Unmunsan-class super-heavy cruiser

Class overview
Builders:	Gyŏngsan Songsu-do Naval Yard
Operators:	Menghe
Preceded by:	Hasŏlsan-class cruiser
Built:	1937-1942
In commission:	1942-2006
Planned:	2
Completed:	2
Retired:	2
Preserved:	2
General characteristics (as commissioned, 1942)
Type:	Cruiser
Displacement:	21,840t standard 26,400t full load
Length:	228.7 m (waterline)
Beam:	24.66 m
Draft:	9.15 m (full load)
Propulsion:	4 sets of steam turbines 160,000 shp total 10 M-type boilers 4 shafts
Speed:	34.7 knots
Range:	10,000 nautical miles (18,500 km) at 16 kts
Complement:	1,171
Armament:	4×II 250mm L/60 Type 37 naval gun 8×II 130mm L/60 Type 32 naval gun in Type 38 DP mount 12×II 37.5mm L/70 Type 38 AA gun in Type 41 mount 19×I 20mm Type 36 autocannon in Type 39 mount
Armour:	Main belt: 200mm Turret faces and barbettes: 200mm Conning tower: 200mm Citadel deck: 100mm Deck over steering: 75mm Torpedo bulkhead: 50mm
Aircraft carried:	3× Chanam Type 41 reconnaissance plane
Aviation facilities:	2× pyrotechnic catapult Hangar Elevator

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 shortly after the outbreak of the Pan-Septentrion War. Conceived to take advantage of Menghe's withdrawal from the Selkiö Naval Treaty, which had limited the displacement and armament of individual signatories' cruisers, they displaced over 26,000 tonnes at full load and carried a main battery of eight 250mm guns. In Menghean wartime sources they were formally classified as "super-heavy cruisers" (chojung sunyangham), though Anglian-language sources sometimes refer to them as battlecruisers or large cruisers.

Owing to Menghe's changing strategic situation in the late 1930s and early 1940s, the Unmunsan-class cruisers underwent a number of dramatic evolutions during their design and construction process. The idea to build a class of super-heavy cruisers upon exiting the Selkiö Naval Treaty in 1935 existed as far back as 1932, with preliminary plans for super-heavy cruisers drawn up in secret under the guise of hypothetical design exercises. In practice, however, these were held up by the development of a suitable 250mm gun, and Menghe's first post-treaty cruisers were the 200mm-armed Hasŏlsan class. Their intended role also evolved during the design process, from "cruiser-killers" to "super-cruiser-killers" and back, before eventually settling on the commerce raiding mission. As such, they were completed with relatively sparse command facilities and a 200mm main armor belt, but an impressive top speed of 34.7 knots.

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 Hyŏngnam. They were handed out to the victorious powers as war prizes, serving until 1955, when they were deemed obsolete and returned to the Republic of Menghe Navy in lieu of reparations. Near the end of the Menghean War of Liberation, the ships' crews mutinied in Gyŏngsan 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 to convert them to fleet flagships. 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

As originally signed in 1923, the Selkiö Naval Treaty imposed no limits on the construction of ships displacing under 12,000 tonnes and carrying guns under 8 inches in caliber, terms which led Menghe to pursue the construction of cruisers and light carriers to offset its relatively small force of eight capital ships. The Avallone Amendment to the treaty, signed in 1930, restricted this loophole by limiting the number of new heavy cruisers each signatory could build between 1931 and 1936. "Heavy cruiser," in the context of this treaty, was defined as a warship displacing over 8,000 tonnes standard. Menghe was limited to six new heavy cruisers, with the four Madaesan-class cruisers already under construction counted against this total.

While the completion of new cruisers within treaty limits led to the Taegisan-class cruisers, the new treaty restrictions quickly led to discussions in the Menghean Navy about what to do once the Avallone Amendment ended. In secret discussions with the upper Navy leadership, Kwon Chong-hoon set 1935 as the date for Menghe to be ready to commence a war against the Western powers, and through further discussion the Navy leadership agreed that Menghe should plan to exit the treaty in 1935 shortly after such a war's outbreak. In order to best take advantage of such a treaty collapse, Menghe would have to design post-treaty ships ahead of time in order to lay them down promptly after the start of such a war.

This requirement quickly raised problems. Most of these early "super-heavy cruiser" proposals incorporated 250mm or even 300mm guns, but development and testing of a naval gun with a caliber in excess of 8 inches (yet still too small to arm a capital ship) would clearly signal intent to violate the Selkiö Naval Treaty. The Taegisan-class cruisers also formed an awkward unit of two ships, whereas the Navy desired a division of four heavy cruisers for the fleet screen role. On this basis, procurement staff concluded that the first cruisers laid down after Menghe's withdrawal from the treaty would be armed with 200mm guns in a 4xIII configuration, becoming the Hasŏlsan class.

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