Even though it was not renewed through the follow-on London Naval Treaty in this alternate history, the Washington Naval Treaty of 1922 was the key factor in capital warship design from the time of its signing through to the end of the battleship era.
The Washington Naval Treaty
Under the 5:4:3 capital ship tonnage ratio agreed upon in this alternate timeline's version of the Washington Naval Treaty, the U.S. Navy is required to scrap all of its early dreadnoughts up to the New York class, but retains all of the Standard type battleships. Since this includes no less less than five 16" armed Colorados, the treaty terms also allow Japan to complete the construction of two Tosa class (for a total of four 16" battleships) and Great Britain to build a single class of four new 16" battleships replacing older tonnage.
In addition to the tonnage ratio, all signatory powers agreed to refrain from building any new battleships for the next ten years (other than those for the 16" agreement), and to subsequently only replace existing battleships at a minimum retirement age of 20 years. These future replacement battleships would be subject to a size limit of 35,000 tons standard displacement and an armament limit of main guns no larger than 16".
Treaty Refits
One of the provisions of the Washington Naval Treaty (both historical and alternate) is that existing warships may be refitted "for the purpose of providing means of defense against air and submarine attack", so long as tonnage does not increase by more than 3,000 tons and no alterations in side armor or main battery are made. The U.S. Navy originally plans to take full advantage of this provision by rotating its Standard type battleships through extensive refits from 1925 to 1931, with two refits being initiated each year in a similar pattern fo the USN's prior two-per-year construction of new battleships. However, the Great Depression led to the partial cancellation of the five Colorado class refits which would have started in 1929, 1930, and 1931; the eight older battleships recieved "full" treaty refits, but the Colorado refits were scaled back to save money.
The most visible element of the treaty era refits is replacing the Standards' distinctive hyperboloid lattice masts with tower foremasts and tripod mainmasts. Although the lattice masts had succeeded in being very lightweight and highly resistant to being shot away by gunfire damage, they could not accomodate the increasingly heavy enclosed tops which were required to support more advanced fire control equipment. The new tripod masts support three-level (one open and two enclosed) spotting tops which house optical sights and rangefinders connected to the central fire control systems located within the armored citadel.
The elements which were applied to all thirteen Standard type battleships are replacing , adding anti-torpedo blisters to the hulls, modifying the main gun turrets for an increased maximum elevation of thirty degrees, adding two catapults and floatplanes to the aft fantail, and finally replacing the tertiary battery of 3" anti-aircraft guns installed for World War I service with eight to twelve 5"⁄25 Mark 10 anti-aircraft guns in single mountings atop the #2 and #4 gun turrets and on the sides of the upper superstructure.
The earlier Nevada and Pennsylvania class battleships also had their forward secondary guns removed so that their hull casemates could be plated over, and their direct drive propulsion machinery replaced with up-to-date turboelectric machinery in an six boiler, three-turbine set, four motor configuration with an output of 32,000 shaft horsepower.
Treaty Battleships
Under the terms of the Washington Naval Treaty, the United States is entitled to begin replacing the thirteen Standard type battleships as soon as the capital ship building holiday ended in 1931. Design work on a treaty-compliant battleship design is started as soon as the treaty provisions are finalized and continues throughout the holiday period.
North Carolina Class (1931)
The North Carolina class are the U.S. Navy's only class of treaty battleships. The Navy's original plan is to construct these battleships at a rate of two per year from 1931 through the Treaty expiration at the end of 1936, thus achieving the full allocation of twelve Treaty-compliant battleships. However, Congress massively curtails battleship construction due to the economic disruption of the Great Depression: one battleship is authorized in 1931, one more in 1932, and then four in 1933 as part of President Roosevelt's economic revitalization.
Design
The North Carolinas have a standard displacement of 35,000 tons and measure 700 feet at the waterline with a beam of 108 feet and a draft of 35 feet. The Navy evaluates a wide variety of potential configurations before settling on a 25-knot battleship armed with nine 16" main guns placed in an "all forward" layout and armored against "regular" 16" AP shells. While this is less speed, less armament, and less armor than the Navy considers desirable, it is a well balanced combination of characteristics within the treaty tonnage limit.
Although both literally and figuratively eclipsed by subsequent post-treaty USN battleships, the North Carolinas are in many ways the definitive American warships of the era, as they pioneer the key features which are echoed on almost every subsequent American battleship, armored cruiser, and light cruiser for the next two decades. Most visually apparent are the all-forward, semi-superfiring main battery arrangement and the unified dual-purpose secondary battery, but here also are the peerless electromechanical fire control directors for both main and secondary batteries, the aggressive adoption of bleeding-edge radar, and the lavish incorporation of STS homogenous armor as structural steel.
(Compared to their historic counterparts, the alternate North Carolinas sacrifice two knots of speed and adopt an all-forward main battery like that of the British Nelson class in order to enable greater armor protection and the Navy's favored turboelectric machinery.)
Armament
The North Carolinas are armed with a main battery of nine 16"⁄45 Mark 6 guns in three three-gun turrets, a secondary battery of seventeen 5"⁄38 Mark 12 dual-purpose guns in four triple and three twin turrets, and a tertiary battery of sixteen 1.1"⁄75 anti-aircraft autocannons in four quad mounts.
The main battery is arranged in a 3-A-0 configuration with all three turrets in a semi-superfiring trio on the forward centerline: #1 at the main deck level, #2 raised by one deck, and #3 raised by two decks. This goes on to become a signature feature for the U.S. Navy's final generations of gun-armed warships; almost all subsequent American battleships, armored cruisers, and light cruisers use this layout.
The secondary battery is arranged with two twin wing mounts on each side of the 01 level superstructure, one triple wing mount on each side of the 02 level superstructure (above and between the lower wing mounts on the same side), and one triple centerline mount at the end of the aft 02 level superstructure.
The tertiary battery is arranged with two 1.1" quad mounts on the sides of the forward deckhouse 03 level and two on the sides of the aft deckhouse 02 level. Throughout World War II, the battleships are repeatedly refitted with steadily increasing numbers of Oerlikon 20mm anti-aircraft autocannons on twin and single pedestal mountings, and later 40mm Bofors anti-aircraft autocannons in quad mountings. In their final wartime configuration, there are ten 40mm mounts (one at the bow, one on top of the #3 turret, one on each side of the forward deckhouse 03 level, one on each side of the aircraft hangar roof, one on each side of the aft deckhouse 02 level, and two on the fantail) and twenty to forty 20mm mounts.
Propulsion
The North Carolinas are fitted with a greatly improved version of the turbo-electric propulsion used on pre-treaty USN battleships, with eight boilers feeding superheated steam to four paired sets of turbines. In order to maximize resistance to battle damage, the machinery is configured in a unit system layout with the boiler rooms and turbogenerator rooms interleaved to form semi-independent units. In addition, the combination of turbo-electric transmission and an "all forward" main battery enables a uniquely compact propulsion layout in which the electrical motors and propeller shafts are located on the bottom deck of the ship and the turbines and turbogenerators are located one deck above them, with both sets of equipment spread across the entire width of the hull inside the armor belt and torpedo bulkheads.
The boilers and their associated machinery are subdivided into eight boiler rooms and four auxiliary machinery rooms, with the boiler rooms in port⁄starboard pairs and the auxiliary rooms on the centerline between each pair. Each boiler room contains an individual oil-fired water-tube boiler operating at a steam pressure of 600 PSI and temperature of 750 F; the steam piping allows any boiler to be routed to any turbine, although the plant is normally operated with the boilers supplying the directly adjacent turbines. The boiler exhausts and air intakes are trunked intoto a rather unusual arrangement of three funnels -- a single funnel for the four forward boilers, and two smaller side-by-side funnels for the two port aft and two starboard aft boilers.
The turbines and generators are subdivided into four turbogenerator rooms which are interleaved with the pairs of boiler rooms. Each turbogenerator room houses one high-pressure turbine, one low-pressure turbine, two 450-volt AC alternators, and three 120-volt DC auxiliary ship’s service turbogenerators. The HP and LP turbines directly drive the alternators, while the turbogenerators have their own integrated geared turbines and are fed steam in parallel to the main turbine sets.
Aft of the boiler and turbogenerator rooms are a set of three electrical switchboard rooms, one on the centerline for the 450VAC main bus which feeds the propulsion motors and two smaller ones flanking it for the 120VDC auxiliary buses provide electrical power throughout the ship. To maximize resistance to battle damage, power distribution is split across four seperate auxiliary buses -- two port and two starboard.
The motor rooms are placed aft of the boiler and turbogenerator rooms. Each of the four propeller shafts has its own motor room, with two 12,500-horsepower double armature electric motors for a total propulsion plant output of 100,000 horsepower.
Post-Treaty Battleships
South Dakota Class (1936)
The South Dakota class battleships are the U.S. Navy's first post-Treaty battleships and its most modern battleships at the beginning of World War II. Two were authorized in 1936, two in 1937, and two in 1938.
Design
The South Dakotas have a standard displacement of 50,000 tons and measure 725 feet long at the waterline with a beam of 108 feet and a draft of 35 feet. The Navy had hoped for 27+ knots, twelve 16" guns and armor protection against 16" superheavy shells for its post-treaty battleships; when it became clear that they could only have two out of three without greatly increasing the size of the next battleship, they chose the traditional U.S. priorities of firepower and protection over speed, producing a battleship with the desired armament and most of the desired armor but a reduced speed of 25 knots.
Armament
The South Dakotas are built with a main battery of twelve 16"⁄50 Mark 2 guns in three four-gun turrets, a secondary battery of seventeen 5"⁄38 Mark 12 dual-purpose guns in three triple and four twin turrets, and a tertiary battery of twenty-four 1.1"⁄75 anti-aircraft autocannons in six quad mounts.
The main battery is arranged in a 3-A-0 configuration with all three turrets in a semi-superfiring trio on the forward centerline: #1 at the main deck level, #2 raised by one deck, and #3 raised by two decks. The four-gun turrets are a tight but workable fit for the Panamax limit beam and give the South Dakotas the heaviest broadside of any battleship in the world to date, especially with 2,700 lbs superheavy shells.
The secondary battery is arranged with two wing twin mounts on each side of the 01 level superstructure (abeam of the forward and aft deckhouses), one wing triple mount on each side of the 02 level superstructure (above and between the twin mounts on the same side), and one centerline triple mount at the end of the aft 02 level superstructure. This is the same configuration already used with great success on the North Carolinas, so the Navy has every confidence in it.
The tertiary battery is arranged with two 1.1" quad mounts on the sides of the forward deckhouse 03 level, two on the sides of the aft deckhouse 02 level, and two on the fantail. Throughout World War II, the battleships are repeatedly refitted with steadily increasing numbers of Oerlikon 20mm anti-aircraft autocannons on twin and single pedestal mountings, and later 40mm Bofors anti-aircraft autocannons in quad mountings. In their final wartime configuration, there are ten 40mm mounts (one at the bow, one on top of the #3 turret, one on each side of the forward deckhouse 03 level, one on each side of the aircraft hangar roof, one on each side of the aft deckhouse 02 level, and two on the fantail) and twenty to forty 20mm mounts.
Propulsion
The South Dakotas utilize the same turbo-electric propulsion as the North Carolinas, scaled up for the increased tonnage of the ships. The number and arrangement of boilers, turbines, and generators remains the same, with four turbogenerator rooms (two port and two starboard) with a high-pressure and low-pressure turbine set, two 450-volt AC main turbogenerators, and two 120-volt DC auxiliary ship’s service turbogenerators. Eight oil-fired boilers are placed in individual boiler rooms outboard of the turbogenerators, and supporting equipment is placed in four axuiliary machinery rooms between the boiler room pairs. Further aft are four motor rooms, each containing two 15,000-horsepower double armature electric motors for a total propulsion plant output of 120,000 horsepower.
Iowa Class Battleships (1938)
The Iowa class is a relatively specialized class of extremely fast battleships built for a top speed of 33 knots in order to escort fast carrier task groups and intercept hostile battlecruisers or fast battleships. Four Iowas are authorized in 1938 and the first two Montanas (see below) are switched for two additional Iowas in 1941, although these last two are not completed until after the war is over.
Design
The Iowa class battleships have a standard displacement of 50,000 tons and measure 850 feet long at the waterline with a beam of 108 feet and a draft of 37 feet. They are essentially a "fast" version of the South Dakota class with the main battery scaled back to the level of the smaller North Carolina class in order to free up tonnage for the absolutely massive power plant required to push a warship of this magnitude to the speed of a cruiser literally a fifth its size.
(The extreme speed requirement for the Iowa class forces strong convergent evolution between the alternate version and her historical counterpart, with the major differences being an all-forward main battery and turbo-electric propulsion. However, one very significant consequence of being built from the start as 50,000 ton post-Treaty battleships rather than as "overgrown" 45,000 ton escalator clause battleships is that the alternate Iowas are built for the cancelled expansion of the Panama Canal and cannot fit through the "old" canal locks. On the other hand, the wider hull is able to accomodate wider bow with a smoother transition to the center hull, which largely eliminates the issue of being extremely "wet" forward.)
Armament
The Iowas are built with a main battery of nine 16"⁄50 Mark 2 guns in three three-gun turrets, a secondary battery of seventeen 5"⁄38 Mark 12 dual-purpose guns in three triple and four twin turrets, and a tertiary battery of eighty 40mm Bofors anti-aircraft autocannons in twenty quad mounts and sixty 20mm Oerlikon anti-aircraft autocannons in twenty twin and ten single mounts.
The main battery is arranged in a 3-A-0 configuration with all three turrets in a semi-superfiring trio on the forward centerline: #1 at the main deck level, #2 raised by one deck, and #3 raised by two decks. The Iowas essentially have the more powerful guns of the South Dakotas in three-gun mounts like those of the North Carolinas.
The secondary battery is arranged with two wing twin mounts on each side of the 01 level superstructure (abeam of the forward and aft deckhouses), one wing triple mount on each side of the 02 level superstructure (above and between the twin mounts on the same side), and one centerline triple mount at the end of the aft 02 level superstructure. This is a direct repeat of what has by this point become the standard secondary configuration for modern American battleships.
The tertiary battery is arranged... The Iowas are the first American battleships to be built from the start with 40mm Bofors anti-aircraft autocannons rather than 1.1" autocannons, although this is something of an academic distinction considering how aggressively older warships were refitted throughout the war.
Propulsion
The Iowas continue the USN preference for turbo-electric propulsion, with twelve oil-fired boilers feeding 600 PSI steam to six paired sets of turbines. The unit systems return to a longitudinal arrangement, with three port and three starboard turbogenerator rooms. Each turbogenerator room contains a high-pressure turbine, a low-pressure turbine, two 450-volt AC main turbogenerators, and two 120-volt DC auxiliary ship’s service turbogenerators. Twelve boiler rooms are stacked vertically above the turbogenerators, and six auxiliary machinery rooms are centered between the pairs of boiler rooms. The control room is placed on the upper machinery deck aft of the boilers and eight motor rooms are on the lower machinery deck aft of the turbogenerators; there are two motor rooms for each shaft, each containing two 15,000 HP double armature electric motors for a total output of 240,000 SHP.
Montana Class (1939)
The Montana class battleships would have been the U.S. Navy's last, largest, and most powerful battleships. Two are authorized in 1939 and two more in 1940, but construction is delayed by the United States' entry into World War II, put on hold in 1942, and ultimately cancelled in 1943.
Design
The Montana class battleships have a standard displacement of 60,000 tons and measure 900 feet long at the waterline with a beam of 130 feet and a draft of 40 feet. With no treaty tonnage limit and the expansion of the Panama Canal enabling increased beam and draft, the Navy is finally able to implement all of its ideal battleship characteristics: a speed of 27 knots, a main battery of twelve 16”/50 guns, a secondary battery of heavier dual purpose guns, and full armor protection against 16" superheavy armor piercing shells.
(Compared to their historical counterparts, the alternate Montanas are shorter and wider due to the use of quad turrets, and slightly lighter due to the use of an all forward main battery arrangement.)
Armament
The Montanas have a main battery of twelve 16”/50 Mark 2 guns in three four-gun turrets and a secondary battery of twenty-one 5”/54 Mark 16 guns in three triple and four twin turrets. Their tertiary battery was originally planned to be forty-eight Bofors 40mm autocannons in twelve quad mounts, but would likely have been changed to between eight and sixteen 3”/50 RF Mark 22 anti-aircraft guns in four to eight twin mounts.
Propulsion
The Montanas utilize turbo-electric propulsion with eight oil-fired boilers feeding 600 PSI steam to four paired sets of turbines. The unit systems are in longitudinal arrangement, with two port and two starboard turbogenerator rooms. Each turbogenerator room contains a high-pressure turbine, a low-pressure turbine, two 450-volt AC main turbogenerators, and three 120-volt DC auxiliary ship’s service turbogenerators. Eight boiler rooms are stacked vertically above the turbogenerators, and four auxiliary machinery rooms are centered between the pairs of boiler rooms. The control room is placed on the upper machinery deck aft of the boilers and four motor rooms are on the lower machinery deck aft of the turbogenerators. Each motor room contains three 15,000 HP double armature electric motors for a total output of 180,000 SHP.