Editing Battle of the Atlantic (section)

==Battle returns to the mid-Atlantic (July 1942 – February 1943)==
[[File:Allied convoy underway in the Atlantic Ocean near Iceland, circa in 1942 (80-G-72409).jpg|thumb|Allied convoy near Iceland, 1942]]
With the US finally arranging convoys in their sector of the Atlantic, ship losses to the U-boats quickly dropped, and Dönitz realised his U-boats were better used elsewhere. On 19 July 1942, he ordered the last boats to withdraw from the United States Atlantic coast; by the end of July 1942 he had shifted his attention back to the North Atlantic, where allied aircraft could not provide cover—i.e. the [[Black Pit]]. [[Convoy SC 94]] marked the return of the U-boats to the convoys from Canada to Britain. The command centre for the submarines operating in the West, including the Atlantic also changed, moving to a newly constructed command bunker at the [[Château de Pignerolle]] just east of [[Angers]] on the Loire river. The headquarters was commanded by [[Hans-Rudolf Rösing]].<ref name="PdlSGM">{{Cite web|url=https://docplayer.fr/9303523-Pignerolle-dans-la-seconde-guerre-mondiale.html|title=Pignerolle dans la Seconde Guerre mondiale - PDF Téléchargement Gratuit|website=docplayer.fr}}</ref>

There were enough U-boats spread across the Atlantic to allow several wolf packs to attack many different convoy routes. Often as many as 10 to 15 boats would attack in one or two waves, following convoys like [[Convoy SC 104|SC 104]] and [[Convoy SC 107|SC 107]] by day and attacking at night. Convoy losses quickly increased and in October 1942, 56 ships of over 258,000&nbsp;tonnes were sunk in the "air gap" between [[Greenland]] and Iceland.

U-boat losses also climbed. In the first six months of 1942, 21 were lost, less than one for every 40 merchant ships sunk. In the last six months of 1942, 66 were sunk, one for every 10 merchant ships, almost as many as in the previous two years together.
[[File:A convoy conference in progress, August 1942. A11796.jpg|thumb|A convoy conference in progress, August 1942]]
On 19 November 1942, Admiral Noble was replaced as Commander-in-Chief of [[Western Approaches Command]] by Admiral [[Max Kennedy Horton|Sir Max Horton]]. Horton used the growing number of escorts becoming available to organise "support groups", to reinforce convoys that came under attack. Unlike the regular escort groups, support groups were not directly responsible for the safety of any particular convoy. This gave them much greater tactical flexibility, allowing them to detach ships to hunt submarines spotted by reconnaissance or picked up by HF/DF. Where regular escorts would have to break off and stay with their convoy, the support group ships could keep hunting a U-boat for many hours. One tactic introduced by [[Frederic John Walker|Captain John Walker]] was the "hold-down", where a group of ships would patrol over a submerged U-boat until its air ran out and it was forced to the surface; this might take two or three days.<ref name="Benedict 2005 93–120"/>

After [[Convoy ON 154]], winter weather provided a brief respite from the fighting in January before convoys [[Convoy SC 118|SC 118]] and [[Convoy ON 166|ON 166]] in February 1943, but in the spring, convoy battles started up again with the same ferocity. There were so many U-boats on patrol in the North Atlantic, it was difficult for convoys<!--convoys were routed evasively, not individual ships--> to evade detection, resulting in a succession of vicious battles.

===Western Approaches Tactical Unit===
{{main|Western Approaches Tactical Unit}}
Between February 1942 and July 1945, about 5,000 naval officers played war games at ''Western Approaches Tactical Unit''. Many game graduates believed that the battle they fought on the linoleum floor was essential to their subsequent victory at sea. In November 1942, [[Max Horton|Admiral Horton]] tested ''Beta Search'' in a wargame. Janet Okell and Jean Laidlaw played the role of the escorts.  Five times in a row Okell and Laidlaw sank the submarine of Admiral Horton, the commander-in chief of Western Approaches.<ref>[https://www.historyextra.com/period/second-world-war/real-life-battleships-game-ww2-german-uboats/] 'Real life battleships': the secret game that countered German U-boat attacks during WW2 | History Extra</ref>

===Ahead-throwing weapons===
[[File:The Battle of the Atlantic 1939-1945 A31000.jpg|thumb|[[Hedgehog (weapon)|Hedgehog]] anti-submarine mortar mounted on the forecastle of the destroyer {{HMS|Westcott|D47|6}}]]

At the start of World War II, the depth charge was the only weapon available to a vessel for destroying a submerged submarine. Depth charges were dropped over the stern and thrown to the side of a warship travelling at speed. Early models of ASDIC/Sonar searched only ahead, astern and to the sides of the anti-submarine vessel that was using it: there was no downward-looking capability. So there was a time lag between the last fix obtained on the submarine and the warship reaching a point above that position. Then the depth charges had to sink to the depth at which they were set to explode. During those two delays, a capable submarine commander would manoeuvre rapidly to a different position and avoid the attack. The depth charges then left an area of disturbed water, through which it was difficult to regain ASDIC/Sonar contact. In response to this problem, one of the solutions developed by the Royal Navy was the ahead-throwing anti-submarine weapon—the first of which was Hedgehog.

====Hedgehog====
Hedgehog was a multiple [[spigot mortar]], which fired contact-fused bombs ahead of the firing ship while the target was still within the ASDIC beam. These started to be installed on anti-submarine ships from late 1942. The warship could approach slowly (as it did not have to clear the area of exploding depth charges to avoid damage) and so its position was less obvious to the submarine commander as it was making less noise. Because hedgehog only exploded if it hit the submarine, if the target was missed, there was no disturbed water to make tracking difficult—and contact had not been lost in the first place.{{sfn|Morison|2001|pp=211–212}}

====Squid====
[[Squid (weapon)|Squid]] was an improvement on 'Hedgehog' introduced in late 1943. A three-barrelled mortar, it projected {{convert|100|lb|kg|abbr=on}} charges ahead or abeam; the charges' firing pistols were automatically set just before launch. The more advanced installations linked Squid to the latest ASDIC sets so that the weapon was fired automatically.{{sfn|Costello|Hughes|1977|p=308}}

===Leigh Light===
{{Main|Leigh Light}}

[[File:Leigh Light.jpg|upright|thumb|[[Leigh Light]] fitted to a Royal Air Force Coastal Command Liberator, 26 February 1944]]
Detection by radar-equipped aircraft could suppress U-boat activity over a wide area, but an aircraft attack could only be successful with good visibility. U-boats were relatively safe from aircraft at night for two reasons: 1) radar then in use could not detect them at less than {{convert|1|mile|km}}; 2) flares deployed to illuminate any attack gave adequate warning for evasive manoeuvres. The introduction of the [[Leigh Light]] by the British in January 1942 solved the second problem, thereby becoming a significant factor in the Battle for the Atlantic. Developed by RAF officer [[Humphrey de Verd Leigh|H. Leigh]], it was a powerful and controllable searchlight mounted primarily to [[Vickers Wellington|Wellington bombers]] and B-24 Liberators. These aircraft first located enemy submarines using [[air-to-surface-vessel radar|air-to-surface-vessel (ASV) radar]]. Then, about {{convert|1|mile|km}} from the target, the Leigh Light would be switched on. It immediately and accurately illuminated the enemy, giving U-boat commanders less than 25&nbsp;seconds to react before they were attacked with depth charges. The first confirmed kill using this technology was ''U-502'' on 5 July 1942.<ref name=":0">{{cite web|url=http://www.historyofwar.org/articles/weapons_leigh_light.html|title=Leigh Light|last=Rickard|first=J|date=1 June 2007|website=History of War|access-date=16 February 2017}}</ref>

The Leigh Light enabled the British to attack enemy subs on the surface at night, forcing German and Italian commanders to remain underwater especially when coming into port at sub bases in the Bay of Biscay.<ref name=":0" /> U-boat commanders who survived such attacks reported a particular fear of this weapon since aircraft could not be seen at night, and the noise of an approaching aircraft was inaudible above the sound of the sub's engines. The common practice of surfacing at night to recharge batteries and refresh air was mostly abandoned as it was safer to perform these tasks during daylight hours when enemy planes could be spotted.

===Metox receiver===
[[File:Depth charges explode astern of HMS STARLING of the 2nd Escort Group in the Atlantic, January 1944. A21992.jpg|thumb|upright|Depth charges detonate astern of the sloop {{HMS|Starling|U66|6}}. She participated in the sinking of 14 U-boats throughout the war]]

By August 1942, U-boats were being fitted with radar detectors to enable them to avoid ambushes by radar-equipped aircraft or ships. The first such receiver, named [[Metox]] after its French manufacturer, could pick up the metric radar bands used by the early radars. This enabled U-boats to avoid detection by Canadian <!--and American? cite them having obsolete radars, please--> escorts, which were equipped with obsolete radar sets,<ref>''North Atlantic Run''; ''The Great Naval Battle of Ottawa''</ref>{{page needed|date=May 2013}} and allowed them to track convoys where these sets were in use.

It also caused problems for the Germans, as it sometimes detected stray radar emissions from distant ships or planes, leading U-boats to submerge when they were not in danger, preventing them from recharging batteries or using their surfaced speed.

Metox provided the U-boat commander with an advantage that had not been anticipated by the British.  The Metox set beeped at the pulse rate of the hunting aircraft's radar, about once per second.  When the radar operator came within {{convert|9|miles|km}} of the U-boat, he changed the range of his radar.  With the change of range, the radar doubled its pulse frequency and as a result, the Metox beeping frequency also doubled, warning the commander that he had been detected and that the approaching aircraft was nine miles away.

===Germans break Admiralty codes===

In 1941, American intelligence informed Rear Admiral [[John Henry Godfrey]] that the UK naval codes could be broken.  In March 1942, the Germans broke Naval Cipher 3, used for Anglo-American communication. Eighty per cent of the Admiralty messages from March 1942 to June 1943 were read by the Germans.  The sinking of Allied merchant ships increased dramatically.

[[Günter Hessler]], Admiral Dönitz's son-in-law and first staff officer at U-boat Command, said:
"We had reached a stage when it took one or two days to decrypt the British radio messages. On occasions only a few hours were required. We could sometimes deduce when and how they would take advantage of the gaps in our U-boat dispositions. Our function was to close those gaps just before the convoys were due."{{sfn|Williams|2003|p=244}} The code breakers of Bletchley Park assigned only two people to evaluate whether the Germans broke the code. After five months, they finally determined that the codes were broken. In August 1942, the UK Admiralty was informed.  The Admiralty did not change the codes until June 1943.

Captain Raymond Dreyer, deputy staff signals officer at Western Approaches, the British HQ for the Battle of the Atlantic in Liverpool, said, "Some of their most successful U-boat pack attacks on our convoys were based on information obtained by breaking our ciphers."<ref>{{cite news |last= Joseph |first= Claudia |year= 2002 |url=https://www.independent.co.uk/news/uk/home-news/revealed-the-careless-mistake-by-bletchleys-enigma-code-crackers-that-cost-allied-lives-5360971.html |title=Revealed: the careless mistake by Bletchley's Enigma code-crackers that cost Allied lives; |newspaper=The Independent |access-date=2019-04-09}}</ref>

===Enigma in 1942===
{{See also|Cryptanalysis of the Enigma#M4 (German Navy 4-rotor Enigma)}}
On 1 February 1942, the {{lang|de|Kriegsmarine}} switched the U-boats to a new Enigma network (TRITON) that used the [[M4 (German Navy 4-rotor Enigma)|new, four-rotor, Enigma machines]]. This new key could not be read by codebreakers; the Allies no longer knew where the U-boat patrol lines were. This made it far more difficult to evade contact, and the wolf packs ravaged many convoys. This persisted for ten months. To obtain information on submarine movements the Allies had to make do with HF/DF fixes and decrypts of {{lang|de|Kriegsmarine}} messages encoded on earlier Enigma machines. These messages included signals from coastal forces about U-boat arrivals and departures at their bases in France, and the reports from the U-boat training command. From these clues, Commander Rodger Winn's Admiralty Submarine Tracking Room{{sfn|Costello|Hughes|1977|p=155}} supplied their best estimates of submarine movements, but this information was not enough.

On 30 October, crewmen from {{HMS|Petard|G56|6}} salvaged Enigma material from {{GS|U-559}} as she foundered off [[Port Said]]. This allowed the codebreakers to break TRITON. By December 1942, Enigma decrypts were again disclosing U-boat patrol positions, and shipping losses declined dramatically once more.