,000yd. The mountings, in triple turrets, had an elevation of 55°, the difficulty in having 70° and HA capability being recognised. The secondary armament was the standard 5in HA in twin mountings, while the light AA consisted of four twin mountings of 25mm and two twin 13mm MGs. It had been intended to ship a pair of 40mm Vickers guns (2pdr) but they were superseded by the 25mm. The triple 24in torpedo tubes were mounted on the upper deck at the after end of the superstructure (instead of near the bridge, as originally planned), and provided with a rapid reloading system of twelve reserve torpedoes, an improved version of that fitted in Takao.

The aircraft arrangements as completed differed from the original design intentions, which envisaged a similar layout to that of the Takao i.e. two catapults, two hangars for single aircraft and provision to embark four aeroplanes. The Tomozuru incident caused a rethink, however and the hangars were deleted, the superstructure deck being extended to provide parking space for aircraft, and their numbers being reduced to three.

Interesting pic of the smokestack of Mikuma, 1938. NH73030
Interesting pic of the smokestack of Mikuma, 1938. NH73030
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Modifications

After the loss of Tomozuru on 12 March 1934, a Board of Inquiry was convened, which reported its findings on 14 June that year. Measures were immediately taken to improve the stability of ships under construction, which included the Mogamis. As a result the bridge structure was redesigned and lowered, the hangars were deleted and the after superstructure reduced. The two later units also had their depths (keel to upper-deck height) reduced, as they were much less advanced than the first pair. As already mentioned, the torpedo tubes were moved further aft and the 5in guns altered to twin mountings. Mogami and Mikuma were quickly found to be still marginal on stability after trials and the 'Fourth Fleet Incident' on 26 September 1935, when many ships were damaged by a hurricane, which showed that they were also lacking in hull strength. By this time Suzuya had just begun initial trials, but these were aborted and all three completed units paid off, disarmed and laid up to await major modifications. Work on Kumano was stopped. After the Board of Inquiry reported, in April 1936, work on Kumano was resumed in the spring and the others reconstructed. Work lasted until October 1937 (Mikuma and Suzuya), and not until January 1938 was Mogami completed. This work saw the hulls considerably strengthened, much of the welded shell plating replaced by riveted plates, and larger bulges fitted. The superstructure was altered to reduce the effects of hull distortion on turret training. Among other things, the number of torpedoes was reduced to eighteen; only six reloads. These modifications added about 1,000 tons to the displacement.

After the collapse of the treaty limitations in the mid-1930s, Japan could implement the plans to up-gun these cruisers by replacing the triple 6.1in with twin 8in turrets. However, the need to manufacture turrets with the required roller path diameter to suit the barbettes of the Mogamis meant that the refits could not be started before 1939. This refit also included the replacement of the catapults by a heavier-capacity model and the fitting of the tubes for the 24in oxygen-propelled 'Long Lance' torpedoes. Twenty-four torpedoes were now to be accommodated.

Little further modification was then carried out until the end of the first year of hostilities, when Mogami, badly damaged by collision and bombing during the Battle of Midway in June 1942, was converted into an 'aircraft cruiser' during repairs at Sasebo between September 1942 and April 1943. This included the removal of the after 8in gun turrets and their replacement by a large aircraft deck, from which it was intended to operate eleven E16A1 'Paul' floatplanes. All eleven could be launched by the two catapults within 30min. The twin 25mm and 13mm guns were replaced by ten triple 25mm, and extra HA fire control equipment was added. Radar was also fitted for airsearch use.

After the Solomons campaign in February 1943, Kumano and Suzuya both received radar sets and had their light AA increased to 20 25mm (4 x 3, 4 x 2). The 13mm twin were landed. Plans to convert both ships to pure AA cruisers by the removal of all or part of the 8in guns and the fitting of extra 5in HA were not carried through.

In the first quarter of 1944 the three survivors had their light AA increased again by the fitting of eight single 25mm to give Mogami 10 x 3 and 8 x 1 mountings and the other pair 4 x 3, 4 x 2 and 8 x I 25mm. In June this was further augmented to 14 x 3, 18 x 1 in Mogami; 8 x 3, 4 x 2 and 24 x 1 in Kumano, and 8 x 3, 4 x 2 and 18 x 1 in Suzuya. Additional radar was fitted.

Service

Kumano
Kumano
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Kumano
Kumano
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Kumano, great photo
Kumano, great photo
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Suzuya
Suzuya
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Mikuma at sea in 1938, seen from a sister ship. Note that the foreground ship still has her original battery of triple 155mm guns. NH73033
Mikuma at sea in 1938, seen from a sister ship. Note that the foreground ship still has her original battery of triple 155mm guns. NH73033
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Mikuma in 1939, anchored in Kagoshima Harbor
Mikuma in 1939, anchored in Kagoshima Harbor
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Mogami
Mogami
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After commissioning, Mogami and Mikuma formed the 7th Cruiser Squadron, temporarily attached to the 4th fleet. As noted earlier, they were rebuilt following this deployment and as a result of the modifications did not see service until the end of 1937. When Suzuya and Kumano joined the fleet, they and Mikuma formed the 7th Cruiser Squadron and deployed to Chinese waters. Not until 1 May 1940 were all four together as the 7th Cruiser Squadron, following which, in January 1941, they were deployed to Indo-China to put pressure on Vichy France after the Thai-French action in that month. Later, in July, they supported Japanese operations in French Indo-China. When war began in the Pacific the 7th Cruiser squadron covered the Japanese landings in Malaya, Borneo, Sumatra, Java and the Andaman islands. In an action off Batavia on 28 February/1 March 1942, Mikuma and Mogami sank the US cruiser Houston and the Perth (RAN) with guns and torpedoes, several of the latter, five out of a spread of six from Mogami, in fact, also sinking a Japanese minesweeper and four army transports totalling 31,461grt. In April 1942 the squadron was part of the Indian Ocean Raiding force which sank eight Allied merchant ships totalling 48,664grt. All ships returned to home waters to refit after the initial successes, and became operational again in May 1942 to participate in the Battle of Midway. The 7th Cruiser Squadron was tasked with covering the invasion convoy, and left Guam on 29 May to join it. On 5 June the Squadron was ordered to bombard Midway itself, but these orders were cancelled, and on their return the cruisers were intercepted by the US submarine Tambour. While this boat was unable to make an attack, Mogami rammed Mikuma in the confusion, causing damage to both.

wreck of Mikuma - afternoon of the 7 June 1942 - she had taken major damage from at least 4 1,000lb bombs, which ignited torpedoes stowed below the HA-gun deck. She sank at some point during the night.
wreck of Mikuma - afternoon of the 7 June 1942 - she had taken major damage from at least 4 1,000lb bombs, which ignited torpedoes stowed below the HA-gun deck. She sank at some point during the night.
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The two undamaged ships retired, leaving the lame ducks to be caught by US aircraft the following day. Attacked by waves of aircraft from the carriers Yorktown, Hornet and Enterprise, Mikuma was reduced to a blazing wreck, sinking that evening, and Mogami very badly damaged, suffering many casualties. She finally limped into Truk on 14 June and arrived in Sasebo on 11 August for repairs. These, and her conversion to an aircraft cruiser, lasted until April 1943. The other two ships joined the 3rd Fleet in July 1942, and in that month made a cruise into the Bay of Bengal. After the US landings in Guadalcanal in August they moved to that theatre, to support Japanese defence of the island. Both took part in the Battle of Santa Cruz in October. After the Guadalcanal campaign they returned to home waters to refit, and later, together with Mogami, were again ordered to the Solomon Islands, where Kumano was damaged by US Marine aircraft on 20 July 1943 off Kolombangara. Repairs at Kure took until the end of October. Mogami, too, was damaged by aircraft off Rabaul on 5 November, putting her out of service until mid-February 1944. In 1944 they participated in the defence of the Marianas and, finally, in the battle for Leyte Gulf, when Mogami was attached to Adm Nishimura's Southern Force and both Suzuya and Kumano to Kurita's Northern Force. Mogami was damaged by gunfire from the cruisers Louisville, Portland and Denver in the Surigao Straits on 25 October, then damaged again in collision with Nachi, and later further damaged by US carrier aircraft of TG77.4 as she limped away to be finished off by a torpedo from Akebono south-west of Panaon Island in Mindanao Sea about midday. Kumano was damaged by a torpedo from Johnston on the morning of 25 October while attacking the US escort carrier force, and retired towards the Bernardino Straits, where she was damaged again by dive bombers. She survived further attacks by aircraft in the Sibuyan Sea on 26 October and reached Manila safely. Suzuya was damaged by carrier aircraft on 25 October, also while attacking the escort carriers. Further air attacks rendered the ship unmanoeuvrable and badly on fire off Samar, and she was sent to the bottom by Okinami after torpedoes and ammunition exploded. Kumano, after provisional repairs at Manila, sailed for Japan on 4 November but was intercepted off Luzon by US submarines, being hit by torpedoes from Ray. She managed to reach Santa Cruz in Luzon, where repairs were begun but on 25 November she was attacked and sunk by aircraft from Ticonderoga in Dasol Bay, having been hit by four bombs and five torpedoes.

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