HMS Mersey as taken over by the Royal Navy in 1914. She and her two siblings were large, shallow draft river gunboats ordered by Brazil in 1912 to patrol the Amazon River. The Ships were built by Vickers, but Brazil was unable to pay. All three were eventually bought by the Royal Navy and commissioned as Humber-class monitors. They were propelled by a triple expansion steam plant which powered two large propellers placed in a tunnel between the rudders. Their armament consisted of a single - Vickers designed - 6" twin turret forward and two 4.7" howitzers aft. Four light 3pdr guns were mounted on the wings of the deckhouse.