Mahan-class destroyer

Mahan-class destroyers of the United States Navy were a series of 18 destroyers of which the first 16 were laid down in 1934. The last two of the 18, Dunlap and Fanning (this pair laid down in 1935), are sometimes considered a separate ship class. All 18 were commissioned in 1936 and 1937. Mahan was the lead ship, named for Rear Admiral Alfred Thayer Mahan, an influential historian and theorist on sea power.

The Mahans featured improvements over previous destroyers, with 12 torpedo tubes, superimposed gun shelters, and generators for emergency use. The standard displacement increased from 1,365 tons to 1,500 tons. The class introduced a new steam propulsion system that combined increases in pressure and temperature with a new type of lightweight steam turbine, which proved simpler and more efficient than the Mahans' predecessors—to such an extent that it was used on many subsequent wartime US destroyers.

All 18 ships saw action in World War II, entirely in the Pacific Theater, which included the Guadalcanal campaign, and the battles of the Santa Cruz Islands, Leyte Gulf, and Iwo Jima. Their participation in major and secondary campaigns included the bombardment of beachheads, amphibious landings, task force screening, convoy and patrol duty, and anti-aircraft and submarine warfare. Six ships were lost in combat and two were expended in the postwar Operation Crossroads nuclear tests. The remainder were decommissioned, sold, or scrapped after the war; none remain today. Collectively, the ships received 111 battle stars for their World War II service.

The Mahan-class destroyers emerged as improved versions of the Farragut class, which incorporated the most up-to-date machinery available. The Navy's General Board had wrestled with the proposed design changes, first they considered 12 torpedo tubes with one fewer 5-inch (127 mm)/38 caliber gun, and then proposed to retain all five guns with the twelve torpedo tubes, but configure those guns only for surface targets, not air targets. The Chief of Naval Operations objected, and recommended against "subordinating the gun to the torpedo", and a compromise was struck that included a new engineering plant and a new battery arrangement for the Mahan class and others. In the final design, No. 3 gun was moved to the aft deckhouse (just ahead of No. 4) to make room for the third quadruple torpedo tube; the two middle torpedo tubes were moved to the sides, and released the centerline space for extension of the aft deckhouse. All five 5 in/38s were kept and remained dual purpose guns, able to target aircraft as well as ships, but only No. 1 and No. 2 had gun shields. The traditional destroyer machinery was replaced with a new generation of land-based machinery. This change ushered in a new steam propulsion system that combined increases in pressure and temperature with a new type of lightweight steam turbine, which proved simpler and more efficient to operate. Double reduction gearing also reduced the size of the faster-turning turbines and allowed cruising turbines to be added. These changes led to a ten percent increase in displacement over the Farraguts.

The Mahans typically had a tripod foremast with a pole mainmast. To improve the anti-aircraft field of fire, their tripod foremast was constructed without nautical rigging. In silhouette, they were similar to the larger Porter-class destroyers that immediately preceded them. The Mahans were fitted with the first emergency generators, which replaced the storage batteries of earlier classes. Gun crew shelters were built for the superimposed weapons, one shelter before the bridge and one atop the shelter deck aft.

The Mahans displaced 1,500 long tons (1,524 t) at standard load and 1,725 long tons (1,753 t) at deep load. The overall length of the class was 341 feet 3 inches (104.0 m), the beam was 35 feet 6 inches (10.8 m), and the draft 10 feet 7 inches (3.2 m). They were powered by General Electric geared steam turbines, driving two shafts that developed a total of 46,000 shaft horsepower (34,000 kW) for a maximum speed of 37 knots (69 km/h; 43 mph). Four Babcock & Wilcox or four Foster Wheeler water-tube boilers generated the superheated steam needed for the turbines. The Mahans carried a maximum of 523 long tons (531 t) of fuel oil, with a range of 6,940 nautical miles (12,850 km; 7,990 mi) at 12 knots (22 km/h; 14 mph). Their peacetime complement was 158 officers and enlisted men. The wartime complement increased to approximately 250 officers and enlisted men.

The Mahans' propulsion plant was considerably improved over that of the Farraguts. The steam pressure was raised from 400 psi (2,800 kPa) to 465 psi (3,210 kPa) in some ships, and the superheated steam temperature was raised from 648 °F (342 °C) to 700 °F (371 °C) in all ships. Double reduction gearing replaced single reduction gearing, and allowed smaller, faster-turning turbines to be used. This saved enough space and weight so that cruising turbines could be fitted, which greatly improved fuel economy at moderate speeds. The boiler economizers, as in previous ships, further improved fuel economy. The ships' range was extended to 6,940 nmi (12,850 km; 7,990 mi) at 12 knots (22 km/h; 14 mph), 1,000 nmi (1,900 km; 1,200 mi) farther than the Farraguts. The design shaft horsepower was increased from 42,800 shp (31,900 kW) to 48,000 shp (36,000 kW) in the same space and weight as in the Farraguts. The relatively compact power plant contributed to the Mahans' ability to carry 12 torpedo tubes instead of eight with only 150 tons of extra displacement. The main turbines were manufactured by the General Electric Company and were the impulse-type, also called the Curtis turbines. Each main turbine was divided into a high-pressure (HP) and a low-pressure (LP) turbine, which fed into a common reduction gear and drove a shaft, in a similar manner to the machinery illustrated at the following reference note. The steam from the boilers was supplied to the HP turbine, which exhausted to the LP turbine, in turn exhausted to the condenser. The cruising turbines were geared to the HP turbines and could be engaged or disengaged as needed. At low speeds, they were operated in a series with the HP turbines to improve the efficiency of the overall turbine arrangement, and also improved the fuel economy. This general arrangement with double reduction gearing became a standard for most subsequent steam-powered surface ships of the US Navy, although not all of them had cruising turbines.

The main battery of the Mahan class consisted of five dual purpose 5-inch (127 mm)/38 caliber guns, equipped with the Mark 33 gun fire-control system. The anti-aircraft battery had four water-cooled .50 caliber machine guns (12.7 mm). The class was fitted with three quadruple torpedo tube mounts for twelve 21-inch (533 mm) torpedo tubes, guided by the Mark 27 torpedo fire control system. The class was initially equipped with the Mark 11 torpedo or Mark 12 torpedo, which were replaced by the Mark 15 torpedo beginning in 1938. The depth charge roll-off racks were rigged on the stern.