Editing Battle of the Atlantic (section)

==The field of battle widens (June–December 1941)==
===Growing American activity===
[[File:Convoy WS-12 en route to Cape Town, 1941.jpg|right|thumb|A [[SB2U Vindicator]] scout bomber from USS ''Ranger'' flies anti-submarine patrol over Convoy WS-12, en route to [[Cape Town]], 27 November 1941.  The convoy was one of many escorted by the US Navy on "[[Neutrality Patrol]]", before the US officially entered the war.]]

In June 1941, the British decided to provide convoy escort for the full length of the North Atlantic crossing. To this end, the Admiralty asked the Royal Canadian Navy on 23 May, to assume the responsibility for protecting convoys in the western zone and to establish the base for its escort force at [[St John's, Newfoundland and Labrador|St. John's]], Newfoundland. On 13 June 1941, Commodore [[Leonard W. Murray|Leonard Murray]], Royal Canadian Navy, assumed his post as Commodore Commanding [[Newfoundland Escort Force]], under the overall authority of the Commander-in-Chief, Western Approaches, at Liverpool. Six Canadian destroyers and 17&nbsp;corvettes, reinforced by seven destroyers, three sloops, and five corvettes of the Royal Navy, were assembled for duty in the force, which escorted the convoys from Canadian ports to Newfoundland and then on to a meeting point south of Iceland, where the British escort groups took over.

By 1941, the United States was taking an increasing part in the war, despite its nominal neutrality. In April 1941 President Roosevelt extended the [[Pan-American Security Zone]] east almost as far as Iceland. British forces occupied Iceland when Denmark fell to the Germans in 1940; the US was persuaded to provide forces to relieve British troops on the island. American warships began escorting Allied convoys in the western Atlantic as far as Iceland, and had several hostile encounters with U-boats.

In June 1941, the US realised the tropical Atlantic had become dangerous for unescorted American as well as British ships. On 21 May, {{SS|Robin Moor}}, an American vessel carrying no military supplies, was sunk by {{GS|U-69|1940|2}} {{convert|750|nmi|km}} west of [[Freetown, Sierra Leone]]. When news of the sinking reached the US, few shipping companies felt truly safe anywhere. As ''Time'' magazine noted in June 1941, "if such sinkings continue, U.S. ships bound for other places remote from fighting fronts, will be in danger. Henceforth the U.S. would either have to recall its ships from the ocean or enforce its right to the free use of the seas."{{sfn|Time}}

A [[Mid-Ocean Escort Force]] of British, Canadian, and American destroyers and corvettes was organised following the declaration of war by the United States in December 1941.

At the same time, the British were working on technical developments to address the German submarine superiority. Though these were British inventions, the critical technologies were provided freely to the US, which then renamed and manufactured them. Likewise, the US provided the British with Catalina flying boats and [[Consolidated B-24 Liberator|Liberator]] bombers that were important contributions to the war effort.

===Catapult aircraft merchantmen===
[[File:The Royal Navy during the Second World War A9421.jpg|thumb|Sea Hurricane Mk IA on the catapult of a CAM ship]]

Aircraft ranges were constantly improving, but the Atlantic was far too large to be covered completely by land-based types. A stop-gap measure was instituted by fitting ramps to the front of some of the cargo ships known as ''catapult aircraft merchantmen'' ([[CAM ship]]s), equipped with a lone expendable [[Hawker Hurricane|Hurricane]] fighter aircraft. When a German bomber approached, the fighter was launched off the end of the ramp [[RATO|with a large rocket]] to shoot down or drive off the German aircraft, the pilot then [[Water landing|ditching]] in the water and—in the best case—recovered by ship. Nine combat launches were made, resulting in the destruction of eight Axis aircraft for the loss of one Allied pilot.{{sfn|Costello|Hughes|1977|pp=131, 180}}

Although CAM ships and their Hurricanes did not down a great number of enemy aircraft, such aircraft were mostly Fw 200 Condors that would often shadow the convoy out of range of the convoy's guns, reporting back the convoy's course and position so that U-boats could then be directed on to the convoy. The CAM ships and their Hurricanes thus justified the cost in fewer ship losses overall.

===High-frequency direction-finding===
{{main|High-frequency direction finding}}
[[File:HMS Kite IWM FL 22973.jpg|thumb|The distinctive HF/DF "birdcage" aerial can be seen at the masthead of {{HMS|Kite|U87|6}}]]

One of the more important developments was ship-borne direction-finding radio equipment, known as HF/DF (high-frequency direction-finding, or [[Huff-Duff]]), which started to be fitted to escorts from February 1942. These sets were common items of equipment by early 1943.<ref name="Mawdsley 2019">{{cite book |author-link=Evan Mawdsley |last1=Mawdsley |first1=Evan |title=The War for the Seas : a maritime history of World War II |date=2019 |publisher=Yale University Press |location=New Haven and London |isbn=978-0-300-19019-9 |edition=Kindle}}</ref> HF/DF let an operator determine the direction of a radio signal, regardless of whether the content could be read. Since the wolf pack relied on U-boats reporting convoy positions by radio, there was a steady stream of messages to intercept. An escort could then run in the direction of the signal and attack the U-boat, or at least force it to submerge (causing it to lose contact), which might prevent an attack on the convoy. When two ships fitted with HF/DF accompanied a convoy, a fix on the transmitter's position, not just direction, could be determined. The standard approach of anti-submarine warships was immediately to "run-down" the bearing of a detected signal, hoping to spot the U-boat on the surface and make an immediate attack. Range could be estimated by an experienced operator from the signal strength.  Usually the target was found visually. If the submarine was slow to dive, the guns were used; otherwise an ASDIC search was started where the swirl of water of a crash-diving submarine was observed. In good visibility a U-boat might try and outrun an escort on the surface whilst out of gun range.

The British also made extensive use of shore HF/DF stations, to keep convoys updated with positions of U-boats. HF/DF was also installed on American ships.

The radio technology behind direction finding was simple and well understood by both sides, but the technology commonly used before the war used a manually-rotated aerial to fix the direction of the transmitter. This was delicate work, took quite a time to accomplish to any degree of accuracy, and since it only revealed the line along which the transmission originated a single set could not determine if the transmission was from the true direction or its reciprocal 180&nbsp;degrees in the opposite direction. Two sets were required to fix the position. Believing this to still be the case, German U-boat radio operators considered themselves fairly safe if they kept messages short. The British developed an oscilloscope-based indicator which instantly fixed the direction and its reciprocal the moment a radio operator touched his [[Morse code|Morse]] key.  It worked simply with a crossed pair of conventional and fixed directional aerials, the oscilloscope display showing the relative received strength from each aerial as an elongated ellipse showing the line relative to the ship. The innovation was a 'sense' aerial, which, when switched in, suppressed the ellipse in the 'wrong' direction leaving only the correct bearing. With this there was hardly any need to triangulate—the escort could just run down the precise bearing provided, estimating range from the signal strength, and use look-outs or radar for final positioning. Many U-boat attacks were suppressed and submarines sunk in this way.

===Enigma cipher===
{{See also|Cryptanalysis of the Enigma#German Navy 3-rotor Enigma|l1=German Navy 3-rotor Enigma}}

The way Dönitz conducted the U-boat campaign required relatively large volumes of radio traffic between U-boats and headquarters. This was thought to be safe, as the radio messages were encrypted using the [[Enigma machine|Enigma]] [[Rotor machine|cipher machine]], which the Germans considered unbreakable. In addition, the {{lang|de|Kriegsmarine}} used much more secure operating procedures than the {{lang|de|Heer}} (Army) or {{lang|de|Luftwaffe}} (Air Force). The machine's three rotors were chosen from a set of eight (rather than the other services' five).{{sfn|Erskine|2004}} The rotors were changed every other day using a system of [[Key (cryptography)|key]] sheets and the message settings were different for every message and determined from [[Cryptanalysis of the Enigma#German Naval Enigma|"bigram tables"]] that were issued to operators. In 1939, it was generally believed at the British [[Government Code and Cypher School]] at [[Bletchley Park]] that naval Enigma could not be broken. Only the head of the German Naval Section, [[Francis Birch (cryptographer)|Frank Birch]], and the mathematician [[Alan Turing]] believed otherwise.{{sfn|Copeland|2004|p=257}}

[[File:Enigma rotors and spindle showing contacts rachet and notch.jpg|thumb|upright=1.0|Enigma rotors and spindle]]

The British codebreakers needed to know the wiring of the special naval Enigma rotors. The capture of several Enigma rotors during the sinking of {{GS|U-33|1936|2}} by {{HMS|Gleaner (J83)}} in February 1940 provided this information.{{sfn|Sebag-Montefiore|2004|p=76}} In early 1941, the Royal Navy made a concerted effort to assist the codebreakers, and on 9 May  crew members of the destroyer {{HMS|Bulldog|H91|2}} boarded {{GS|U-110|1940|2}} and recovered cryptologic material, including bigram tables and current Enigma keys. The captured material allowed all U-boat traffic to be read for several weeks, until the keys ran out; the familiarity codebreakers gained with the usual content of messages helped in breaking new keys.

In August 1940, the British began use of their "[[bombe]]" computer which, when presented with an intercepted German Enigma message, suggested possible settings with which the Enigma cipher machine had been programmed. A reverse-engineered Enigma machine in British hands could then be programmed with each set of suggested settings in turn until the message was successfully deciphered.<ref name=Wenger1945>{{cite report |author=Wenger, J. N. |date=12 February 1945 |title="Appendix II: U. S. Army Cryptanalytic Bombe", Solving the Enigma: History of the Cryptanalytic Bombe. National Archives and Records Administration Record Group 457, File 35701. |publisher=United States National Security Agency |url=http://ed-thelen.org/comp-hist/NSA-Enigma.html |url-status=dead |archive-url=https://web.archive.org/web/20141002211707/http://ed-thelen.org/comp-hist/NSA-Enigma.html |archive-date=2 October 2014 |access-date=11 July 2023}}</ref>

Throughout late 1941, Enigma intercepts (combined with HF/DF) enabled the British to plot the positions of U-boat patrol lines and route convoys around them. Merchant ship losses dropped by over two-thirds in July 1941, and the losses remained low until November.

This Allied advantage was offset by the growing numbers of U-boats coming into service. The [[German Type VII submarine|Type VIIC]] began reaching the Atlantic in large numbers in 1941; by the end of 1945, 568 had been [[Ship commissioning|commissioned]].<ref>{{cite web |url=http://www.uboat.net/types/viic.htm |title=Type VIIC |last=Helgason |first=Guðmundur |website=German U-boats of WWII – uboat.net |access-date=13 February 2010 }}</ref> Although the Allies could protect their convoys in late 1941, they were not sinking many U-boats. The Flower-class corvette escorts could detect and defend, but they were not fast enough to attack effectively.

===U-boat captured by an aircraft===
A Coastal Command [[Lockheed Hudson|Hudson]] of [[No. 209 Squadron RAF]] captured ''U-570'' on 27 August 1941 about {{convert|80|miles|km}} south of Iceland. Squadron Leader J. Thompson sighted the U-boat on the surface, immediately dived at his target, and released four depth charges as the submarine crash dived. The U-boat surfaced again, crewmen appeared on deck, and Thompson engaged them with his aircraft's guns. The crewmen returned to the conning tower under fire. A few moments later, a white flag and a similarly coloured board were displayed. Thompson called for assistance and circled the German vessel. A Catalina from 209 Squadron took over watching the damaged U-boat until the arrival of the armed trawler ''Kingston Agate'' under Lt Henry Owen L'Estrange. The following day the U-boat was beached in an Icelandic cove. No codes or secret papers were recovered, but the British now possessed a complete U-boat. After a refit, ''U-570'' was commissioned into the Royal Navy as {{HMS|Graph}}.{{sfn|Costello|Hughes|1977|p=168}}

=== Mediterranean diversion ===
In October 1941, Hitler ordered Dönitz to move U-boats into the Mediterranean Sea to support German operations in that theatre. The resulting concentration near Gibraltar produced a series of battles around the Gibraltar and Sierra Leone convoys. In December 1941, [[Convoy HG 76]] sailed, escorted by the 36th Escort Group of two sloops and six corvettes under Captain [[Frederic John Walker]], reinforced by the first of the new [[escort carrier]]s, {{HMS|Audacity|D10|6}}, and three destroyers from Gibraltar. The convoy was immediately intercepted by the waiting U-boat pack, resulting in a brutal five-day battle. Walker was a tactical innovator and his ships' crews were highly trained. The presence of an escort carrier meant U-boats were frequently sighted and forced to dive before they could get close to the convoy, at least until ''Audacity'' was sunk after two days. The five-day battle cost the Germans five U-boats (four sunk by Walker's group), while the British lost ''Audacity'', a destroyer, and only two merchant ships. The battle was the first clear Allied convoy victory.{{sfn|Costello|Hughes|1977|p=183}}

Through dogged effort, the Allies slowly gained the upper hand until the end of 1941. Although Allied warships failed to sink U-boats in large numbers,<ref>{{Cite book |last1=Morgan |first1=Daniel |title=U-boat attack logs: a complete record of warship sinkings from original sources 1939 - 1945 |last2=Taylor |first2=Bruce |date=2011 |publisher=Seaforth |isbn=978-1-84832-118-2 |edition=1. publ |location=Barnsley}}</ref> most convoys evaded attack completely. Shipping losses were high, but manageable.<ref>{{Cite book |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9781118325018 |title=A Companion to World War II |date=2012-12-10 |publisher=Wiley |isbn=978-1-4051-9681-9 |editor-last=Zeiler |editor-first=Thomas W. |edition=1 |language=en |doi=10.1002/9781118325018 |editor-last2=DuBois |editor-first2=Daniel M.}}</ref>