With a substantial force of modern dreadnought armored cruisers in commission, the United States is unwilling to accept treaty limits which would see those vessels immediately scrapped. The compromise settled on is to define a treaty limit for "capital cruisers" with an armament no larger than 10" (255mm) and a standard displacement of up to 20,000 tons.
Capital cruisers are not subject to the ten-year pause on the construction of new capital ships, but they are subject to the twenty-year replacement limit. Until then, the United States retains its six existing dreadnought armored cruisers, Great Britain retains its two slightly larger Lion class battlecruisers, Japan retains its three surviving Tsukuba and Ibuki class armored cruisers (and may construct four capital cruisers to replace them), and France is permitted to complete complete three of its laid-down Normandie class small battleships as armored cruisers.
Treaty Era Research & Development
10"⁄45 Mark 5 Naval Gun
Since the terms of the Treaty freeze armored cruiser armament at the ten-inch (255mm) caliber, the U.S. Navy is forced to abandon plans for a twelve-inch armored cruiser (resurrected post-treaty as the Alaska class) and instead engages in a comprehensive development effort to maximize the effectiveness of the ten-inch caliber. This effort ultimately produces the first "gun system" in which the turret, mounting, gun, and ammunition are all developed together as a coordinated whole rather than as separate projects.
As originally made, the Mark 5 gun system can throw 550-pound AP and HE shells out to an effective range of 36,000 yards, with a rate of fire of 4-5 rounds per minute per barrel. With the later development of superheavy AP shells, muzzle velocity and thus maximum range is sacrificed (to 33,000 yards) for a heavier 660-pound shell with better penetration.
Turrets
Both a three-gun turret and a four-gun turret are developed as part of the Mark V project, with the intent of making immediate use of the three-gun turret for the initial class of treaty armored cruisers and having the four-gun turret as an option for later classes of treaty cruisers and post-treaty cruisers.
The turrets have an elongated hexagonal gunhouse similar to those on the later classes of Standard type battleships, mounted on a cylindrical barbette of 30' diameter for the four-gun version or 26' diameter for the three-gun version. The gunhouse is subdivided by longitudinal bulkheads between the guns and a transverse bulkhead dividing the guns from the sighting equipment and rammers installed at the rear of the compartment. Below the gunhouse and within the barbette is the turret stalk, a multi-story cylindrical structure which rotates with the gunhouse. The stalk is four levels high in the “regular” deck level turret and five levels high in the taller superfiring variant.
The first and largest level of the stalk is the turret pan, which provides additional working space for the guns and their associated equipment. The gunhouse floor (shelf plate) is cut away to allow the gun breeches to depress and recoil into the pan when at higher elevations, and the pan floor is fully armored to to isolate the guns from the ammunition housed in the lower levels. The turret assembly is supported on a ring of tapered roller bearings directly under the pan floor, which in turn rests on a circular foundation extending all the way through the bottom of the hull. The lower levels of the stalk project down through the roller ring and are suspended within the foundation.
The second level of the stalk is the electrical deck, which houses most of the hardware for the powered operation of the turret. This level is slightly smaller than the pan deck since it fits within the inner circular foundation rather than the entire barbette tube, and is divided into a circular inner compartment and a ring-shaped outer compartment. The inner compartment is the powder transfer room, which houses the lower end of the upper propellant hoists, the upper end of the two lower propellant hoists, and the manned transfer equipment used to move sets of propellant bags between these hoists. The outer compartment houses the electric-hydraulic drive systems for turret traverse, gun elevation, and ammunition hoisting.
The third, fourth and (on superfiring installations) fifth levels of the stalk are projectile flats, which house the ammunition load of ten-inch cannon projectiles. Like the electrical deck, these levels are are divided into inner and outer compartments by a circular bulkhead. The small inner compartment provides a protected space for the two lower propellant hoists to pass through. The outer compartment houses shells on two independently rotating storage rings, one along the outer perimeter and one along the inner perimeter.
Mountings
The gun mountings developed for the Mark 5 utilize a modernized variant of the "grasshopper" counter-recoil system fitted to prior previous ten-inch and twelve-inch naval guns. Each gun is mounted on a pair of pivoting lever arms which transmit its firing recoil to a set of hydro-pneumatic recoil cylinders and recuperators mounted below and slightly to the rear. While more mechanically complex than a counter-recoil system mounted directly to the gun slide or carriage, this system minimizes the elevating mass of the system and moves the substantial bulk of the counter-recoil mechanism from the gunhouse to the turret pan.
Due to the combination of the grasshopper recoil mounting and the high maximum elevation specified for these mountings, the elevation gear uses a diagonal translating screw operated by an electro-hydraulic drive. The elevation range is -5 to +40 degrees, although the turret gun house openings go to 41 degrees because the gun barrels tip upward slightly when recoiling. The loading angle for the guns is +5 degrees and the elevation gear is configured with a hardwired preset to semi-automatically return the gun to this angle after firing.
Each gun mounting is serviced by a single-stage projectile hoist and a two-stage powder hoist, both electro-hydraulically driven. Projectiles are hoisted directly in line with the gun and transferred from vertical to horizontal by the loading tray as it unfolds to align with the open breech, then rammed into the firing chamber by a rigid chain drive. Powder bags are hoisted to the outboard side of the gun and placed in the breech by hand, then rammed to fit tightly against the projectile.
Gun
The 10"⁄45 Mark 5 naval gun itself is a "clean sheet" design which breaks away from the pre-dreadnought era Mark 3 and the improved but derivative Mark 4 which are used on previous dreadnought armored cruisers. Because prior research and operational experience had already demonstrated that excessive dispersion and short barrel life made muzzle velocities beyond 2,900 FPS impractical, the Mark 5 project focuses on developing as compact and lightweight a gun as possible and placing it in an optimized mount with high elevation capability and handling equipment for improved shells.
10-inch is far too large a caliber for a radially expanded monobloc barrel to be practical, so the Mark 5 guns are built using a hybrid approach in which the progressively rifled barrel liner and "A" tube are radially expanded, but the "B" jacket and two "C" reinforcing hoops are then built up over them. This is combined with a downward-opening Welin interrupted screw breech operated by the new Smith-Asbury geared action, which unifies the unscrewing, withdrawing, and swinging-open steps of the Welin breech into a single continuous lever movement.
Ammunition
The Mark 5 guns utilize seperate-loading bag ammunition with AP and HE shells propelled by a 225-pound charge of double-base smokeless powder (SPD).
In order to maximize the Mark 5's effective range and penetration against foreign battlecruisers, the Bureau of Ordnance develops a "double capped" armor piercing shell with both a relatively blunt penetrator cap and a highly streamlined (9crh) ballistic windscreen, as well as a tapered boat-tail base. This shell weighs 550 pounds and has a bursting charge of 20 lbs of ammonium picrate based "Explosive D" triggered by a base delay fuze. Fired at a muzzle velocity of 2,900 FPS and an elevation of forty degrees, it can reach ranges of up to 35,000 yards.
The high explosive shell is a high capacity projectile with the same physical dimensions as its AP counterpart. It weighs 550 pounds and has a bursting charge of 50 lbs of Explosive D, triggered by both a base delay fuze and a nose contact fuze. Muzzle velocity and range are the same as for the AP shell, which also enables the use of a blind loaded and plugged (BLP) version of this shell to be used as the sole practice round for Mark 5 armed cruisers.
Treaty Armored Cruisers
The U.S. Navy commissions three classes of dreadnought armored cruisers under the limitations of the Washington Naval Treaty: two Boston class are laid down in 1925 to replace the California class of 1905, two New York City class are laid down in 1929 to replace the Los Angeles class of 1909, and two Portland class are laid down in 1933 to replace the Phoenix class of 1913.
Boston Class (1925 ⁄ 1929)
The Boston class cruisers are the U.S. Navy's first treaty-era armored cruisers.
Design
The Boston class cruisers have a standard displacement of 20,000 tons. In most respects they are enlarged and improved repeats of the pre-Treaty Phoenix class, with the notable addition of floatplanes for long-range scouting and spotting.
Armament
The Bostons are armed with a main battery of twelve 10"⁄50 Mark 5 guns, a secondary battery of twelve 5"51 guns, and a tertiary battery of six 5"⁄25 anti-aircraft guns.
The main battery is placed in a conventional 2-A-2 configuration with superfiring pairs of three-gun turrets on the forward and aft centerline.
The secondary battery is arranged in six twin casemate mounts in the 01 level superstructure: two in the forward corners, two in the amidships sides, and two in the aft corners.
The tertiary battery is placed in high-angle pedestal mounts, one on each side of the forward 03 level, one on each side of the hangar roof, and one on each side of the aft 02 level. An additional sixteen 1.1"⁄75 anti-aircraft autocannons in four "Chicago Piano" quad mounts are added in the early 1940s, with further refits adding increasing numbers of 40mm Bofors and 20mm Oerlikon anti-aircraft autocannons.
Propulsion
Because of their constrained tonnage, the Bostons utilize geared turbine propulsion. Twelve oil-fired boilers feed superheated steam to four paired sets of high-pressure and low-pressure turbines, with reversing sections integrated into the low-pressure turbines. Combined output is 30,000 horsepower per shaft, producing a top speed of 33 knots.
Aircraft
The Bostons> have a modest aircraft hangar placed amidships between the funnels and the mainmast, flanked by two catapults and two cranes. Four floatplanes are carried, two on the catapults and two in the hangar.
New York City Class (1931)
Design
The New York Cities have a standard displacement of 20,000 tons and measure 750 feet long at the waterline with a beam of 80 feet and a draft of 25 feet.
Armament
Propulsion
Aircraft
Portland Class (1933)
The Portland class are the U.S. Navy's final treaty armored cruiser class. Although built in scrupulous adherence to the Treaty tonnage limits, the Portlands are intended from the start to serve as the basis for larger post-treaty cruisers and are the direct parent design for all subsequent American "gun cruisers".
Design
The Portlands have a standard displacement of 20,000 tons and measure 700 feet long at the waterline with a beam of 90 feet and a draft of 25 feet. The hull is flush deck with a high freeboard and moderate forward sheer, tapering to a clipper bow over a bulbous forefoot; the stern has a simple rounded transom with moderate aft sheer. The superstructure extends forward to contain the barbette of the #3 turret, aft to a vestigial quarterdeck, and laterally across the full width of the hull; a tower foremast is placed immediately aft of the conning tower and a shorter tripod mainmast is placed well aft, flanked by two side-by-side funnels.
(The Portland class armored cruisers carry the same name as the original timeline's third class of treaty ⁄ heavy cruisers, but as "true" CAs they are twice the size and a completely different design.)
Armament
The Portlands are armed with a main battery of twelve 10"⁄50 Mark 5 guns, a secondary battery of eleven 5"⁄38 Mark 12 dual-purpose guns, and initially a tertiary battery of sixteen 1.1"75 Mark 1 autocannons and twelve .50-caliber heavy machine guns. Like all warships which served into World War II, they are repeatedly refitted with increased tertiary AAA, with 20mm Oerlikon autocannons in single and twin pedestal mounts starting in late 1940 and 40mm Bofors autocannons in twin and quad mountings starting in 1942.
The main battery is arranged in a 3-A-0 configuration with three four-gun 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 secondary battery is arranged with two wing twin mounts on each side of the 01 level superstructure and one centerline triple mount at the end of the aft superstructure.
The original tertiary battery is arranged with the 1.1" autocannons in four "Chicago Piano" quad mounts and the .50-caliber machine guns on six twin pedestals. The Chicago Pianos are placed one at the bow, one on each side of the forward superstructure 03 level, and one on the stern fantail, while the machine gun pedestals are placed one on each bridge wing, one on each side of the forward spotting top, and one on each side of the aft spotting top.
The final tertiary battery is eight quad Bofors, six twin Bofors, twelve twin Oerlikons, and four single Oerlikons. The quad Bofors are arranged one at the bow, one atop the #3 main battery turret, two on each side of the main deck forward and aft of the wing turrets, and two on the stern fantail; the twin Bofors are arranged three on each side between and raised over the wing turrets; the twin Oerlions are arranged one on each bridge wing, four on each side of the hangar roof, one on each side of the aft deckhouse 03 level; and the single Oerlkons are arranged one one on each side of each spotting top.
Propulsion
Despite the tight tonnage constraints imposed by the Treaty, the Portlands revert to turbo-electric propulsion. This is primarily because the ability to split the propulsion plant across multiple decks enables a vertically stacked configuration ideal for the volume constraints of the all-forward layout. Twelve boilers feed steam to six sets of turbines, which power eight motors on four shafts.
Twelve boiler rooms and six auxiliary machinery rooms are placed on the third deck, split port and starboard with four boiler rooms on each side and the machinery rooms interleaved between 1-2, 3-4, and 5-6. Each boiler room contains a single oil-fired M-type water tube boiler with separately fired superheater, producing dry steam at a pressure of 600 PSI and a temperature of 850 F. The air intakes and boiler exhausts are longitudinally trunked to two side-by-side funnels with angled openings directing the exhaust streams aft and outward. The boilers are piped to supply steam to any turbine set; in typical cruising operation, four boilers are active at a time with two more on warm standby.
Six turbogenerator rooms are on the centerline of the third deck between the boiler and machinery rooms, each containing a high-pressure turbine (two velocity-compounded impulse stages followed by eight reaction stages), a low-pressure turbine (ten reaction stages), two AC alternators, and three DC ship’s service turbogenerators. The main turbines and alternators are directly coupled and operate at a constant speed of 3600 RPM, while the SSTGs have their own integral reduction-geared turbines and operate in parallel to the mains. Two DC switching rooms are placed between the 2-3 and 4-5 turbogenerator rooms, distributing power from the SSTGs via two fully independent buses.
Four motor rooms are placed on the bottom deck, one in line with each propeller shaft. The outboard motor rooms are partially forward of the inboard motor rooms, with the AC switch room enveloped between them. Each room contains two externally excited synchronous AC motors producing 15,000 equivalent horsepower at 240 RPM.
Aircraft
With the incorporation of radar into their fire control, the role of floatplanes in spotting is diminished but their value as long-range scouts remains vital.
Post-Treaty Era
The priority placed on new battleship production in the years immediately following the expiration of the Washington Naval Treaty led to a period of several years in which no new armored cruisers were authorized. This changed with the large-scale naval expansion program of the late 1930s into the early 1940s; four extremely large armored cruisers were authorized in 1940 as part of the Two-Ocean Act.
Alaska Class (1940)
The Alaska class cruisers are the U.S. Navy's sole post-Treaty armored cruisers, with considerably improved armament and protection over prior classes while maintaining the same high speed and maneuverability. This came at a substantial cost in size and tonnage, and even with war budgets only four were built, with two being completed during the war and two being converted into missile ships post-war.
Design
The Alaska class cruisers have a standard displacement of 30,000 long tons and measure 750 feet at the waterline by a beam of 95 feet and a draft of 30 feet.
Armament
The Alaskas are armed with a main battery of nine 12"⁄50 Mark 8 guns in three three-gun turrets and a secondary battery of eighteen 5"⁄38 dual purpose guns in two triple and six twin turrets.
The primary battery is 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. Although the all-forward battery layout was adopted on treaty era armored cruisers primarily as a function of weight savings, it was repeated in the post-treaty Alaskas on post-treaty as an optimally concentrated "chase" battery.
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 twin mount on each side of the 02 level superstructure (above and between the twin mounts on the same side), one centerline triple mount on the forward 03 level superfiring over the #3 main battery turret, and one centerline triple mount at the end of the aft 02 level.
Propulsion
The Alaskas are large enough for the USN's preferred turbo-electric propulsion to be reasonably weight-efficient, utilizing a more compact variant of the high pressure plant developed for the North Carolina class battleships. This has six oil-fired boilers feeding 600 PSI, 850 F superheated steam to three paired sets of turbines.
In order to fit efficiently within the confines of the smaller cruiser hull, the unit systems are arranged in a two-deck stacked configuration, with three turbogenerator rooms longitudinally across the centerline of the machinery deck, six boiler rooms and three auxiliary machinery rooms on the mid-deck directly above them. Each generator room contains a high-pressure and low-pressure turbine pair, one 450-volt AC main turbogenerator, and two 120-volt DC ship’s service turbogenerators.
Three motor rooms are placed aft of the generator rooms, with the centerline motor room containing the six 12,500 horsepower double armature electric motors for both inboard shafts and the port and starboard motor rooms contianing three of the same motors for one outboard shaft each. Total system output is thus 150,000 SHP.