Light and heavy monitors filled different fire-support roles

The Royal Navy used the Humber class as mobile close-support platforms while larger monitors carried battleship-calibre weapons for distant fixed targets. Their 6-inch guns suited troops moving along a coast or river and could oppose destroyers in confined waters. Britain bought the ships after Brazil declined them because officials feared that a neutral intermediary might resell them to Germany. The unplanned purchase gave the Royal Navy a light-monitor capability it had not intended to build for itself. Humber served off the Belgian coast and at the Dardanelles, while Mersey and Severn later fought Königsberg in East Africa. HMS Humber also received a third 6-inch gun aft, increasing her close-support firepower without changing the class's basic role.