Convoys ONS 20 / ON 206 ONS 20 and ON 206 were North Atlantic convoys of the ONS / ON series which ran during the battle of the Atlantic in World War II . They were the subject of a major U - boat attack in October 1943 , the third battle in the German Navy ’ s " Kriegsmarine " KM ’ s autumn offensive . Background . Despite the losses suffered by " Rossbach " and poor results achieved during its attack on SC 143 . Still believing their new weapons and tactics gave them an advantage , BdU ( " Befehlshaber der Unterseeboote " , Commander of the Submarines ) re - organized the boats then in the North Atlantic into patrol line " Schlieffen " with the intention of intercepting the expected west - bound convoys . These were ONS 20 and ON 206 . Ships involved . ONS 20 departed Liverpool on 9 October 1943 . Composed of 52 ships it was bound for Halifax , and was escorted by B - 4 escort group , which comprised 6 destroyer escorts led by HMS " Bentinck " ( Cdr HR Paramor as Senior Officer Escort ) . ON 206 departed Liverpool on 11 October 1943 ; Composed of 62 ships , mainly in ballast , it was bound for New York and was escorted by B - 6 escort group , comprising 2 destroyer s , " Fame " ( Cdr RA Currie SOE ) and " Vanquisher " , the frigate " Deveron " , and 3 corvette s . Also in the vicinity was Escort group B - 7 , in the role of a support group ; it comprised 2 destroyers , " Duncan " ( Cdr P Gretton ) and " Vidette " , and 4 corvettes . Group " Schlieffen " comprised a number of U - boats from group " Rossbach " , re - inforced by new - comers from bases in France and Germany . It had already come under attack whilst re - fueling by aircraft from USS " Card " ; one boat was damaged and forced to retire , and 2 boats ( " U - 402 " and " U - 603 " ) had been sunk . Another 2 boats had collided , and one of these was also forced to return to base for repair . At the time of the attack on convoys " Schlieffen " consisted of 14 boats , with a mix of experience and type . When Allied intelligence learned of the position of " Schlieffen " , Western Approaches Command decided to seek battle , increasing the defence by joining the two convoys and their escorts , and sending B - 7 as reinforcement . Action . On 15 October 1943 the convoys were sighted by " U - 844 " , and group " Schlieffen " was direct to converge on the position . By evening a number of boats were in position and commenced the attack , which became one of the most calamitous of the entire campaign for the U - boat Arm ( " U - Bootwaffe " , UBW ) . During the night of 15 / 16 October the U - boats attacked but were repelled ; " U - 844 " was damaged by " Duncan " and " Vanquisher " and driven off . Later on 16 October " U - 844 " was attacked and sunk by two Liberator s , from RAF 86 and 59 squadrons ; one Liberator was shot down , but the crew rescued by the corvette " Pink " . Also on 16th " U - 964 " was sunk by a Liberator of RAF 86 ; her survivors were later picked up by " U - 231 " . Early that day " U - 470 " was damaged by a Sunderland of RCAF 422 sqdn ; the Sunderland was also damaged , ditching later near the ships of ONS - 20 and its crew rescued by HMS " Drury " . Later , " U - 470 " was attacked again , by 2 Liberators from RAF 120 and another from RAF 59 , and sunk ; 2 of her crew only were picked up by " Duncan " . The only success for the " Schlieffen " boats that day was when " U - 426 " sank the freighter " Essex Lance " , which was straggling from ONS - 20 . During the night of 16 / 17 October the attack was renewed , but repelled with no success ; " U - 540 " was attacked and damaged by a Liberator of 59 sqdn ; later that day she was attacked again and sunk by 2 Liberators from RAFs 59 and 120 . Also on 17 October U - 841 was sunk by " Byard " of B - 4 group . During the night of 17th / 18th the shadower " U - 631 " was sunk by " Sunflower " of B - 7 group . At this point the convoys made a drastic alteration in course , in order to shake off any shadowers ; this was successful in that BdU received two conflicting reports from U - 91 and U - 413 , leading to Schlieffen being sent in the wrong direction . After pursuit was shaking off , " Schlieffen " was unable to re - gain contact , and in view of the losses sustained BdU ordered the group to retire . On 20 October , with no further attack developing , B - 7 Group detached from the convoys to join ON 207 , which was following ; ONS 20 was handed over to its Western Local Escort Group on 22 October , and arrived at Halifax without further loss on 26 October 1943 . ON 206 met its Local Escort Group on 24 October and arrived at New York on 27 October 1943 . Conclusion . BdU regarded this operation as a setback , but remained confident that their new weapons and tactics would achieve success . However the loss of 6 U - boats for the sinking of just one ship was a major defeat for the UbW and seen as a victory for the Allies . Tables . Allied ships sunk Axis submarines destroyed