The Portland-class cruisers are the U.S. Navy's final class of treaty-limited "capital cruiser". These warships are officially classified as armored cruisers and assigned hull numbers in the same sequence as pre-dreadnought armored cruisers, although some foreign navies categorize them as de facto battlecruisers. The British notably refer to them as "pocket battleships" akin to the German Deutschland-class panzerschiffes, and this term gains considerable popularity in the general press despite its technical absurdity.
While often compared to France's Dunkerque-class due to their all-forward main battery layout and four-gun turrets, the Portlands are best understood as "fast little sisters" to the North Carolina-class treaty battleships. They are also the parent design for all subsequent American gun cruisers, since the large physical size of the armored cruisers enables them to serve as operational testbeds for the panoply of advanced features the U.S. Navy envisones for the next generation of cruisers.
Two cruisers of this class are laid down in 1933 to replace the Phoenix class of 1913, and four more are laid down in 1936 when the Washington Naval Treaty expires. The six Portlands are the largest and most powerful U.S. Navy cruisers to see combat service during World War II; all are assigned to the Pacific Fleet and serve as cruiser squadron flagships.
Design
As treaty armored cruisers, the Portlands have a standard displacement of exactly 20,000 tons and an actual full-load displacement of roughly 25,000 tons (a substantial increase over prior cruisers of the "same" standard displacement). They measure 725 feet long with a beam of 90 feet and a draft of 25 feet.
The overall hull configuration is a flush deck design which emphasizes seaworthiness with high freeboard, prominent forward sheer, and moderate aft sheer. Since the hull is relatively broad-beamed for a high speed cruiser, the bow is carefully shaped for the best possible hydrodynamic efficiency, with a tapered clipper design incorporating a large bulbous flare below the waterline. The stern is a a traditional elliptical "cruiser" design with four propeller shafts and two semi-balanced rudders are aligned with the inboard shafts.
The superstructure is a semi-unified design with an 01 level which extends continuously forward to enclose the barbette of the #3 turret and aft almost to the transom, leaving only a vestigial quarterdeck; 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 with slanted exhausts.
Details
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 twenty 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.
Fire Control
A key innovation in the Portlands is the introduction of remote power control (RPC) for both the main battery and secondary battery turrets. This enables the ship to use its centralized fire control director equipment to directly transmit traverse and elevation commands to the turret drives rather than merely setting pointers for the on-mount crew to manually follow. Manual operation of the turrets via either pointer or purely local control remains available as a backup method of operation.
The original Portlands have two Mark 31 directors for the main battery and two Mark 33 directors for the secondary battery: the main battery directors are located on the foremast top and the roof of the conning tower, while the secondary battery directors are port and starboard on the mainmast. The second batch of Portlands is fitted with newer and more advanced Mark 37 secondary directors, and the two original ships are refitted with these as well in 1938. In the 1940s, all six armored cruisers have both their primary and secondary directors augmented with SG surface-search and SK air-search radar, vastly enhancing their accuracy and enabling full "blind fire" in night and adverse weather conditions.
Main Battery
The main battery is arranged in a 3-A-0 configuration with three four-gun turrets on the forward centerline. The turrets are a "semi-superfiring" trio with #1 installed at main deck level, #2 raised by a half deck, and #3 raised by a full deck; they are spaced along the centerline so that the gun barrels of the #2 and #3 turrets partially overlap the gunhousees ahead of them, but are able to able to traverse and fire overhead at elevation angles of five degrees or more.
The adoption of all-forward four-gun turrets in the Portlands saves substantial tonnage over the 2-A-2 three-gun turret configuration on preceding armored cruisers while maintaing the same number of guns, and also enables the entire main battery to be brought to bear in a maneuvering chase against enemy cruisers instead of only half. In addition, the barbette diameter of the four-gun ten-inch turret is large enough to fit a three-gun twelve-inch turret, opening up the possibility of an post-treaty upgunning.
Secondary Battery
The secondary battery is arranged in two twin wing mounts on each side of the main deck and one centerline triple mount at the aft end of the 01 level superstructure. The forward wing mounts are placed slightly aft of the conning tower so that they are clear of the #3 main turret firing "over the shoulder" at its maximum traverse of 150 degrees, while the aft wing mounts are placed slightly forward of the mainmast to similarly clear the centerline secondary turret at maximum traverse.
Since the North Carolina class battleshihps are already entering service with 5"⁄38 Mark 12 dual purpose guns in twin and triple mountings as their secondary battery, fitting lighter versions the same mounts as secondaries for the Portlands and other new cruisers in place of the previous mixed battery of 5"⁄51 and 55"⁄25 guns is an obvious decision. This provides American cruisers with the most capable anti-aircraft suite in the world while also massively easing logistics.
Tertiary Battery
The original tertiary battery is arranged with the 1.1" autocannons in five "Chicago Piano" quad mounts and the .50-caliber machine guns on six twin pedestals. The Chicago Pianos are placed one at the bow, one atop of the #3 turret, 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 six quad Bofors, eight 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, one on each side of the main deck forward of the wing turrets, and two on the stern fantail; the twin Bofors are arranged with one on each side of the forward superstructure 03 level and three on each side of the main deck between 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.
During post-WW2 service, all tertiary armament was removed.
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 at the aft end of the third deck, split port and starboard with six boiler rooms and three machinery rooms on each side, the latter interleaved between boilers 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 placed on the centerline of the third deck directly 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 450-volt AC main electrical alternators, and three 120-volt DC ship's service turbogenerators (SSTGs). 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.
Three motor rooms are placed on the aft bottom deck, with a single shared center motor room for both inboard propeller shafts and individual motor rooms for the outboard propeller shafts. The port and starboard motor rooms are placed outward and forward of the center motor room, with the AC switch room between them. Each shaft is operated by two externally excited synchronous AC motors producing 15,000 equivalent horsepower at 240 RPM.
Aircraft
Due to their primary role as cruiser squadron flagships, the Portlands carry a larger complement of scout aircraft than other American cruisers. This is initially six Curtiss SOC-1 Seagull seaplanes, from 1942 onwards six Vought OS2U Kingfishers, and from 1945 onwards six Curtiss SC Seahawks. The floatplanes are stored and serviced in a large cross-deck hangar incorporated into the amidships superstructure, directly aft of the conning tower and foremast. A catapult and recovery crane are located on each side of the hangar roof, and aircraft are deployed to the catapults via a large centerline elevator.
In the 1960s, the Portlands are converted to operate helicopters in place of their floatplanes, with the catapults and cranes removed and helipad markings added to the hangar roof. After this conversion, they operated six Kaman SH-2 Seasprite utility helicopters. Due to the size of their hangar elevators, they are unable to operate the larger UH-60 Seahawk and retain the Seasprite until final retirement.