Sperrbrecher were used extensively by the Germans in World War I. The Imperial Fleet had a total of thirty Sperrbrecher for clearing mine streets – eight were lost during the war. Some of these ships were equipped with airplanes, such as Rio Negro, Plauen or Wigbert. In World War II officially designated as 'Special Purpose Merchant Ships',[1] although termed by the Royal Air Force as Heavy Flak Ships,[3] the Sperrbrecher were converted from merchant ships for their special role, were primarily crewed by merchant seamen.