(AO-45: length 441'8"; beam 64'0"; speed 13 knots; armament five 4-inch guns)
Gulfdawn, a single-screw oil tanker, was built in 1936 at Chester, Pa., by the Sun Shipbuilding & Drydock Corp. and operated by the Gulf Oil Corp. Acquired by the Navy on 31 March 1942, she was renamed Big Horn and designated AO-45 on 3 April 1942; converted to Navy use at the Bethlehem Shipyard in Brooklyn, New York; and commissioned on 15 April 1942, Comdr. James A. Gainard, USNR, in command.
Sailing to Boston on 23 April, Big Horn entered the Boston Navy Yard for conversion to a "Q-ship." A disguised heavily armed merchantman, the decoy ship was intended to lure unsuspecting U-boats to the surface and sink them with gunfire. While at Boston, Big Horn completed her disguise as a fleet oiler and was given extra watertight integrity, in case she was torpedoed, by the installation of thousands of sealed empty drums in her cargo tanks.
Completing her shakedown cruise to Casco Bay, Maine, on 26 August, Big Horn underwent a repair period at Boston until 12 September. As German U-boats had been attacking bauxite ore cargo ships in the West Indies, the Q-ship sailed south to help defend the convoy routes there on the 27th. After joining south-bound convoy GAT-11 at Guantanamo Bay, Cuba, the ship, using her old call sign of Gulf Dawn, purposefully lagged behind the convoy enroute to Trinidad. No U-boats were tempted to attack, however, and the ship moored in Port of Spain on 9 October.
After waiting for another convoy to form up, Big Horn joined TAG-20 and, on 10 November, sailed for Curacao in the Dutch West Indies. Unfortunately, she trailed so far behind the convoy that, when U-163 torpedoed gunboat Erie (PG-50) on 12 November, she could neither fire a shot in defense nor render assistance. After suffering damage in heavy weather, the Q-ship sailed north and arrived in New York on 1 December.
The next day, Big Horn moved to Bethlehem Steel in Hoboken, New Jersey, for what proved to be nine weeks of repair work and alterations. The latter included the installation of a "hedgehog" depth charge projector and a DF (direction finding) radio receiver. She departed New York on 17 February 1943 and arrived at New London, Connecticut, the following day. There, she conducted shakedown training until mid-March. This training included submarine tracking exercises with Mingo (SS-261) in Long Island Sound. After a short trip to New York between 20 and 28 March, the Q-ship then joined PC-617 and PC-618 in a "Hunter-Killer" group and conducted antisubmarine training against Muskallunge (SS-262) until 9 April. Big Horn and the two subchasers then sailed south in Task Group (TG) 21.8 to New York, arriving there on 9 April.
Sailing with Mediterranean-bound convoy UGS-7A on 20 April, Big Horn and the two subchasers crossed the Atlantic until reaching a position about 500-miles south of the Azores. During this voyage, the subchasers practiced refueling at sea. On 30 April, the Q-ship dropped astern of the convoy in a straggling position. Her two accompanying subchasers also dropped astern, lurking even farther behind over the horizon.
On 3 May, Big Horn made radar contact with a suspected U-boat at a range of about six miles and sent the two PCs to investigate. Shortly thereafter, Big Horn made radar contact on a periscope at a range of 3,400 yards, closed the position, and made five hedgehog and depth charge attacks. After the third hedgehog attack, five of the bombs exploded about 12 seconds after hitting the water. According to Big Horn's action report, a "considerable amount of light oil came to surface," and the Q-ship lost contact with the suspected submarine. A postwar review of German U-boat losses, however, indicated that no submarines were sunk on that date in this area.
After returning to New York on 17 May, the ship underwent another overhaul between 19 May and 16 July. Big Horn and TG 21.8 then joined convoy UGS-13 and sailed for the Mediterranean on 20 July. Big Horn made another attack on a suspected U-boat south of the Azores on 8 August, but again failed to confirm any damage. After stopping at Recife, Brazil, between 27 August and 6 September, the Q-ship cruised unsuccessfully in the mid-Atlantic before returning to New York on 7 October.
Big Horn then steamed to New London on the 30th, whence she conducted training in Long Island Sound, before departing on a third "decoy" cruise on 19 December. Although she operated near a suspected U-boat concentration in the waters off Bermuda, the Q-ship and her two subchasers had no contacts and returned to New York empty-handed on 30 December.
Given the cancellation of the Q-ship program, following similar meager results and even losses by other Q-ships, Big Horn was ordered to Boston for another assignment. Arriving there on 17 January 1944, the ship was transferred to the Coast Guard for duty as an ocean weather station ship. Her name was struck from the Navy List on 22 January 1944. The ship was assigned to the 1st Naval District and operated out of Boston. Her main duty was to conduct 25-day patrols on the Coast Guard's mid-ocean weather stations and report on surface and aerial weather conditions. These reports were used to determine air-ferry routes across the Atlantic and to reroute shipping around storm concentrations.
Although she kept her Coast Guard crew, Big Horn was returned to Navy control on 1 February 1945 and redesignated IX-207 two days later. Over the next five weeks, the ship was converted into an oil shuttle and storage vessel before departing for the Pacific on 11 March. After loading 84,000 barrels of oil at Aruba, Netherlands West Indies, on the 18th, she passed through the Panama Canal on 21 March and reported to the Service Force, Pacific Fleet, that same day.
Steaming west, Big Horn stopped at Pearl Harbor in early April before sailing on to the Marshall Islands, where she anchored at Ulithi on 1 May. Assigned to Service Squadron (ServRon) 10, the shuttle tanker carried oil to Kossol Roads and Peleliu in the Western Carolines in early May before moving on to Tacloban in the Philippines later in the month. Returning to Ulithi on 3 June, she loaded more oil and delivered it to Leyte on the 9th. Over the next eight weeks, Big Horn carried out three more of these shuttle missions from Ulithi to Leyte. The tanker sailed to Okinawa on 11 August and she was at that island on 15 August 1945 when her crew heard the news of the Japanese surrender.
Departing Okinawa on 29 September, Big Horn steamed to Japan, where she was assigned duty as a station tanker at Nagoya on 3 October. She remained there through January 1946. After transferring her cargo of oil to Beagle (IX-112)