Furutaka-class cruiser

The Furutaka-class cruisers (古鷹型巡洋艦) were a class of two Japanese heavy cruisers which saw service during World War II. Both vessels of this class were sunk in 1942.

Design
The Furutaka class cruisers were the first heavy cruisers in the Imperial Japanese Navy, also referred to as “A class” cruisers in the I.J.N. Like the Yūbari, their design was the work of Constructor Captain Yuzuru Hiraga, assisted by Lt. Cmdr. Fujimoto Kikuo.

Designed to beat the U.S. Omaha-class cruisers and the British Hawkins class cruisers, they were as fast as the Omaha's (and nearly 4 knots faster than the Hawkins class), while firing a heavier broadside, and carrying a larger torpedo battery, than either one.

Their flush deck resulted in both weight savings and increased strength by allowing the hull's longitudinal members to be continuous. As with the Yūbari, the design featured side and deck armour integrated into the ship's structure, saving additional weight.

Despite the weight-saving efforts, as built the Kako was more than 900 tons heavier than its design weight. As a result, draft was increased by more than 1 metre, subsequently reducing top speed, and the height of the belt armour above the waterline. The portholes of the lowest-level crew quarters were near enough to the waterline that they needed to be closed when the ships were at sea, reducing ventilation and making the living spaces less habitable.

They were the first of the I.J.N. cruisers to feature a substantial bridge, with 6 distinct levels, providing support for navigation, fire control, communication and command.

Due to the high freeboard of these ships, mounting the torpedo tubes on the main deck would have caused the torpedoes to enter the water at too steep of an angle. Instead, they were mounted on the middle deck in 3 pairs of fixed tubes on each side. Captain Hiraga argued against this kind of mounting, concerned that during battle either a direct hit or fires could detonate the torpedoes, causing severe damage, as indeed happened with Furutaka of this class during WWII; additionally, the cruisers Mikuma, Suzuya and Chōkai, all of which featured similar arrangements, would all be sunk or severely damaged by their own exploding torpedoes.

Modernization
Japanese naval strategists since the early 1900s had planned for a defensive war in the Pacific, with the U.S. Navy as their main opponent. To take advantage of their superior long-range torpedoes, and offset the numerical superiority the U.S. Navy enjoyed, they extensively trained their crews in night torpedo tactics. In 1930, the Naval General Staff, further concerned by the limitations on the size of their navy by the London Naval Treaty won approval for an extensive modernization program of the “A class” cruisers. Planned upgrades to the ships included the latest weapons, protection, fire control systems, and communication equipment.

From 1931 to 1933 the two ships had their 4 original anti-aircraft guns replaced by improved 12 cm HA electro-hydraulically operated guns, with directors and range finders for them. The original airplane takeoff platform was replaced with a catapult for a reconnaissance seaplane.

They were substantially rebuilt in 1936-1937 (Kako) and 1937-1939 (Furutaka) as follows:

The 6×1 20 cm. main battery was replaced by 3 twin turrets housing the 8 in/50-cal guns. Light antiaircraft protection was augmented with 8 25 mm machine guns in 4 twin mounts. The 6 pairs (3 per side) of fixed torpedo tubes mounted on the middle deck were replaced with 2 quadruple mounts using the powerful Type 93 torpedo, located on the upper deck, one on each side of the catapult.

The bridge structure was completely rebuilt to accommodate the latest rangefinders and fire control equipment for the main battery, antiaircraft and torpedoes. Platforms were redesigned for aircraft spotters.

The twelve original mixed-fuel boilers were replaced by 10 large oil-fired units, along with a redesign of all the boiler rooms, and replacement of coal bunkers with fuel-oil tanks.

All of the new equipment resulted in increased electrical power requirements, so 3 more generators were added to increase power output from 315 kW to 885 kW.

The above modifications added 560 tons to the ships. To prevent the draft from increasing even more, and to improve stability, bulges were added, simultaneously enhancing antitorpedo protection. As a result, the ships' beam was increased to 16.92 m.