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, and (this pair laid down in 1935), are sometimes considered a separate ship class. All 18 were commissioned in 1936 and 1937. 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.
Design The Mahan-class destroyers emerged as improved versions of the ,Reilly p. 28 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 /38 caliber gun,Friedman p. 86 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.Friedman pp. 87 & 88 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.Friedman p. 88
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 s that immediately preceded them.Reilly p. 28 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 at standard load and at deep load. The overall length of the class was , the beam was , and the draft . They were powered by General Electric geared steam turbines, driving two shafts that developed a total of for a maximum speed of . Four Babcock & Wilcox or four Foster Wheeler water-tube boilers generated the superheated steam needed for the turbines. The Mahans carried a maximum of of fuel oil, with a range of at . Their peacetime complement was 158 officers and enlisted men.Friedman p. 465 The wartime complement increased to approximately 250 officers and enlisted men.Roscoe p. 20
Engineering The Mahans' propulsion plant was considerably improved over that of the Farraguts. The steam pressure was raised from to in some ships, and the superheated steam temperature was raised from to in all ships.USS Mahan (DD-364) and USS Cummings (DD-365) General Information Book with as-built data at Destroyer History Foundation USS Dunlap (DD-384) and USS Fanning (DD-385) General Information Book with as-built data at Destroyer History Foundation 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 at , farther than the Farraguts. The design shaft horsepower was increased from to in the same space and weight as in the Farraguts.Friedman, pp. 88, 465–467Comparison of 1500-ton classes at Destroyer History Foundation 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.Bauer and Roberts, p. 184USS Mahan (DD-364) and USS Cummings (DD-365) General Information Book with as-built data 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.
As-built armament
The main battery of the Mahan class consisted of five dual purpose /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).Hodges and Friedman p. 111 The class was fitted with three quadruple torpedo tube mounts for twelve 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.
Wartime modifications In early 1942, the Mahan-class destroyers began a wartime armament refitting process to increase the ships' light anti-aircraft (AA) armament.Reilly p. 73 These followed the standard pattern of other five-gun destroyer classes, removing the No. 3 5-inch/38 mount and replacing it with ideally two twin Bofors 40 mm guns (1.6 in): sacrificing 20% of the anti-surface gun battery was preferable to a third of the torpedo battery.Friedman p. 208-209 Generally between four and seven 20 mm Oerlikon (0.79 in) mounts were installed, with the intended fit three forward of the bridge and two abreast the second funnel. Due to early shortages of Bofors mounts most of the class was not fully refitted until 1944, thus additional Oerlikons were installed in 1942–1943 in the intended Bofors positions.Hodges & Friedman p. 145 As weight compensation, the bridge wings were cut back, tripod foremast replaced by a pole mast, and several other elements eliminated, including the mainmast, torpedo tube blast shields, gun shelter on the after deckhouse, and the potato locker.Friedman p. 208-209
In January 1945, as part of the anti-Kamikaze refit program, removal of the two waist torpedo tubes was authorized, providing enough weight margin to replace the two 40 mm twin mounts with quads. Ultimately only Lamson was modified to this configuration.Friedman p. 217-218 In June, removal of the third centerline tube was authorized to make way for two 40 mm twin mounts abreast of the aft stack for a total of two quad and two twin mounts. All Oerlikons were removed except two mounts forward of the bridge, upgraded from singles to twins. Only Shaw received the twelve Bofors configuration, and when inclined after the refit she was found to be too top-heavy and had to land one of her 5-inch/38s. This largely negated the improved anti-aircraft armament and Commander Destroyers Pacific insisted she be reassigned to the Atlantic Fleet.Friedman p. 217-218 All ships receiving these AA modifications were to have additional directors installed with their new 40 mm mounts; these Mark 51s were to be replaced by new blind-firing GFFC Mark 63 installations with radar,Reilly p. 75 but Shaw at minimum retained four Mark 51s.Friedman p. 217-218
There were some unique modifications to individual ships, but the most noteworthy are the rebuilt Cassin and Downes. The original hulls were damaged beyond repair at Pearl Harbor and scrapped, but the machinery and main batteries were largely undamaged and installed in two new-built hulls at Mare Island. These rebuilt ships retained only two quadruple torpedo tubes (both mounted on the centerline above the main deck) and had six rather than five Oerlikons (four rather than three on the bridge), but otherwise had the same four 5-inch/38s and two twin Bofors of a typical Mahan in 1943/1944. These ships also had a new British-style bridge like those being mounted on 5-inch destroyer escorts, rebuilt 1,850 ton "leaders", and the new Sumner class destroyers. These were also the only two Mahans to mount the Mark 37 director for their 5-inch guns.Friedman p. 89-90
Dunlap class The Dunlap class was a two-ship destroyer class based on the Mahan design, listed as a separate class in some sources.Bauer and Roberts, pp. 184–186 The ships were and , the last two Mahans. Unlike the Mahans, the Dunlaps had the new Mark 25 enclosed mounts for the two forward 5-inch/38 caliber guns, with base rings housing projectile hoists that rotated with each of the guns; their ammunition was fed from a handling room below each mount.McComb p. 11 Dunlap and Fanning were the first US destroyers to use enclosed forward gun mounts rather than shields; their light pole foremast and lack of a mainmast visibly distinguished them from the Mahans.Reilly p. 35
Construction