 The navy's existing minesweepers were obsolete, while the increasing sophistication of modern mines meant the mine warfare forces could not be supplemented by requisitioned fishing vessels as had been done in previous wars.Large orders were placed for coastal minesweepers (the ) and for smaller inshore minesweepers and minehunters intended to operate in inshore waters such as river estuaries (the and classes).As the navy did not have sufficient manpower to operate all the required ships in peacetime, it was planned to lay a large number up in reserve, so they could be manned by reservists (in may cases the crews of the fishing boats which would previously have been used in the same role) in time of emergency.