Ton - class minesweeper The Ton class were coastal minesweeper s built in the 1950s for the Royal Navy , but also used by other navies such as the South African Navy and the Royal Australian Navy . They were intended to meet the threat of seabed mines laid in shallow coastal waters , rivers , ports and harbours , a task for which the existing ocean - going minesweepers of the " Algerine " class were not suited . Description . The design of the class was led by the shipyard John I . Thornycroft & Company , and drew on lessons learnt in the Korean War , and numbered 119 vessels . They were diesel powered vessels of 440 tons displacement fully laden , constructed of wood and other non - ferromagnetic materials . Their small displacement and shallow draft gave them some protection against pressure and contact mines , and allowed them to navigate in shallow inshore waters . Primary armament was one Bofors 40 mm gun , although the South African variants also had an Oerlikon 20 mm cannon behind the funnel . RN vessels also had the same but they were gradually removed and an M2 Browning machine gun mounted midships . Sweeping equipment was provided for moored mines and magnetic mines . Many of the class were converted to minehunters by the incorporation of active rudders and the installation of the Type 193 minehunting sonar and associated equipment , including a very welcome enclosed bridge ( the exception being HMS " Highburton " who retained her open bridge until de - commissioning in the 1970s , this actually becoming a source of manliness to her crew when meeting other Ton crews ) . These vessels only retained mechanical " Oropesa " sweep capability . The class served as patrol vessels in Borneo , Malaysia , Northern Ireland and Hong Kong . The minehunters played a significant role in the Suez Canal clearance after the Yom Kippur war . They also provided the backbone of the UK ' s Fishery Protection Squadron ( 4th MCM ) . Five of the class in Royal Navy Service were permanently converted to patrol craft for service policing Hong Kong ' s territorial waters in 1971 . These vessels , comprising HM Ships " Beachampton " , " Monkton " , " Wasperton " , " Wolverton " and " Yarnton " had their minesweeping gear removed and were fitted with a second Bofors 40 mm gun aft of the funnel . They also received new pennant numbers : " Beachampton " P1007 , " Monkton " P1055 , " Wasperton " P1089 , " Wolverton " P1093 and " Yarnton " P1096 . It was originally planned to name the ships after insects , with names like " Red Ant " , " Green Cockchafer " and so on , but this plan was abandoned and the Royal Navy ships of the class were given names of British towns and villages ending in " - ton " , hence the name of the class . With the rundown of the Royal Navy fleet in the 1960s , many were sent to become base ships for the Royal Naval Reserve allowing reserve crews to get to sea for short periods without a lot of effort to organise a crew of significant size . Some of these had their names changed to reflect the RNR Division they were attached to . The RNR vessels lasted until the introduction of the River - class minesweepers in 1984 . The remainder of the RN ships paid off in the 1990s . Ships . Royal Navy : South African Navy : Royal New Zealand Navy :