Sperrbrecher A Sperrbrecher ( German ; informally translated as " pathfinder " but literally meaning " minefield breaker " ) , was a German auxiliary ship of the Second World War that was intended to serve as a type of minesweeper , by sailing ahead of other vessels through minefields , intending to detonate any mines in their path . Also used as anti - aircraft ships , the " Sperrbrecher " suffered heavy losses in the war . Operational history . Officially designated as ' Special Purpose Merchant Ships ' , although termed by the Royal Air Force as ' Heavy Flak Ships ' , the " Sperrbrecher " were converted from merchant ships for their special role , were primarily crewed by merchant seamen , and often had their cargo hold s filled with buoyant material to aid in flotation in case of hitting a mine . The bows were also strengthened . Ships converted to the " Sperrbrecher " type were usually fitted with heavy anti - aircraft armament , and often carried barrage balloons . The primary use of the " Sperrbrecher " was to escort other vessels through cleared paths in defensive minefields , for the purpose of detonating any mines that might have strayed into the passageways . In addition , the ships of the " Sperrbrecher " type would , early in the war , be used to clear suspected enemy minefields by simply sailing through them . Even with the strengthened hull and buoyant material , however , the ships suffered heavy losses in this role , and with the advent of acoustically and magnetically fused mines , this role became ineffective . Later in the war the " Sperrbrecher " type ships were used to escort U - boat s in and out of harbour . Due to their heavy , dual purpose armament and respectable fire control a " Sperrbrecher " was also a capable surface combattant , significant enough to deter a RN escort destroyer from engaging for fear of " a bloody nose " . In an attempt to counter newer , magnetically fused mines , some ships of the " Sperrbrecher " were equipped with a large electromagnet fitted in their bows . Referred to as the VES system , this was intended to detonate magnetic mines well clear of the vessel , the design specifications calling for a distance of from the hull at detonation . However , careful military intelligence work by the Royal Navy resulted in a method to defeat this method of minesweeping , sinking several " Sperrbrecher " through the careful fusing of mines laid as traps , their fuses desensitised to be activated only when the sweeping vessel was directly above them . Over one hundred vessels , mostly merchant ships of around and larger displacement , were converted to serve in the " Sperrbrecher " role , and it is estimated that around 50 percent of the vessels converted were lost during the war . During World War II only one commander received the Knight ' s Cross of the Iron Cross for services on a " Sperrbrecher " . Korvettenkapitän of the Reserves Karl Palmgreen received the award on 3 August 1941 as commander of " Sperrbrecher " IX and I . Following the end of the war , some of the surviving " Sperrbrecher " were converted back to merchant duties , and some of these vessels continued in their more peaceful service until the 1970s .