J3 Class battlecruiser

The J3 class battlecruiser was a design study conducted during the Royal Navy's 1921 Fleet modernization programme. As a follow-on to the, the J3 class incorporated all the lessons learned from the First World War, specifically the battle of Jutland. The design was seen as an improvement to the Admiral class by virtue of its heavier and improved armouring scheme, specifically the deck armour, although the offensive armament remained roughly the same. The design was superseded by the I3 and G3 battlecruiser designs, as both mounted a heavier main armament, and further improved the protection scheme, on roughly the same tonnage. The 1921 fleet program was cancelled due to signing of the Washington Naval Treaty in 1922, which limited the size and armament of battleships to 35000 LT and no gun bigger than 16 in.

Background
In 1916 the US had declared its intention to create a Navy "second to none"; Congress had authorized the building of a large number of battleships and battlecruisers. In response, the Japanese government also began a large programme of warship building (the 8-8 fleet). Two improved hulls were rebuilt into the two s by the Royal Navy during the war. The only new capital ships laid down during the war were the s. Their design had been called into question after the Battle of Jutland in 1916, and three ships of this class were cancelled, leaving only HMS Hood (51) to be completed to a modified design.

The US plan had been delayed by the wartime need to build smaller vessels. Nevertheless, estimates by the Admiralty were that by the early 1920s the Royal Navy would be behind in ships. By the beginning of 1920 the Americans had completed one battleship since the end of World War I and had five more building. Seven more were intended to be laid down in 1920–21, six of which were the very large and powerful, armed with twelve 16-inch guns. The Japanese had finished one battleship since the end of the war and had three more under construction. To respond to this state of affairs, the Admiralty initially planned to build three battleships and one battlecruiser in the fiscal year 1921–22 and again in 1922–23, but this was changed to four battlecruisers to be built first, presumably to be followed by the same number of battleships the following year.

The British did have access to German technology through ships such as the battleship SMS Baden which had been saved from the scuttling of the interned German High Seas Fleet in Scapa Flow and the experiences of the war. A committee concluded that any new ship should be able to match the speed of the new US s, expected to make 32 knots. Consequently, a series of designs was prepared of ships with displacements ranging from 53100 to 44500 LT, the only limitations being the ability to use British dockyards and passage through the Suez Canal. These designs were given letters of the alphabet running backwards from K to G. The related battleship designs under consideration at the same time had design letters from L upwards.

The first two design proposals, 'K2' and 'K3', had a general layout similar to Hood, but were armed with either eight or nine 18-inch guns, in four twin or three triple gun turrets, respectively. The numeral in the designation came from the number of guns in each turret. These ships were very large, displacing 52000 to 53100 LT, could only reach 30 kn, and could only be docked in a single ex-German floating dock, and one dock, Gladstone Dock in Liverpool. The 'J3', the next proposal, saved nearly 10000 LT by reducing the main armament to nine 50-calibres long 15 in guns and the main deck armour to 4 in. Total armour was less than Hood but with increased power was expected to reach 32 kn. This reduction in size allowed the ship to dock anywhere that Hood could, and to pass through the Suez and Panama Canals.

The configuration was considered poorly armoured and a more compact design I3 - with machinery aft, magazines centrally disposed and the main armament forward -was drawn up by the DNC and presented in late 1920. A variation of I3, the G3 design, with reduced gun calibre and thinner armour over machinery was tendered as a means to reduce weight and hence increase speed.

General characteristics
The J3 battlecruisers were of very similar size to the Admiral class. They had an overall length of 860 ft, a beam of 104 ft, and a draught of 32 ft at deep load. They would have displaced 43100 LT normally and 48000 LT at deep load, over 2500 LT more than the older ships. The main benefit of the design was a reduction in the weight of the main armament but increase in firepower by converting from twin to triple mount turrets, with the additional weight savings being used to improve protection and armouring.

Propulsion
The J3 battlecruiser design had four geared steam turbine sets, each driving one propeller shaft, and arranged in two engine rooms. The forward engine room held the two turbines for the wing shafts, while the aft engine room contained the turbine for the port and starboard inner shafts. The turbines were powered by 18 Yarrow small-tube boilers divided between nine boiler rooms. They were designed to produce a total of 151000 shp at a working pressure of 200 psi and temperature of 200 °C with superheat. Maximum speed would have been 34 kn.

Armament
Housing the main armament in triple turrets was new to the Royal Navy, though British companies had been involved in the production of triple gun turret designs for other navies. The choice of a high muzzle velocity with a lighter shell was taken from the German practice; it ran counter to previous British guns such as the BL 15-inch Mark I gun of 42-calibre length, which were lower-muzzle-velocity weapons firing heavy shells.

The J3 design mounted nine 15-inch 50-calibre guns in three triple hydraulically powered Mark I gun turrets, designated 'A', 'B', and 'Z' from front to rear. The guns could be depressed to −3° and elevated to 40°. The ships' maximum stowage was 116 shells per gun. They fired 2048 lb projectiles at a muzzle velocity of 2670 ft/s. Their maximum range was about 38000 yd at maximum elevation. These weapons would have been very similar in construction to the BL 16-inch Mark I produced for the Nelson class battleships. The J3s carried a secondary armament of 12 BL 6-inch Mark XXII guns in twin turrets instead of casemates. the six turrets were sited amidships behind the main bridge structure and ahead of the Z turret. The guns could elevate between –5° and +60°.

An anti-aircraft battery of six QF 4.7-inch Mark VIII guns was included. They had a maximum depression of -5° and a maximum elevation of 90°. They fired a 50 lb high explosive shell at a muzzle velocity of 2457 ft/s at a rate of eight to twelve rounds per minute. The guns had a maximum anti-aircraft ceiling of 32000 ft, and effective range of 16,160 yards (14,780 m). A maximum of 256 rounds per gun could be carried. The ships were intended to carry four 8-barreled mountings for the 40 mm QF 2-pounder Mark VIII gun (commonly known as a "pom-pom"), two abaft the funnels and two at the stern. Each barrel was provided with 1,300 rounds of ammunition. The gun fired a 40 mm 2 lb shell at a muzzle velocity of 1920 ft/s to a distance of 3800 yd. The gun's rate of fire was nearly 100 rounds per minute.

Fire-control
The main guns of the J3 battlecruisers could be controlled from any of the three director-control towers (DCT). The primary DCT was mounted at the top of the forward superstructure. Another was mounted on the roof of the conning tower in an armoured hood, and the third was aft. Each main gun turret was provided with a 41 ft coincidence rangefinder in an armoured housing on the turret roof. The secondary armament was primarily controlled by three DCTs. Two mounted on each side of the bridge and the third was located aft of the secondary armament. The anti-aircraft guns were controlled by a high-angle control system mounted on the very top of the forward superstructure. Each "pom-pom" mount had its own director and there was also a height-finder aft. Two 15 ft torpedo rangefinders were located on the sides of the funnels.

Armour
The J3 was armoured similarly to the Hood, with bands of varying thickness. The armour was angled to up to 25 degrees on the belt, to increase relative thickness. One of the considerations that led to the cancellation of the J3 design was that the protection scheme did not use an "all or nothing" layout as later designs, such as the G3; this led to the J3 wasting tonnage on armour that would not have withstood incoming fire, a situation that the Admiralty found unacceptable.