Hammac was a steam tank ship built in 1920–1921 by Bethlehem Shipbuilding Corporation of Alameda for the United States Shipping Board as part of the wartime shipbuilding program of the Emergency Fleet Corporation (EFC) to restore the nation's Merchant Marine. Early in 1923 the vessel together with two other tankers was sold to General Petroleum Corporation and renamed Emidio. The tanker spent the vast majority of her career carrying oil along the West Coast of the United States as well as between West and East coast. In December 1941 she was shelled and damaged by the Japanese submarine I-17 and eventually wrecked with a loss of five crewmen.

After the United States entry into World War I, a large shipbuilding program was undertaken to restore and enhance shipping capabilities both of the United States and their Allies. As part of this program, EFC placed orders with nation's shipyards for a large number of vessels of standard designs. Design 1047 tank ship was a standard tanker of approximately 10,100 tons deadweight designed by Bethlehem Shipbuilding Corp. and adopted by USSB.

Hammac was part of the order for six vessels placed by USSB with Bethlehem Shipbuilding Corporation on 30 April 1918 and was laid down on 30 November 1920 at the builder's shipyard at Potrero Point and launched on 25 May 1921 (yard number 5274). The ship was built on the Isherwood principle of longitudinal framing providing extra strength to the body of the vessel and had two main decks. The tanker had her machinery placed aft and a cargo pump room located amidships, and had cargo tanks constructed throughout the vessel with a total capacity to carry 3,369,862 gallons of oil. In addition, the ship had four booms and four winches to handle deck load and dry hold cargoes. The vessel had electric lights installed along the decks and was also equipped with wireless of De Forest type.

As built, the ship was 435.0 feet (132.6 m) long (between perpendiculars) and 56.2 feet (17.1 m) abeam, and had a depth of 33.5 feet (10.2 m). Hammac was originally assessed at 6,900 GRT and 4,248 NRT and had deadweight of approximately 10,000. The vessel had a steel hull, and a single 422 nhp (3,100 ihp, 2,300 kW) vertical triple expansion steam engine, with cylinders of 27-inch (69 cm), 47-inch (120 cm) and 78-inch (200 cm) diameter with a 48-inch (120 cm) stroke, that drove a single screw propeller and moved the ship at up to 11 knots (20 km/h; 13 mph). The steam for the engine was supplied by three single-ended Scotch boilers fitted for oil fuel.

The sea trials were held outside the Heads on 12–13 July 1921 with the tanker performing satisfactorily. Upon completion of the twelve hour long trial run, the ship returned to port and after applying final touches was handed over to the USSB.

Following the delivery to her owners the tanker was laid up in Benicia due to scarcity of cargo and overabundance of tonnage. Hammac remained idle until the end of January 1922 when demand picked up and it appeared the recession was easing. After undergoing maintenance, the ship was chartered for one trip by Union Oil to deliver gasoline to United Kingdom. At the time there was widespread confusion as to why the delivery was effected from the West Coast when it was cheaper and quicker to do the same from the East Coast of the United States. Nevertheless, after loading over 3,000,000 gallons of gasoline the tanker departed San Francisco on 14 March 1922 bound for Thames Haven. Hammac passed through the Panama Canal on 28–30 March and reached her destination on 21 April. After unloading her cargo, the tanker left England on 27 April and arrived at Port Eads on 19 May, successfully concluding her maiden voyage.

The tanker conducted one more trip carrying oil for Union Oil from the Gulf ports to Philadelphia before being switched to molasses trade. In that role Hammac carried molasses in bulk from Cuba to Gulf ports or Europe. For example, in December 1922 she brought in approximately 9,500 tons of molasses to Rotterdam. On her next trip the ship brought in a cargo of beet molasses from Holland to Baltimore. Due to lack of storage space for this type of cargo, it had to be unloaded into a discarded tanker Brindina. In March 1923 several tankers including Hammac, still in the USSB possession were sold to Union Oil and General Petroleum Corporation. Following the tanker's arrival in Baltimore she was inspected and transferred to her new owners and departed for West Coast on 26 May reaching San Francisco on 15 June. Under new ownership, Hammac conducted one trip with a cargo of gasoline to Philadelphia in July 1923 before returning to West Coast where she was renamed Emidio after the San Emidio oil fields.

In 1923-1924 the tanker was largely involved in transportation of gasoline from General Petroleum refineries on California to London, with occasional trips up the West Coast. For example, in January 1924 she left San Pedro with 75,000 barrels of gasoline bound for London, and in November 1924 she carried 66,000 barrels of oil to Seattle. In 1925 Emidio was shifted from her international duties to domestic, carrying oil and gasoline to ports both on East and West Coast of the United States, although she made one trip to Hamburg in September 1925. For example, the tanker brought 74,000 barrels of gasoline from San Pedro to Baltimore in August 1925. In 1926 through major part of 1928 the tanker continued to mainly serve her West to East Coast route. Early in 1926 rumors spread about possible merger between General Petroleum and Standard Oil of New York. These rumors were confirmed when representatives from both companies met in March 1926 discussing the proposed merger. Following these successful negotiations both companies held shareholder meetings on 17 May 1926 which approved the proposed merger between the oil concerns. As a result, all assets including vessels of General Petroleum passed to Standard Oil of New York, however, General Petroleum continued to operate in California under its own name.