New Mexico-class Battleship, USS Idaho (BB-42) - A Complete Modernization

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USS Idaho
As Outfitted in 1945
USS Idaho off of Kwajalein in January, 1944.

USS Idaho was the third ship of the New Mexico-class of battleships. She was laid down on the 20th of January, 1915, launched on the 30th of June, 1917, and commissioned into the US Navy on the 24th of March, 1919.

The New Mexico-class was the third battleship class under the USN’s Standard-type umbrella. In most respects, the New Mexicos were copies of the preceding Pennsylvanias, but with the gun length increased from 45-caliber to 50-caliber. All ships of the New Mexico-class would be modernized throughout their service lives, with modifications such as new radar sets, and increasing the main guns’ elevation angle to 30 degrees. Unlike her sisters, Idaho received true dual purpose secondaries in late 1944, in the form of ten single-mount 5"/38s replacing her eight 5"/25 guns.

Idaho would serve throughout the interwar period and to the end of WW2, and she earned seven battle stars for her service. She was decommissioned on the 3rd of July, 1946, and scrapped the next year.

Service History

Commissioned on the 24th of March, 1919, USS Idaho did not participate in the closing diplomacy of WW1 like her sister New Mexico. shakedown cruise was a trip to Guantanamo Bay and back to New York City.

USS Idaho under the Brooklyn Bridge, 1919.

There, she took the then President of Brazil, Epitácio Pessoa, to Brazil, leaving on July 6th and arriving at Rio de Janeiro on July 17th. After the President disembarked, she continued to the Panama Canal enroute to join the Pacific Fleet, where she participated in exercises and a fleet review for President Woodrow Wilson.
She remained in the Pacific until 1931, when she departed for Norfolk Navy Yard for an overhaul. She received new turbines and boilers, along with some 5"/25 AA guns. Fleet maneuvers increased in frequency with the rising Japanese expansionism, until 1940 where Idaho and her sisters, among others, were transferred to the Atlantic to participate in the Neutrality Patrols. Idaho was in Iceland when Japan attacked Pearl Harbor on December 7th, 1941, bringing war to the United States.

USS Idaho anchored at Hvalfjörður, Iceland, October 1941.

With the United States now being active in WW2, Idaho and her sister Mississippi left Iceland for the Pacific, arriving on the 31st of January, 1942. Idaho was in combat training for much of 1942, but in October she went to the Puget Sound Navy Yard for replacements for her worn out guns, along with general repairs. The original secondary 5"/51 guns were all removed, as they were needed to arm merchant ships, and she continued training until August 1943, when she departed for the Aleutians. There, she was the flagship of the bombardment force, and participated in shore bombardment until returning to San Francisco in preparation for deployment to the central Pacific.
Idaho then participated in more bombardment missions in the Gilbert Islands, returning to Pearl Harbor in late 1943 to prepare for attacks on the Marshal Islands. In January 1944, she returned for more shore bombardment. In March, she and a group of escort carriers steamed for the Marianas, where Idaho bombared both Saipan and Guam. She remained with the invasion fleet during the Battle of the Philippine Sea, and continued bombarding Guam until returning to Espiritu Santo on August 15th for repairs in a floating drydock.

Following these repairs, she went to Guadalcanal for training and then Peleliu, where she provided support until late September, at which point she left for an overhaul. During this refit, she received 5"/38 guns, the only one of her class to receive such weapons. These required the removal of all the 5"/25s and the last of the old 5"/51s, as the new guns needed continuous ammunition hoists.
She then returned to the Marianas with a new bombardment group, and participated in the Battle of Iwo Jima. She remained for nearly a month, from February 19th to March 7th, before returning to Ulithi for resupply. On the 21st of March, she joined the bombardment group for the Battle of Okinawa, as the flagship of Bombardment Unit 4. She began bombardment on the 25th of March, and the battle reached land on the 1st of April.

USS Idaho about to be hit by a kamikaze strike, on April 12th, during the Battle of Okinawa. View from USS Texas.

This battle marked the height of kamikaze attacks in the Pacific, by increasingly desperate Japanese defenders, with Idaho shooting down five kamikazes during a massed attack on the 12th. In return, she was damaged by a near-miss, and she had to return to Guam for repairs. She was back at Okinawa on the 22nd of May, remaining there until the 20th of June. She then went to the Philippines, which is where she was when Japan surrendered on August 15th.

Idaho was among the ships that entered Tokyo Bay on the 27th of August, carrying occupation troops, and was present during the signing of the Japanese Instrument of Surrender, on the 2nd of September, 1945. Idaho earned seven battle stars during the war.
With that, Idaho’s role in WW2 was over, and she left Japanese waters on the 6th of September, for the East Coast of the United States. She arrived there in October, where she remained until being decommissioned on the 3rd of July, 1946, and ultimately sold for scrap on the 24th of November, 1947.

USS Idaho docked before transiting the Panama Canal, enroute to the Eastern United States, 1945.

Inboard and outboard profiles of USS Idaho, 1945.


General Information
Displacement 36,737 tons (full load)
Length 624ft (190m)
Beam 97ft 5in (29.69m)
Draft 30ft (9.1m)
Speed 21 knots (39 km/h)
Complement 1443 officers and enlisted
SK-1 Air Search
SG Surface Search
Mk 8 Radar Fire Control
Gun Turret/Mount
12 × 14"(356mm)/50 Mk 11 4 × Triple
10 × 5"(127mm)/38 Mk 12 10 × Single Mk 30
40 x 40mm/60 Mk 1/2 10 x Quad
43 x 20mm/70 Oerlikon Mk 2 ?
2 x OS2U-3 Kingfisher Floatplanes 2 x Aircraft Catapult
14" (356mm) Ammunition
Designation Mass Bursting Charge Muzzle Velocity Notes
AP Mk 16 1,500lbs (680.4kg) 22.9lbs (10.4kg) Exp. D 2,700f/s (823m/s)
AP Mk 8 1,400lbs (635kg) 34.3lbs (15.6kg) Exp. D 2,800f/s (853m/s)
AAC Mk 19 1,275lbs (578kg) 104.21lbs (47.3kg) Exp. D 2,825f/s (861m/s) HC fuses could be loaded with PD (Point Detonating) or MT (Mechanical Time) fuses, and were considered AAC rounds if using MT. What would ordinarily be called HC Mk 19 is designated AAC Mk 19 here, for this reason.
HC Mk 22 1,275lbs (578kg) 104.21lbs (47.3kg) Exp. D 2,825f/s (861m/s) Essentially the same as the preceding AAC Mk 19, but instead using a PD fuse.
Belt 8-13.5" (203-343mm)
Deck 3.5" (89mm)
Turrets 5-18" (127-457mm)
Barbettes 13" (330mm)
Conning Tower 11.5" (292mm)

USS Idaho, I think, would be the perfect New Mexico-class to be added to the tree. She fulfills the role of a proper battleship while also gaining access to true dual-purpose weapons, allowing defense against small boats and aircraft alike with ease. At the same time, she represents probably the apex of the New Mexico-class, and as such I think deserves to be added.


Wikipedia - USS Idaho (BB-42)
Wikipedia - New Mexico-class Battleship
Naval-Encyclopedia - New Mexico-class Battleship
NavWeaps - US 14"/50 Mk 4 and Mk 6
NavWeaps - US 14"/50 Mk 7 and Mk 11
History.Navy.Mil - USS Idaho (BB-42)
NavSource - USS Idaho

Should be pointed out that USS Idaho was the only battleship ever to be fitted with 5"/38 single mounts. Several battleships (of both earlier and later classes) were given 5"/38 twin mounts in their modernizations, but Idaho’s secondary armament configuration with Fletcher style turrets was entirely unique.