Convoy ON 122 Convoy ON 122 was a trade convoy of merchant ships during the second World War . It was the 122nd of the numbered series of ON convoys Outbound from the British Isles to North America . The ships departed Liverpool on 15 August 1942 and were joined on 17 August by Mid - Ocean Escort Force Group B - 6 consisting of V and W class destroyer " Viscount " with the Norwegian - manned Flower class corvette s " Potentilla " , " Eglantine " , " Montbretia " , and " Acanthus " and the Convoy rescue ship " Stockport " . Background . As western Atlantic coastal convoys brought an end to the second happy time , Admiral Karl Dönitz , the " Befehlshaber der U - Boote " ( " BdU " ) or commander in chief of U - Boats , shifted focus to the mid - Atlantic to avoid aircraft patrols . Although convoy routing was less predictable in the mid - ocean , Dönitz anticipated that the increased numbers of U - boats being produced would be able to effectively search the area with the advantage of intelligence gained through " B - Dienst " decryption of British Naval Cypher Number 3 . However , of the 180 trans - Atlantic convoys sailing from the end of July 1942 until the end of April 1943 , only 20 percent lost ships to U - boat attack . The Norwegian - manned corvettes of escort group B - 6 fought three of these convoy battles in sequential voyages with convoys SC 104 , ON 144 , and HX 217 . Discovery . " U - 135 " discovered and reported the convoy on 22 August while patrolling a formerly assigned station after having missed the signal to change position . The initial report caused some confusion because of the unexpected position and a coding error , but after " U - 135 " sent two clarifying messages while shadowing the convoy , the wolf pack " Lohs " was ordered to converge on the convoy . Stalking . While the Norwegian corvettes investigated HF / DF bearings provided by " Viscount " and " Stockport " on 23 August , " Viscount " conserved fuel by declining to engage in long daylight stern chases with U - boats . " Viscount " and " Potentilla " attacked HF / DF contacts more aggressively through the hours of darkness , but were satisfied by simply forcing the U - boats to submerge rather than conducting sustained depth charge attacks . Attack . Visibility was reduced to 7 , 000 yards with patchy squalls under overcast skies on 24 August . As dusk approached , the escort had located only four of the nine U - boats in contact with the convoy . The convoy ' s course was altered to 267° at 2300Z . " U - 605 " torpedoed " Katvaldis " and " Sheaf Mount " on the starboard side of the convoy an hour after the course alteration . " Viscount " obtained a RADAR contact and forced the submarine to submerge . As " Viscount " was dropping depth charges , " U - 176 " and " U - 438 " entered the front of the convoy to torpedo " Trolla " and " Empire Breeze " . Disengagement . The convoy escorts effectively intercepted attacks through the pre - dawn hours of 25 August . The calm sea conditions were favourable for the Type 271 centimeter - wavelength RADAR with which all the escorts were equipped , and prompt counter - attacks prevented the U - boats from reaching torpedo launch positions . A depth charge attack by " Eglantine " holed the conning tower of " U - 605 " . " U - 135 " , " U - 174 " and " U - 438 " were also damaged by depth charges . The shadowing U - boats lost contact after the convoy entered heavy fog after daybreak on 25 August , and discontinued pursuit on 26 August . " U - 256 " was under repair for more than a year after being bombed in the Bay of Biscay on 31 August following depth charge damage from " Viscount " and " Potentilla " . " U - 438 " aided " U - 256 " reaching port , and " U - 174 " refueled three " Lohs " U - boats before returning to France to repair damage . " U - 705 " suffered several casualties when hit by gunfire from the convoy escorts ; and was sunk in the Bay of Biscay by Armstrong Whitworth Whitley s of No . 77 Squadron RAF on 3 September . The ships in the convoy dispersed off Cape Cod on 3 September to proceed independently to North American ports .