Design
In 1936, the Naval Board began design of what became the Atlanta-class cruiser. It is sometimes said a camel is a horse designed by a committee, the result of conflicting and mutually exclusive requirements. This description fits the Atlanta class. The most fixed requirement in its design was tonnage. Due to informal agreements with the Royal Navy and formal London Treaty conditions, tonnage was to be between 5,000 and 7,000 tons, dictating a maximum size.

Next in priority was speed, but that depended on its ultimate purpose: a 36knt top speed for working with destroyers or 33–34knts if intended to operate with cruisers. Two knots was not a trivial difference. Water resistance grows as the cube of the speed, so a 2knt difference in top speed could require doubling the horsepower. The extra engine size had to come out of a limited weight budget at the expense of armament and range.

A lighter broadside and less bunker space were accepted. The Navy’s perceived primary foe was Japan; fighting on the wide expanse of the Pacific Ocean was expected. A minimum 8,000nm range was required; a 10,000nm range desirable. Armament had a lower priority. The General Board wanted a design with the heaviest main battery possible, while carrying both torpedoes and depth charges. A strong surface and antiaircraft battery was desired.

Construction on Atlanta started on April 22, 1940, when the keel was laid in Kearny, NJ. By June 1940, much of the bottom plating and framing was installed. The Atlantas were built using traditional shipbuilding techniques, with each frame and hull plate attached on the building ways. (USNHHC)

These contradictory speed, range, and armament requirements had to be folded into a single design. That design had to be completed in time to add the vessels to the Fiscal Year (FY) 1939 building program. Over the next two years, the General Board considered 25 different possibilities for this new cruiser. The largest hit 8,400 tons, the smallest 3,500 tons. Speeds of the different designs ranged from 32.5–36knts.

The main batteries alternated between 6in. and 5in. guns. Most opted for a single size, although five designs mixed a 6in. main battery with a dual-purpose 5in. secondary battery. These were discarded. Mixed battery designs required hulls too large for the tonnage allocated or delivered a main battery or antiaircraft battery too weak to be effective. Since the dual-purpose 6in. would be unavailable by FY 1939, in February 1938 the Naval Board settled on a design with 16 5in. guns and three quad 1.1in. guns.

This design had a standard displacement of 5,990 tons. It was intended to go 32.5–34knts, which required engines developing 70,000SHP. It carried fuel bunkers with fuel to take it 9,000nm at 15knts. Guns, machinery, and hull left little available tonnage for armor. Only a thin layer of belt and deck armor over the magazines and machinery spaces could be installed. Turret protection was limited to stopping splinters. A direct hit by a 5in. or larger shell would penetrate the turret.

An alternative design eliminated four 5in. guns to allow better protection. This design increased belt and deck armor, and shielded the gun houses with 4in. of armor. However, the desire for greater gun power prevailed and this final design was discarded.

Powerplant size also generated considerable debate. The Naval Board ruled 70,000SHP was too light for the upper ranges of top speed. To reach 33knts, a 75,600SHP was required; for 34knts, 87,600SHP. Either increase in horsepower would require extra weight in a tight design. The 75,600SHP design increased weight by at least 60 tons, while the 87,600SHP option added 200 tons. Ultimately, the Naval Board opted for the lower horsepower option, 75,600SHP.

The preliminary design was approved in July 1938. Standard displacement was now 6,000 tons, with 7,400 tons design displacement. This allowed the larger powerplant, and provided tonnage for auxiliary diesel-powered electrical generators, two triple torpedo tubes, sonar, and depth-charge racks. The design initially had a 100kw generator aft and a 25kw generator forward, but these were upgraded to two 250kw generators.

It placed the main guns in eight twin turrets. Six were mounted centerline, with three superfiring turrets forward and three aft. Two others were mounted on the weather deck aft of the machinery spaces to starboard and port. The ships also carried three quad 1.1in. guns: one placed on each side of the bridge and one centerline just forward of the aft 5in. turrets. To round out the antiaircraft suite there were eight .50cal machine guns.

The bow bulb on the Atlanta-class cruisers can be plainly seen in this pre-launch photograph of Atlanta. It filled the low-pressure area that formed ahead of the bow at high speeds, reducing turbulence resistance. (USNHHC)

The hull mimicked the lines of the Brooklyn class and the Wichita, the last cruisers built before the Atlantas. The hull incorporated a bulbous bow and a transom stern. Bulbous bows were relatively new. Naval architects began adding it after towing tank tests indicated the bulb improved performance at high speeds. It reduced bow turbulence at speed by filling a low pressure area in the stem which induced turbulence.

The transom stern was an artifact of previous cruiser designs, incorporated to provide hangar space for floatplanes. The Atlantas did not carry aircraft or catapults, but it was simpler to reuse a tested hull form. A feature unique to the Atlanta-class hull was a knuckle in the bow that ran to aft of the second 5in. mount. This broadened the bow above the second deck (the deck below the main deck), which increased that deck’s available space and also improved seakeeping, deflecting waves away from the bow, leaving the forecastle drier.

The design’s most significant advance was the unseen powerplant. US Navy warships used steam turbines, with steam generated by fuel oil-fired boilers. The higher the boiler pressure, the greater the efficiency of the boiler. To fit a powerplant capable of generating 75,600SHP in the volume and weight allocated, a new generation of boilers was installed, which ran at a higher temperature and pressure.

The first Atlantas were fitted with a high-pitch three-bladed propeller. It was supposed to increase thrust, but it created cavitation that caused severe vibration when the ships traveled in a 20–30knts speed range. (USNHHC)

The Atlantas were also fitted with deep-pitch three-bladed propellers to maximize thrust. The designers believed the combination of high-pressure boilers and deep-pitch propellers would let the Atlantas reach a top speed of 35 or 36knts. Atlanta and Juneau, the two Atlantas built at Kearny, New Jersey, were fitted with these propellers, which proved unsuccessful. They were too powerful, each blade moving enough water to create severe cavitation. This occurs when water pressure drops enough to cause dissolved air in water to come out of solution.

The resulting bubbles collapse when the water returns to normal pressures, creating turbulence. During sea trials, the resulting turbulence created vibration severe enough to trip circuit breakers throughout the ship and threaten damage to radio, radar, and gyrocompass. Vibrations shook the after-three turrets badly enough to render them unusable. Although Atlanta hit a trial speed of 34.22knts with these propellers, they were soon replaced with lower-pitch four-bladed propellers. These solved the vibration issue at the cost of a 2knt reduction in top speed.

A more significant propulsion innovation for cruisers was introduced with Atlanta, the first cruiser design with alternating boiler rooms and engine rooms. Benson-class destroyers built in 1938 introduced alternating boiler and engine rooms. Previous cruiser designs had two boiler rooms located ahead of two engine rooms. Alternating the boiler and engine rooms increased combat survivability.

Like all Treaty designs, the final design was a compromise, attempting to cram as much capability as possible within a limited displacement. As with all compromises, there were prices to be paid. With the Atlantas, their biggest design weakness was stability. The tall smokestacks, three levels of guns, gun directors even higher than the guns, and deck-mounted torpedoes and depth charges yielded an alarmingly small metacentric arm.

The metacentric arm is the difference between a ship’s center of gravity (CG) and its center of buoyancy (CB). A ship tips around its CB. The CG, which has to be below the CB, serves to right the ship as it rolls. The greater the distance (or metacentric height) between the two, the greater the righting moment produced. The short metacentric distance on the Atlantas left them vulnerable to capsizing. Adding more weight topside would further decrease the distance, limiting future growth.

All eight Atlantas were built in pairs. Typically, one ship in each pair was laid down two weeks to two months before the second. This picture shows Juneau with Atlanta behind it, both in an advanced stage of construction. (USNHHC)

The first four Atlantas were ordered in April 1939, part of construction authorized by the Naval Act of 1938. Two – Atlanta (CL-51) and Juneau (CL-52) – were built at the Kearny, New Jersey, Federal Shipbuilding and Drydock yard. San Diego (CL-55) and San Juan (CL-54) were laid down at Bethlehem Steel’s Quincy, Massachusetts, yard on Fore River. Both pairs were built in adjacent slips. Construction on all four began in spring 1940.

You can read more in NVG 340 US Navy Atlanta-class Light Cruisers 1940–49