During the Battle of the Atlantic , British merchant shipping was formed into convoys for protection against German submarine attack. Patrolling aircraft restricted the ability of submarines to converge on convoys during daylight. The North Atlantic winters offered the longest periods of darkness to conceal surfaced submarine operations. The winter of β43 saw the largest number of submarines deployed to the mid-Atlantic before comprehensive anti-submarine aircraft patrols could be extended into that area.
During March, there was a series of fierce convoy battles which became, for the Allies, the crisis point of the whole campaign. SC was a slow eastbound convoy of 60 ships, routed from New York to Liverpool. This was during the period when SC convoys were switched from Sydney , Cape Breton , to New York; this was reversed later due to congestion problems there. It sailed on 5 March , protected at first by one destroyer and five corvettes of the Western Local Escort Force.
On 6 March, off Cape Cod , two ships put back to New York due to heavy weather, and on 8 March, another six abandoned the crossing, and put into Halifax. The convoy pressed on, changing escorts on 13 March off Cape Race. HX was also eastbound and sailed from New York on 8 March, with 40 ships and the local escort. A further 34 ships which should have been included were delayed due to congestion at New York; they sailed the following day as HX A.
Luther, although an ASW specialist, had recently joined the group and this was only his second crossing. The other ships of B4 were the destroyers HMS Beverley , Mansfield and Witherington and the corvette Anemone , although Witherington had to detach on 15 March, to be replaced by the corvette Pennywort for the crossing. This comprised six sloop-type warships under Cdr. Taking a more northerly route than HX , the convoy remained undetected by German patrol lines and made a safe and timely landfall on 26 March.
The voyage was not without its perils though. The more northerly route took it into an icefield and two ships were damaged by icebergs, rerouting to Iceland.
