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).

[1][2] 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.

[6] 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.

[10][11] After unloading her cargo, the tanker left England on 27 April and arrived at Port Eads on 19 May, successfully concluding her maiden voyage.

[15] In March 1923 several tankers including Hammac, still in the USSB possession were sold to Union Oil and General Petroleum Corporation.

[16] 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.

[19] 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.

[20][21] 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.

[25] Starting in August 1928 Emidio was shifted largely to West Coast operation, carrying oil and petroleum products to ports of Seattle, Portland and Tacoma.

[36] Subsequently, Emidio made one trip between Aruba and Balboa bringing approximately 10,500 tons of fuel oil for fleet operations there.

On her final Atlantic voyage she passed through the Panama Canal in early February 1940 carrying about 10,300 tons of fuel oil from Aruba bound for Los Angeles.

[38] In October 1940 Emidio made another trip to Pearl Harbor during an extensive ongoing expansion of Naval facilities in the Hawaii.

[40] With the hostilities raging in Europe and the Atlantic as well as deteriorating relations between United States and Japan, all General Petroleum tankers had American flag painted on their sides as neutrality markers in June 1941.

At approximately 15:15 the Japanese submarine I-17 fired a shot at the vessel, forcing the captain to stop and order the crew to abandon ship.

As the crew rushed to lower the lifeboats, the submarine continued shooting at the tanker from its 14 cm (5.5 in) deck gun hitting it five times.

One of the shells knocked off an unlaunched lifeboat causing its three crew members to fall into water and eventually drown in choppy seas.

Emidio was left by the crew in the sinking condition with her stern submerged, however, the tanker managed to stay afloat and slowly drifted up the coast of California until it ran aground on the rocks off Crescent City in early January 1942.

[51][52] Louis George Finch, able seaman aboard SS Emidio was given the Merchant Marine Distinguished Service Medal by the president of the United States.