Raubgraf 10 U-boatsStürmer 18 U-boatsDränger 11 U-boats 1939 1940 1941 1942 1943 1944 1945 During the Battle of the Atlantic, British merchant shipping was formed into convoys for protection against German submarine attack.

The North Atlantic winters offered the longest periods of darkness to conceal surfaced submarine operations.

The winter of 1942–43 saw the largest number of submarines deployed to the mid-Atlantic before comprehensive anti-submarine aircraft patrols could be extended into that area.

[3] One hundred merchant ships in trade convoys HX 229 and SC 122 encountered three wolfpacks of 38 submarines in a single sprawling action, which German radio reported as "the greatest convoy battle of all time" (Die grösste Geleitzugschlacht aller Zeiten).

[4] A Royal Navy report later concluded "The Germans never came so near to disrupting communications between the New World and the Old as in the first 20 days of March 1943".

(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.)

B5 Escort Group consisted of eight warships, led by Commander RC Boyle in the destroyer HMS Havelock, the destroyer USS Upshur, the River-class frigate HMS Swale, the Flower-class corvettes Buttercup, Godetia, Lavender, Pimpernel and Saxifrage, and a trawler as rescue vessel.

It was led on this occasion by Lieutenant Commander Gordon John Luther of HMS Volunteer, as its regular leader was in dock for repairs.

Taking a more northerly route than HX 229, the convoy remained undetected by German patrol lines and made a safe and timely landfall on 26 March.

[6] Arrayed against them were three patrol lines (rakes) of U-boats:[7] The German B-Dienst signals intelligence group, had given notice of an east-bound convoy and by 8pm on 13 March had a location for SC 122.

Admiral Karl Dönitz, commanding the U-boat fleet, directed Raubgraf to intercept, forming a new rake to the west.

This resulted in the British code breakers being starved of the cribs necessary to break "Shark", the cipher used by the German U-boats.

At the north-eastern end of Stürmer's rake, German submarine U-338 had sighted SC 122 heading east, about 120 miles from HX 229's position.

HX 229's escort suffered a blow as HMS Mansfield was forced to detach during the night of 17/18 March.

Arriving on 18 March, Day, as a senior and more experienced officer, took command of B4 Group for the rest of the engagement.

Also en route from Hvalfjord, in Iceland, were the destroyers HMS Vimy and USS Babbitt, for HX 229, and the US Coast Guard cutter USCGC Ingham for SC 122.

Further changes to the escort occurred on 20 March as reinforcement arrived in the form of the corvette HMCS Sherbrooke, while Upshur and Ingham were detached.

A Royal Navy report later concluded "It appeared possible that we should not be able to regard convoy as an effective system of defence".