During the night of 17th/18th the attack on both convoys, now just 70 miles apart, continued. U-338 sank the freighter Granville, of SC122 in the evening, surviving a fierce counter-attack by escorts, and after midnight U-305 sank 2 more ships ( Port Auckland and Zouave); however, HX229 escaped further losses that night.
HX 229s escort suffered a blow as HMS Mansfield was forced to detach during the night of 17th/18th; however help was on its way in the form of destroyer HMS Highlander, under Cdr ECL Day RN. Arriving on the 18th, Day, as a senior and more experienced officer, he would take command of B4 group for the rest of the engagement. Also en route from Hvalfjord, in Iceland, were HMS Vimy and USS Babbitt, for HX 229, and USCG Ingham for SC 122. These were dispatched on the morning of the 18th, and arrived the following day. On the afternoon of the 18th U-221 succeeded in sinking 2 ships of HX 229, but further losses were avoided. HMS Highlander joined that afternoon, a welcome addition as B4 was by this time reduced to 5 ships.
During the night of 18th/19th the two convoys were running in tandem, though sailing independently. All attacks on both convoys were repelled this night, and 6 firm contacts were attacked, but little damage was inflicted. One ship from HX 229 was lost, a romper which broke away to proceed independently; this ship Matthew Luckenbach ran into the melee around SC 122 and was torpedoed, to be sunk later on the 19th. A straggler from SC 122, Clarissa Radcliffe was also lost; she disappeared without trace.
On 19th the escorts were reinforced by the arrival of Vimy and Babitt, for HX 229, and Ingham for SC 122. HX 229 was also joined by HMS Abelia, detached from another convoy. Also on the 19th U-384 was attacked by air patrol to the north of SC 122 and sunk. There were no further losses on the 19th; faced with stiffening resistance, and sensing nothing further would be achieved without disproportionate losses, Donitz called off the assault.
The convoys continued east; further changes to the escort occurred on the 20th as reinforcement arrived in the form of HMS Sherbrooke, while Upshur and Ingham were detached.
The local escort groups met on the 23rd,and HX 229, with 27 ships surviving, arrived at Liverpool on 23 March; SC 122, with 42 ships, arrived later the same day.