Genesis and construction
The First World War saw the United States assume the mantle of one of the world’s leading naval superpowers. As an international mediator in naval re-armament, the US played a leading role in the determination of naval battle fleets post-war.
After the Five-Power Treaty was signed in Washington DC on 6 February 1922, there remained 18 ships under American flag in service, including as many as 12 new super-dreadnoughts.
During the mid 1930s US Navy planners gave serious consideration to the significance of the tactical capabilities of fast assault naval groups, and to the growing importance of air power and aircraft carriers. There was a greater emphasis placed on the need to construct new battleships, capable of attaining high speeds and combining great fire power and armor.
In the face of constantly deteriorating international relations in the mid 1930s, a decision was made to introduce small changes to the technical specifications of a battleship then on the drawing boards. In its final version the ship’s standard displacement met treaty limitations, with a hull length of 217.6 m (728 ft), a beam of 32 m and a draught of 9.8 m (approaching 38 feet). The ship’s steam turbine engines were capable of developing 115,000 SHP. The battleship was to be equipped with twelve 356mm (14”) L/50 Mk-11 guns mounted in 3 quadruple turrets. The full technical specification of the new battleships was approved on May 4, 1937 and as early as August of the same year, shipyards were contracted to start laying down the keel. The battleship, hull number BB-55, to be named the USS “North Carolina”, was to be built by the New York Navy Yard, and an identical sister vessel (BB-56), the USS “Washington”, by the Philadelphia Navy Yard.
Following Japan’s renunciation of the disarmament treaties and intelligence reports that Japan had commenced work on new, big battleships2 (their details were unknown at the time) the United States felt able to take advantage of an escalation clause. In mid-1937 a decision was made to arm the battleships with 9 new, powerful 406 mm (16”) L/45 Mk-6 guns. This tardy decision made it impossible to use the armor protection designed for the main armament3. Some armor plate sheets were already manufactured and delivered to the yards, and using new ones would not only increase the displacement to well over an estimated 35,000 ts4, but also result in delays in construction.
All in all, some small improvements to the armor system were introduced, especially in ammunition chamber areas. It had become evident that the standard displacement of 35,000ts as stated in the treaty could not be met. As a result there were attempts made to make weight reductions. Among other things, this was largely achieved through modern production technologies, including the mating of the hull plates and the assembly of mechanical elements. As a result as many as 35% of all joints were electrically welded, which, together with the lighter and more powerful than expected engines, enabled considerable weight savings of 330 ts (335 t)5. As the new, harder homogenous STS armor was used, it was possible to reduce the thickness of the hull plates from 25 mm (1”) to 19mm (3/4”) while maintaining more or less the same strength and resilience. Further weight reductions could only be achieved by reducing the main armor, an option which was obviously not retained.
As construction neared an end, actual standard displacement (BB-55) reached 37,818 ts (38,415 t), which was above the Washington Naval treaty limits6. However given the very tense international political situation at that time many politicians simply turned a blind eye to this fact. Hull number BB-55 was finally launched and named on 13 June 1940. It was sponsored by the daughter (Isabel Hoey) of the-then Governor of North Carolina,. For the next 10 months the battleship was fitted out and tested in the shipyard, to be commissioned on 9 April 1941 after a series of acceptance trials7 – one of the most powerful machines afloat in any navy.
The construction of the USS North Carolina was preceded by a number of model tests carried out in a special Taylor deck. The results made it possible to determine the hull’s drag and the power plant needed to reach a speed of 27 knots at battle displacement as was estimated in the project. The tests led to the development of a relatively long hull (222.1 m) with slender theoretical lines which caused some problems while designing the ship’s interior. To reach high stability and to increase the level of underwater protection, the hull was equipped with up-to-date streamlined anti-torpedo bulges. These bulges were over 1.3 m wide on each side of the hull. Starting at a lower edge of the board armor, they extended as far as the triple bilge. As with older ships, the board armor was mounted to the side plating. To reduce the hydrodynamic drag, impacted by the board armor mounting and bulges, a large cast bow bulb was mounted.
To avoid flooding of the bow deck, a special raked stem was designed (a slightly different design from older battleships) with widely open bow frames at the top. To ease the waterflow and to improve explosion durability of the shaft and propeller system two solid skegs were mounted at the stern. These were specially strengthened under-hull covers enclosing the end parts of the inner propeller shafts. There were two end pieces of outer drive shafts running on both sides of the skegs (one on each side). In their stream there were two solid rudder blades mounted with the area of 28.1 m2 extending over 36.5 degrees towards each of the boards. This resulted in the creation of a hull with very good naval characteristics at battle (44,795 ts) or full load displacement (46,800 ts). The battleship was navigable and easily manoeuvrable. At the maximum speed of about 27 knots a complete turning radius was merely 625 m, which made it one of the most easily manoeuvrable battleships in the world.
As with earlier US Navy battleships, the hull’s beam reached its maximum not far from bow and stern. This shape and construction enabled, amongst others, the ability to deepen the port and starboard anti-torpedo defence systems and its unification on particular frames8, and proved its worth over many years of service when the ship was at a sea for months in all possible weather conditions – even in hurricanes and typhoons. Deep draught (ca. 10.5 m at battle displacement) and a beam of 31.852 m at the waterline made the USS North Carolina a very stable artillery platform, a characteristic feature of the majority of American battleships.
The outer hull plating comprised sheets of special 19‑25 mm STS armored steel (homogenous), and 16-19 mm thick tensile strength (HTS) steel was used for underwater sections. There were six decks on the ship: main deck – bomb deck, second deck – main armored deck, third deck – plunging fire deck, two discontinuous platform decks (called first and second respectively), and a hold water deck. The distance between the decks, with the exception of plunging fire deck, reached 2.52 m (2.41 m at the edges, near the boards).
Innovative and compact turrets were designed for the USS North Carolina to enable the ship’s guns the widest possible firing angles. The vessel was a formidable weapons platform – with all of her main and subsidiary guns in action, the North Carolina could bring a full complement of 128 gun barrels to bear.
Along with the associated and modern opto-electronics this made cabin space very limited. There was a solid, elliptic main command post tower in the frontal part of the conning tower. The inside of the tower and the citadel were connected by a 406 mm (16”) communication tube. The main command post was partially built over by a navigation bridge with a wheelhouse. At the top of the bridge there was an Mk-37 sight director to control secondary artillery fire (as well as main artillery if need be). Unlike other types of American battleships, the bridge was not paned. There was only a row of slightly bigger than usual portholes, with special steel plunging fire covers on the outside.
Mounted atop the large fire control tower was a bow Mk-38 sight director for the main artillery. Not far from the bow conning tower there were two relatively thin standalone funnels for exhaust gases from the boilers (the first funnel was the slightly taller of the two). On both sides of the first funnel was located a Mk-38 sight director for controlling main and secondary artillery fire. Caps were fitted to the tops of both funnels to direct exhaust fumes towards the stern and protect fire control posts from smoke. The space between the funnels was initially used to mount boats and launches. At a later stage they were replaced with 40 mm (1.75”) L/56 Mk-2 Bofors anti-aircraft guns and their associated sight directors and fire control radars.
On the roof of a small stern conning tower another main artillery Mk-38 sight director was installed. It was based significantly lower than the one at a bow to protect the fire control post from smoke and the necessity to reduce the so called lifted weight. At the very end of the stern conning tower there was an Mk-37 long-range fire control sight, and on its sides the were solid cranes used for the lifting and lowering of boats. Interestingly both of these heavy devices survive on the battleship up to this day, although all life boats and launches were removed at the start of the Second World War.
Armor protection system
The USS North Carolina’s armor was to fully protect the battleship against 356 mm (14”) Mk-16 projectiles fired from Mk-10 and Mk-11 guns and 406 mm (16”) Mk-5 projectiles fired from Mk-5 L/45. The development of heavier Mk-8 APC projectiles and the new Mk-6 L/45, Mk-8 L/45 and Mk-7 L/50 guns in June 1939, caused the necessity of strengthened the ships’ passive defence. However, it was too late to introduce major adjustments to the design and new battleships were not fully defended against new types of projectiles. It has to be remembered, though, that the latest American heavy guns were exceptionally powerful weapons and there was no match for them in the world. Bearing this in mind, the North Carolina’s armor deserves the highest grades, especially that many of its elements could not be damaged with the latest guns.
Main 305 mm (12”) thick [under water, 1.6 m off the lower edge, the thickness decreased to 168 mm (6.6”)] and 5.5 m wide belt armor made of A3 class armor based upon „all or nothing” system was 136 m long. Starting at the front of main artillery gun turret number 1’s barbette, it ended at the stern barbette of tower number 3. The ship’s side armor was angled outwards 15 degrees to increase its effective thickness against incoming enemy shells.. Armored sheets were bolted to the side plating (19 mm of STS steel) with huge double-sided bolts made of NS steel, welded at nuts on both ends. There was a 50 mm wide gap filled with concrete between armored sheets and side plating, which served as a sealer. Near the ammunition chambers behind the main board armor, there was yet another belt, 95-51 mm (3.7-2”) thick, inclined by 10 degrees. There were two vertical cross bulkheads, 282 mm (11.1”) thick, protecting the citadel at the bow and at the stern.
From the very beginning, North Carolina was designed for medium and long distance combat, and an appropriate system of horizontal armor was incorporated. The first (upper) deck was designed to cause the detonation of lighter bombs and projectiles, before they could reach the second (main armored) deck. It was 37 mm (1.45”) thick and was made of STS steel. The second deck, that is the main armored deck, was constructed from a combination of 92-104 mm (3.6-4.1”) B2 class special armor and 36 mm (1.4”) STS steel. Its function was to stop (shatter or damage) heavy bombs and armor-piercing projectiles before they reached the lower deck. The third (lower) deck was made of 16-19 mm (5/8-3/4”) STS sheets. Its function was to stop the remains of the projectile, minimize the explosive’s effects and catching the shrapnel which could be propelled off the construction elements of the main armored deck. At the stern, near the steering gear, the lower deck was covered with two layers of sheets: 166 mm (4.6”) STS on top of 36mm (1.4”) STS.
The main armament and turrets were very heavily armored. The barbettes were made of 406 mm (16”) A3 class armor, sides of turrets of 249 mm (9.8”) on 19 mm (3/4”) STS plating, rear walls of 300 mm (11.8”) on the same kind of plating; the roofs made of 178 mm (7”) B2 class armor were attached to solid MS steel girders with 19 mm STS plating. Amazingly thick (406 mm of superior B2 class armor on 19 mm STS base), inclined at 35 degrees turrets’ front plates need to be noticed. The main command post was also heavily armored. There was 406-373 mm (16-17.7”) thick armor on the sides and 178 mm (7”) B2 class armor plating on the roof.
Secondary artillery gun turrets were equipped with light, 49.5 mm (1.95”) thick armor, the pilot house and navigation bridge were covered with 51 mm (2”) armor, and there was 37 mm (1.47”) plating protecting funnels, conning tower and sight directors. All these elements were made of STS Mode 2 class steel armor.
Underwater protection system
The USS North Carolina’s underwater defence was one of the strongest points of this otherwise outstanding ship. The hull was divided into 22 main compartments (sections), these in turn divided by 21 watertight solid cross bulkheads (without culverts) extending from the triple bottom to the upper deck. As with older battleships, the North Carolina owed its ‘unsinkability’ to its five hull sections – in practice this meant that even the complete flooding of any two out of five hull’s sections would not result in the loss of the ship. Lengthwise the battleship had a triple bottom designed to protect it from exploding mines and deep-set torpedoes. It was also a foundation for boilers, machines, generators and other mechanical equipment located near the boiler room and engine room. The tanks of the triple bottom were filled with fuel or ballast water.
Underwater site protection system consisted of five light, flexible longitudinal bulkheads and an up-to-date anti-torpedo bulge. The space between particular bulkheads was used as fuel or ballast water tanks, increasing the defence value of the entire system. The two peripheral compartments were always left empty (although it was possible to flood it with water, e.g. to counter-ballast the ship) and the three inner compartments were filled with liquid (fuel or water). According to the design, the minimal toughness point for the board defence system were 317 kg torpedoes filled with pure TNT, in practice however, the system could stop much more powerful torpedoes.
The layered underwater protection system proved to be the best possible solution against underwater weapons for large ships. It was effective, easy to repair and it provided the ship with the possibility of balancing any list quickly by flooding the appropriate tanks on the opposite side, and if all the bulkheads were damaged, it effectively reduced the scope of combat damage, an incredibly important feature for emergency squads. The system passed the test on September 15, 1942 when the USS North Carolina was hit by a powerful 95-shiki torpedo (550kg head filled with Hexyl – heksanitro-diphenylamine – and pure TNT – trinitrotoluene)9 launched from a Japanese I-15 submarine. The explosion tore open a large hole near turret number 1, at a location where the hull narrowing made the bulkhead system narrow as well (in addition there were only four bulkheads in this area). However, despite an area some 60-square meters being damaged, only a couple of compartments aside from the main anti-torpedo bulkhead were flooded (longitudinal bulkhead number 4). Only a total of 970 ts of water flooded the ship’s hull, and its flow brought about a 5.5 degree list, soon balanced by flooding tanks on the opposite board. Despite being hit, the damaged USS North Carolina remained at operational readiness and in line (although turret number 1 could be used only in extreme necessity). In comparison, when in December 1943 the big Japanese battleship Yamato was hit by a Mark 14 torpedo (270 kg of TNT only) launched from an American submarine the USS Skate, some 3000 ts of water flooded the hull although the explosion took place at the widest and thus best protected section of the ship’s hull!
In the early 1930s, the US Navy resumed development of reliable and damage-resistant steam turbine propulsion. While designing new motive units, years’ of experience with high-pressure steam boilers proved very useful. The Americans were constantly up-rating the engine rooms of their ships improving energy efficiency, thus increasing endurance for a given fuel supply. As there was a need to achieve huge power, the only option was to make use of the traditional propulsion arrangements, yet which were different from what was used by other countries. The key to the American’s success was the development of holicoidal, two-stage reduction gear coupled with high-speed steam turbines built by the General Electric Co. This solution enabled the designers to keep propeller rotations within limits acceptable from the hydrodynamic point of view. High-speed turbines were easily powered by high-specs steam (pressure, temperature), which, in turn, made it possible to use high-pressure steam boilers. The outstanding specifications of the latest generation of US Navy battleship engine rooms was made possible by progress in the science of metallurgy. The up-to-date construction of all the propulsion system’s elements made it possible to greatly overload the engine room. An advantage of all US Navy’s battleships built from 1920s on, was the implementation of 450 V tri-phase alternating current to power the machines.
The USS North Carolina’s engine room was of a “clean” type (Melville-MacAlpine system). That means that most of the machines, like turbines or motor-generator, were put into special containers. This increased the staff’s work comfort and safety and, to some extent, reduced bothersome vibrations. The entire power unit was placed in five compartments, divided by solid transverse bulkheads (without culverts). Four compartments housed two Babcock & Wilcox boilers each, adjacent General Electric steam turbine (with 1150 kW tri-phase alternating power generator and a switchboard) and a condenser. Each of the water tube, three shaft boilers produced steam at a pressure of 40.43 kG/cm2 and a temperature of 454.4 degrees centigrade. There was an auxiliary power room in the fifth compartment, together with spare 200kW combustion turbo-generators. The turbines were coupled with four asymmetrically placed propeller shafts (both propeller’s type and radius altered during the ship’s service). Propulsion control and supervision was run from the Power Room Control Centre located in engine room number 3. This automated room was at the same time the main Damage Control Centre.
When developing full engine room power of 121,000 SHP, the battleship could reach the speed of 27 knots in deep waters. The up-to-date construction, however, made possible one-hour overload by 20-25% to reach the power of 145,000 SHP and the maximum speed of 29 knots at the displacement of 44,794.7 ts. Unfortunately problems with vibrations caused by, amongst other things, insufficient care while constructing shafts line and choosing propellers, speeds exceeding 27 knots were shunned (the fastest recorded battleship speed at combat displacement reached 27.3 knots). The new battleships’ range was outstanding: at the maximum fuel reserve of 6859.7 ts USS North Carolina could travel 17,450 nautical miles at 15 knots without re-fueling.
406 mm (16”) L/45 Mk-6 guns – were the main weapons of the battleships of the North Carolina class (and also the following South Dakota class). Designed in 1936, the Mk-6 guns were easily among the best heavy naval guns produced – in deck penetration they were unmatched – not for nothing were they nicknamed „deck smashers”. Although they are not the biggest caliber guns ever – in the 20th century they were second only to 460 mm (18.1”) Japanese guns from the Yamato class battleships – they had the best ballistic specifications and records in their class while being very quick firing for such a big caliber. Launching Mk-8 armor-piercing hard-capped projectiles (APC) they were able to penetrate 10-meter thick reinforced concrete or pierce 664mm vertical armored plate made of A3 class face-hardened armor from a distance of 9.1 km. There were Mk-21 fuzes used with a delay of 0.033 second. 38 mm thick armored plate at the hit angle of 0 degrees or 9.5 mm at 65 degrees was sufficient to detonate each of the fuzes. […]
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