Unmunsan-class super-heavy cruiser

<imgur w="300px">u0y203m.png</imgur> Unmunsan as she appeared upon commissioning.
Class overview
Name:	Unmunsan-class cruiser
Operators:	Greater Menghean Empire
Preceded by:	Hwaju-class battlecruiser
Succeeded by:	none
Built:	1932-1936
In service:	1935-2005
Planned:	2
Completed:	2
Retired:	2
General characteristics Unmunsan, 1935
Type:	Battlecruiser
Displacement:	20,430 tonnes standard 23,700 tonnes full load
Length:	210.7 m at waterline 217.9 m overall
Beam:	24.6 m
Draught:	8.78 m (normal) 9.18 m (full load)
Propulsion:	4 steam turbines, 150,000 shp total 8 three-drum boilers 4 shafts
Speed:	34.4 knots
Range:	8,000 nm (14,800 km) at 15 knots
Complement:	1,042
Armament:	4 × 2 250mm L/55 Type 32 naval gun 6 × 2 100mm L/40 Type 28 AA gun 6 × 1 40mm pom-pom 16 × 1 12.5mm mg (8×1) 2 × 3 550mm trainable torpedo tube
Aircraft carried:	4 × Donghae Type 32 floatplane
Aviation facilities:	2 × catapult, recovery crane

The Unmunsan-class super-heavy cruisers (Menghean: 운문산급 초중 순양함 / 雲門山級超重巡洋艦, Unmunsan-gŭb Chojung Sunyangham) were a pair of warships built by the Greater Menghean Empire immediately prior to the outbreak of the Pan-Septentrion War. They were originally designed as replacements for the two Hwaju-class battlecruisers, which were set to reach the ends of their service lives in 1935 and 1936 respectively; however, as the Septentrion Nine-Power Naval Treaty collapsed in the middle of 1935, Menghe retained the Hwaju-class in service even after it launched the newly completed vessels. Designed as "cruiser-killers" with the guns and armor to defeat any treaty-limited heavy cruiser, they saw extensive service early in the Pan-Septentrion War, and engaged in commerce raiding missions in 1939 and 1940.

With standard displacements of 20,000 tonnes, the Unmunsan and her sister ship Sudŏksan were considerably larger than any other cruiser class in service or under construction at the time, the result of a special clause allowing Menghe to retain two ships in between cruiser and capital ship size. In Tyrannian-language service, they are sometimes referred to as battlecruisers, though they did not meet the original definition of a ship with battleship-grade guns and cruiser-grade armor. In Menghean wartime sources they were formally classified as "super-heavy cruisers" (chojung sunyangham), though in 1964 they were reclassified as "battlecruisers" (jŏntu sunyangham).

When Menghe surrendered to the Allied powers in November 1945, both ships in the class were still intact and fully serviceable, having waited out the final years of the war at anchor in Anchŏn. They were handed out to the victorious powers as war prizes, serving until 1955, when they were judged obsolete and returned to the Republic of Menghe Navy to serve as flagships. Near the end of the Menghean War of Liberation, the ships' crews mutinied and handed over the vessels to the newly-formed Democratic People's Republic of Menghe. There, they served as flagships for the Menghean People's Navy, undergoing additional modernizations in the 1970s. By the time of their retirement in 2006, they were the only surviving ships of the former Imperial Menghean Navy in service anywhere in the world. Both have been converted to museum ships.

Development

During the negotiation of the Septentrion Nine-Power Naval Treaty, Menghe successfully lobbied to retain its two Hwaju-class battlecruisers in service, as they had only recently been delivered from Tyran after emergency confiscation during the War of the Sylvan Succession. Under the compromise clause surrounding their inclusion, both Hwaju-class ships were to be re-fitted with guns not larger than 10 inches in caliber, and classified as "super-heavy cruisers." In 1932, the Imperial Menghean Navy declared that because 20 years had passed since the Hwaju was laid down, Menghe was permitted to begin construction of a replacement ship meeting the same armament and displacement regulations.

In contrast to the Hwaju-class, which were pre-war battlecruisers with replacement turrets, the new "super-heavy cruisers" were designed from the keel up as well-rounded modern combatants. Like the Madaesans and Taegisans before them, they were intended to fight enemy heavy cruisers, but the increased displacement limit allowed the Navy to invest in substantially improved capabilities. The armor scheme was considerably stronger, allowing engagements at closer ranges, and the main armament was increased to twelve 250mm guns in four triple turrets. Top speed on this initial proposal was projected at 30 knots, comparable to previous Menghean cruiser classes, and range was stated to be in excess of 5,000 nautical miles at a cruising speed of 15 knots.

The design of the Unmunsan class was interrupted by the April 11th incident, which resulted in the reshuffling of the Navy's doctrinal planners. While the old leadership favored a slow defensive fleet, the incoming officers considered offensive operations and particularly commerce raiding essential to a Menghean war against colonial powers, and ordered that the super-heavy cruiser be modified for increased speed and range. In order to keep standard displacement within the 20,000-ton limit, the main armament was reduced to eight 250mm guns in four twin turrets, and the armor scheme was slightly modified. The resulting design was projected to exceed 34 knots at full power, allowing it to outrun most heavy cruisers in rival navies.

In the context of Menghe's escalating war against Themiclesia in Dzhungestan and its leadership's aggressive rhetoric, the prospect that Menghe alone would possess state-of-the-art "super cruisers" stirred up heated international debate. The treaty did not formally specify whether Menghe could replace the Hwaju and Baekjin, leaving it ambiguous as to whether this was an interim category or a permanent one. Menghean diplomats dismissed legal challenges, arguing that they were politically motivated, and maintained that the ships' construction would continue with planned commissioning in 1935 and 1936. Some historians credit the "super cruiser debate" as one of the first steps on the road to the naval treaty's eventual collapse, along with Dayashina's falsification of displacement numbers and Rajland's concern over Ostish aggression.

Characteristics

Armament

When the Unmunsan was laid down in 1932, the new 200mm L/55 naval gun planned for the Hasŏlsan-class cruisers was already deemed sufficient to defeat all existing heavy cruisers' armor at probable combat ranges. The designers of the Unmunsan nevertheless insisted on a 250mm weapon, so that any future increase in cruiser armor would not render the new ship obsolete. As an added bonus, these shells could in theory penetrate many battleships' belt armor at ranges of under 10,000 meters, if the cruiser managed to close to such a hazardous range.

The resulting weapon was designated "250mm L/55 Type 34 naval gun." It had a theoretical maximum range of 38,210 meters when elevated to 40 degrees, the maximum elevation permitted by the turrets. The theoretical rate of fire is reported as 4 rounds per minute, but achieved rates of fire in trials were closer to 3 rounds per minute. Maximum ammunition stowage was 150 rounds per gun, and barrel life was 300 rounds. The following figures listing range and penetration characteristics are taken from Dae Meng Jeguk Haegun-e Jŏnham, and represent official Menghean figures for the Type 34 armor-piercing shell used during the war; belt armor penetration figures appear to have been rounded to the nearest 5 or 0.

Elevation Angle	Range (Type 34 AP)	Penetration (belt)	Penetration (deck)	Angle of fall
2.4 degrees	5,000 meters	440 mm	- - -	2.8 degrees
4.9 degrees	10,000 meters	325 mm	27 mm	6.1 degrees
8.2 degrees	15,000 meters	280 mm	45 mm	11.2 degrees
11.8 degrees	20,000 meters	230 mm	52 mm	19.5 degrees
17.7 degrees	25,000 meters	185 mm	64 mm	28.9 degrees
24.1 degrees	30,000 meters	150 mm	76 mm	36.8 degrees
33.2 degrees	35,000 meters	130 mm	102 mm	45.2 degrees
40 degrees	38,210 meters	- - -	117 mm	52.5 degrees

Like all late-interwar Menghean cruisers, the Unmunsans had an anti-aircraft armament built around the 100mm L/40 Type 28 dual-purpose gun, in this case mounted in six twin turrets. These had an AA ceiling of about 10,000 meters when firing timed-fuse HE shells, and a surface range of 14,600 meters when firing armor-piercing shells. These were "D-type" turrets, with assisted loading arms and ready racks on the sides. Like 100mm L/40 AA guns on other Menghean ships, they made an impressive impact early on due to their high rate of fire and thin-walled shells, but their poor range and accuracy limited their usefulness later in the war.

In the initial design proposal, short-range anti-air defense would have consisted of six 12.5mm water-cooled machine guns in single mounts. Combat experience off the coast of Innominada quickly demonstrated that 12.5mm guns were inadequate, and both ships were completed with sixteen 12.5mm MGs and six 40mm pom-pom guns removed from reserve storage as an interim measure. In 1939, after the Battle of the Porcullia Strait, they were refitted with Type 38 anti-aircraft guns, which greatly improved their effectiveness. Refits to intermediate-range AA would continue throughout the war as the threat from Allied carrier aviation grew.

Initially, torpedo armament consisted of two triple 550mm launchers, one per side. Six above-deck reloads were carried, but they had to be handled and loaded manually, a laborious process that could only be conducted while out of combat. The torpedoes used were the Type 23-II, with a maximum range of 11 kilometers at their lowest speed setting and 4 kilometers at their highest. After 1938, Dayashina agreed to sell Menghe designs for the Type 93 torpedo, which was produced in Menghe under the designation Type 38. This weapon was installed on the Unmunsan and Sudŏksan during their refits in late 1939, in quadruple launchers with an automatic reloading system.

Protection

The armor scheme of the Unmunsan-class cruisers was exceptionally strong, in many respects rivaling that of WSS-era battlecruisers. The main belt armor was 200 millimeters thick and angled inward at 15 degrees, with 100mm and 50mm belts extending beneath it to catch shells which penetrated through the water. Behind this was a 100mm turtleback plate angled at 30 degrees from the horizontal, decreasing to a 40 degree angle at the forward and aft ends of the citadel. These plates combined yielded a line-of-sight armor thickness of 400 millimeters, and the inner angled plate had a high likelihood of deflecting any shells which passed through the main belt, especially if the initial impact reduced the shell's velocity and stripped off its cap. In theory, this scheme allowed the cruisers to withstand close-range hits from battleship-caliber guns without suffering damage to the critical internal systems.

Ri Se-yŏng, a Vice-Admiral of the IMN who survived the war, alleged in postwar interviews that the aim of this design scheme was to allow the Unmunsan and Sudŏksan to attack enemy battleship formations in night battles, using the cover of darkness to close undetected to a range of 10,000 kilometers where their guns, armor, and torpedoes would be most effective. Subsequent research has not turned up any corroborating evidence of this claim, as many of the Navy's records were lost during the firebombing of Anchŏn, and several prominent historians consider it an exaggeration. Ri Se-yŏng himself passed away in 1957, and while he did serve at the Imperial Naval College in a doctrinal post from 1931 to 1934, his connections to the Unmunsan project remain unclear.

Apart from the thickened turtleback sections, the deck armor was 75 millimeters thick over the entire citadel and 50 millimeters thick over the entire quarterdeck, to cover the rudder and steering gear. The barbettes were 250 millimeters thick, though their effective protection was weaker than that of the citadel as they lacked the behind-armor layers. Turret armor was 300 millimeters thick on the face and cheeks, 125 millimeters thick on the sides, and 75 millimeters thick on the top and rear. Altogether, horizontal armor protection was comparable to that on the Madaesan and Taegisan classes, able to withstand plunging fire from 8-inch guns but still vulnerable to heavier weapons, especially with the addition of the angled sections.

Torpedo protection was also unusually strong for a cruiser, though still less effective than the protection schemes used on contemporary battleships. Air-filled watertight compartments formed the outermost layer, with the oil bunker providing additional cushioning further inward and the lower section of the main belt passing between them. A 20-millimeter bulkhead formed the innermost protective layer. As on many Menghean warships, the port and starboard boilers in each section were separated by a watertight bulkhead, which limited the volume of flooding but could create a dangerous list if one side flooded but not the others.

Propulsion

Power was provided by eight water-tube boilers, which fed steam to four geared turbines. The maximum design output was 150,000 horsepower, enough to reach a top speed of 34.4 knots. Though lower than the 35 knots projected on a few intermediate proposals, this was still more than adequate for pursuing enemy heavy cruisers, and enabled the Unmunsans to outrun all battleships and battlecruisers then in service. The hull design was also relatively modern for the time, incorporating a small bulbous bow and transom stern to reduce drag.

Cruising range was approximately 8,000 nautical miles (14,800 kilometers) at 15 knots, a high figure which reflected the Navy's plan to use the Unmunsans as commerce raiders in the Helian Ocean. With alongside refueling from a fleet oiler, this could be extended even further.

In spite of all their other refits, the Unmunsan and Sudŏksan retained their original engine machinery throughout their operational lives, as all of their subsequent operators deemed it prohibitively expensive to cut open the deck armor and install new purpose-built turbines or boilers. Already worn thin by long ocean patrols in the first half of the Pan-Septentrion War, the engines became a source of persistent maintenance problems from the 1960s onward, by which time replacement parts for them were no longer in production.

Aircraft

As the Unmunsans were intended to operate independently, or as the flagships of smaller cruiser formations, during long-range ocean raids, they were designed with relatively good on-board reconnaissance capabilities. Each ship could carry four single-engine reconnaissance floatplanes, two on the catapults and two behind them on rotating stands. Movement and recovery was accomplished by means of one large crane on each side. The initial reconnaissance floatplanes were Donghae Type 32 models, but over time the ships were assigned Namtong Type 36 and Yusin Type 41 floatplanes, reverse-compatible with the same handling equipment. The lack of enclosed hangars did leave the planes exposed to the elements - the Type 32 and Type 36 lacked folding wings - and increased the need for maintenance, somewhat limiting patrol time.

Ships in the class

Throughout their existence, the Menghean Navy and Imperial Menghean Navy did not have a unique naming system for super-heavy cruisers or battlecruisers. The Hwaju-class battlecruisers were named for cities, like Menghean battleships; yet the Unmunsans, which followed them, were named for mountains, like contemporary heavy cruisers.

The first ship of the class was named Unmunsan, after Mt. Unmun in Gangwŏn province, and her sole sister ship was named Sudŏksan after Mt. Sudŏk in Pyŏngsu province. Translated literally, the mountains' names are "gate to the clouds" and "restoration of virtue" respectively. According to surviving documents, the names Taesan and Hwasan were originally reserved for these ships, as they are the two most famous mountains in Menghe; but the Navy later reverted to the more conventional regulation of using three-syllable mountain names, possibly to avoid confusion with the cruisers Taegisan and Taean.

Name	Name (Stuart-Lavender)	Mengja	Laid down	Commissioned	Fate
Unmunsan	Woon Moon San	雲門山	1933	1935	Museum ship; docked in Anchŏn.
Sudŏksan	Soo Tohk San	修德山	1934	1937	Museum ship; docked in Sunju.

See also

• Hasŏlsan-class cruiser