Hyangchun-class battleship

The Hyangchun-class battleships (Menghean: 향춘급 전함 / 香春級戰艦, Hyangchun-gŭb Jŏnham) were a group of three battleships produced in the Greater Menghean Empire during the Pan-Septentrion War. They were the last battleship class completed in Menghe, with the larger Insŏng-class battleships scrapped on their slipways to free up more steel for the Army's manufacturing needs. They also rivaled the preceding Chŏngdo-class battleships in displacement, though they were shorter in length and carried a lighter broadside.

When design work began on the Hyangchun class, the Septentrion Nine-Power Naval Treaty was still in effect, and new battleships in the 14-inch gun category were confined to a maximum displacement of 35,000 long tons. Working under this constraint, Menghean naval engineers produced a series of increasingly innovative designs intended to obtain the maximum possible performance from the displacement limit. Two main schools of thought emerged: one calling for a slow (27-knot) but heavily armed and well-protected design, and one calling for a fast (32-knot) ship which traded some armor and armament for speed. With the collapse of the treaty order in July 1935, shortly before the new ships were scheduled to be laid down, Menghean naval engineers were left with no limit on displacement and merged characteristics of both proposals into a well-balanced fast battleship displacing 43,000 tonnes at standard load. The first two ships following the revised design were laid down in 1936 and commissioned in 1941, and a third was commissioned in 1942.

Development

Treaty limitations

Under the terms of the Septentrion Nine-Power Naval Treaty, signed in 1923, the Menghean Navy was limited to seven battleships: three 35,000-ton ships with 14-inch guns (the Songrimsŏng class), two 35,000-ton ships with 16-inch guns (the Anchŏn class), and two 45,000-ton ships with 16-inch guns (the Chŏngdo class). Each battleship was given a minimum lifetime of 20 years, meaning that work on its replacement could only begin 20 years after the original ship's keel was laid down. Similar provisions were applied to the other signatories, though with different overall fleet limits.

The oldest ships in Menghe's "treaty fleet," the three Songrimsŏng-class battleships, had been laid down in 1915, 1916, and 1917, meaning that the first would become eligible for replacement in September 1935 and the other too soon afterward. Beginning in 1931, the Imperial Menghean Navy began to organize a series of competitions to solicit new battleship designs. Following the treaty rules, the new vessel would have to displace under 35,000 long tons when loaded with all supplies save for fuel and reserve boiler water, and it could not carry any gun over 14 inches (355.6 mm) in caliber. Speed, armor, and secondary armament were not limited.

The design competition received particular state emphasis in light of Menghe's changing political situation. Following Kwon Chong-hoon's military coup in 1927, Menghe's relations with the major Casaterran colonial powers had soured, and the Imperial Menghean Navy was preparing for the possibility of a war involving Rajland or New Tyran. This demanded maximum combat efficiency from any new battleship design, as it might have to go up against a numerically superior battle line composed of state-of-the-art Western warships. Navy officers and staff, however, remained divided over the best way to respond.

Equal Armament proposals

The first faction of design staff, headed by Rear Admiral Kim Gyŏng-sŏn, produced a series of "Equal Armament" proposals. These were based on the premise that the replacement battleships should carry the same firepower as the Songrimsŏngs: twelve 350mm guns in centerline turrets, and eight 150mm secondary guns on each broadside. Speed, likewise, was to be around 27 knots, allowing the new ships to keep up with the Anchŏns (and their replacements) in a combined battle line. Any spare displacement left over from more modern and efficient design practices would be used to improve the ships' armor and torpedo protection, with main belt thicknesses of 350mm and 400mm floated in early discussion papers.

A wide variety of Equal Armament proposals circulated in the early 1930s, including some proposed by civilian engineers. A few featured quadruple main battery turrets, and one, which received serious consideration, featured 200mm secondary turrets. All shared a main battery of twelve 350mm guns, as per the core requirements.

By 1933, the Equal Armament proposals had coalesced around a single conservative design known as "Battleship J1." This carried 350mm L/50 guns in four three-gun superfiring turrets, following a balanced AB+XY arrangement. The secondary battery was grouped into four quadruple 150mm turrets, two per side, at deck level. These had a maximum elevation of 40 degrees, and were exclusively for use against surface targets; anti-aircraft fire came from six twin 100mm L/40 turrets, three per side, one deck above the secondary turrets. The main belt was 350 millimeters thick, and the armor deck was 150 millimeters thick; against existing 16-inch shells, this was claimed to yield an immunity zone of 18,000 to 25,000 meters, though improved gun and shell designs introduced in the 1930s would have reduced the former figure. A 100,000-shp powerplant yielded a design speed of 27 knots, comparable to many contemporary treaty battleship designs and slightly faster than the ships they were intended to replace.

Fast Battleship proposals

Unfortunately for Kim Gyŏng-sŏn's "Equal Armament faction," by the time the J1 design reached maturity, political winds within the Navy had already shifted. The April 11th Incident threw out the Navy's former leadership, which had favored slow, short-ranged warships oriented toward a defensive battle, and installed younger officers who favored a more aggressive approach based on convoy raiding and fast interception. Kim Gyŏng-sŏn himself was spared due to his design expertise, but his new superiors rejected Battleship J1 and called for a faster design.

Two major proposals emerged from this requirement. The first, designated Battleship J2, borrowed from the J1 design but deleted the "X" turret, freeing up more space for a larger powerplant. The 150mm quadruple turrets were also removed; to allow for a sleeker hull form, the redundant anti-surface and anti-air guns were replaced by a single battery of sixteen 130mm guns in eight twin dual-purpose turrets, four per side. These changes were projected to yield a top speed of 31 knots, faster than any existing battleship or battlecruiser.

An alternative design, Battleship J3, pursued the more radical route of placing all three main battery turrets on the forward deck, with the B turret elevated to fire over the A turret. The armor scheme was also modified: 350mm belt and 150mm deck over the magazine spaces, and 300mm belt and 125mm deck over the machinery spaces. These changes led to further weight savings, and a projected top speed of at least 32 knots. The all-forward turret arrangement, though unconventional, was justified on the basis that it would still permit good firing angles when pursuing a target. Interestingly, the J3 design also introduced aft hangars for compact folding-wing floatplanes, though these appear to have been purely notional at the time; the Donghae Type 36 floatplane, planned as the next-generation replacement, was still in development and did not feature folding wings.

Initially, Kim Gyŏng-sŏn and the Equal Armament design faction reacted with strong opposition to the fast battleship concept, arguing that a switch to a nine-gun battleship would decrease the firepower available to the smaller Menghean fleet and undermine its effectiveness in a decisive line battle. The new designs did, however, prompt Kim to further streamline his original proposal, using a dual-purpose battery in place of the large 150mm quad turrets and a somewhat longer hullform with larger engine spaces. The resulting "Battleship J4," which still had four triple turrets in an AB+XY arrangement, had a projected top speed of 28 knots, fast enough to keep up with the Royal Navy's planned construction but still below the new leadership's expectations.

The final synthesis of the two factions came in late 1934, as the Navy leadership pressured them to reach a compromise which would offer the best of both sides. This resulted in the impressive "Battleship J5," the final culmination of the design process. It had three turrets in an AB+Y arrangement, but each of these carried four 350mm L/50 naval guns, keeping the total at twelve. This was still a more efficient layout than the J1 and J4 designs, and when combined with a redesigned hull and a dual-purpose flak battery, it allowed the installation of a 140,000 shp powerplant and a design speed of 30 knots. Belt armor was 350 millimeters thick across the entire citadel, and deck armor was 150 millimeters thick over both magazine and machinery spaces. These designs also introduced an angled internal belt, which the IMN had experimented with on the Taegisan-class cruisers; this increased the effective thickness of the belt armor against shells descending at moderate angles, and the 75mm lower belt doubled as torpedo protection.

Post-treaty modifications

Even after Menghe issued its declaration of war against Sylva in May 1935, the Imperial Menghean Navy's design board continued to refine the Battleship J5 design, with an eye to improving its capabilities. Turret side armor was thickened to 250 millimeters and magazine capacity increased to 130 shells per gun, and the size of the fuel bunkers was increased to allow for a longer cruising range. According to postwar estimates derived from surviving documents, the final iteration of the J5 design probably would have exceeded its official displacement by at least 2,000 tons if built, though the contemporary Hasŏlsan-class cruisers already surpassed their own limits by a higher margin.

Rajland and Ostland both withdrew from the Nine-Power Naval Treaty in the summer of 1935, followed by Dayashina; seeing that the treaty order's collapse was imminent, Menghe withdrew shortly afterward. As work had not yet begun on the first J5 battleship, Navy leaders decided to expand the design further to exploit the absence of a tonnage limit. The main battery was revised from quadruple 350mm turrets to triple 400mm turrets, yielding better armor-piercing performance at range...

See also

• Hanmaesan-class cruiser
• Boryŏng-class cruiser