SS Atrato was a 3,184 tons iron built paddle steamer built by Messrs Caird & Co of Greenock, in 1853 for the Royal Mail Steam Packet Company. She ran on the South American service for 17 years. She was sold to John Morrison & Co London and in 1870 converted into a single-screw vessel with compound engines and three double-ended boilers. She has been listed as the largest passenger ship from 1853 to 1858.