Built by R. Thompson & Sons at Sunderland in 1930, Harmonic was ordered for J. & C. Harrison's National Line, all of whose ships bore names prefixed with 'Har'. This particular vessel, their third Harmonic, was designed with a distinctively long bridge-deck and was broadly similar to seven others all launched for the company in the same year. Registered at 4,558 tons gross (2,709 net), Harmonic measured 395½ feet in length with a 54½ foot beam and was powered by triple-expansion engines from the North East Marine Engine Co., also of Sunderland. Equipped with five holds, she [and her sisters] were mostly employed in the South American (River Plate) grain trade, usually carrying coal on the outward journey, although they were often to be seen with either locomotives or rolling stock lashed to their decks and destined for the British-owned Central Argentine Railways.

Harrison's suffered a total of nineteen losses during the Second World War, one of which was Harmonic, which was torpedoed and sunk off the Brazilian coast on 15th July 1943 whilst on passage from Buenos Aires to the U.K. with a bulk cargo of 7,368 tons of linseed oil; fortunately only one crewman out of the forty-seven persons aboard lost his life.