As has already been mentioned , the German submarine commander liked to attack the largest convoys with the maximum number of U-Boats he could muster , even if this meant other contacted convoys getting through unmolested. His objective was the highest possible number of convoy ships sunk and to do this he was convinced he needed as many U-Boats as possible.
From 1940 the Germans had used some of their comparatively few U-Boats to attack unescorted independent US coastal shipping. These had practically no air defence and negligible surface combat ships to protect them. The results during the first months of 1942 were disastrous for the Allies. By late spring however the campaign had begun to lose effectiveness and attacks on the convoys were renewed. Until the end of 1942 there were not really enough U-Boats to have a concerted effort. By December the Germans could put , on average , 35 U-Boats in the area used by the convoys and this number rose to 70 by March 1943. The number of convoy ships lost became formidable. The Allied landings in North Africa in November 1942 diverted attention somewhat away from the North Atlantic and from December 1942 onwards , around 15 U-Boats were placed west of Gibraltar to intercept convoys between that point and the US / Caribbean.
By the end of 1941 submarine command knew the general rhythm of both the eastbound and westbound convoys and the routes they followed. Decryptions of messages from Allied shore stations had greatly helped building this picture. Without specific intelligence on individual convoys the Germans were still able to make a reasonable estimate of probably positions on any given date. There were up to 6 or 7 convoys underway at any particular time. The Germans could guess a position of within 500 to 600 miles in a north to south direction and 150 to 200 miles in an east west direction. A pack of 10 boats 15 miles apart could sweep this probable area in about 2 days , weather permitting. If this was a point where convoys crossed each others’ path , the probability of contact was increased. The German Special Intelligence Service had computed these areas of probable maximum convoy density and , generally , throughout the convoy war , there were between 2 and 6 U-Boat groups patrolling these areas. They were dispersed in 3 lines and the packs were shifted along these lines as the intelligence , or lack of it , suggested.
In the east , the boats were deployed along a line from south of Iceland at about the 50° level down to the 25° level. These packs were intended to intercept the westbound convoys and chase them across the Atlantic to about 45° west , the limit of aircraft patrol. In the west a line was established in a nearly east-west direction from north of Newfoundland to the Flemish Gap. A third line extended from the south east tip of Greenland in a south easterly direction to the 40° parallel. This third line could be used against both east and west bound convoys. In addition boats in transit traversed the regions not patrolled be the packs and in several cases convoys were contacted by these transiting submarines. Contact in this context means spotting and reporting.
By the middle of May 1943 the U-Boat losses forced a rethink of this strategy. From February to May 1943 , the U-Boats sank 123 convoy ships but lost 60 themselves , 34 in May alone. Of these 60 , half were sunk by land based aircraft , 27 by surface ships and 3 by carrier based aircraft. Submarine command had to get their U-Boats away from this lack of success and withdrew the boats to an area southwest of the Azores in the hope of attacking the US-Gibraltar convoys. During June and July they did not manage to sink any ships so they then dropped anti-convoy operations and concentrated on coastal shipping and independent in more distant areas – the Caribbean , off Brazil , Freetown , the Cape of Good Hope and the Indian Ocean.
The U-Boats stationed in mid Atlantic did avoid land based aircraft , bases in the Azores were not available to the Allies until August. The Allied ships made a total of 44 attacks during the summer of 1943 , sinking 15 and damaging 9 boats.
The U-Boats had to transit the Bay of Biscay to and from their French ports. During these 3 months , 31 were sunk in the Biscay , 10 more were sunk in the Atlantic south of the Azores and 9 north of the Azores , 6 lost in the Caribbean area and 9 in the Mediterranean and the Indian Ocean. While losing these 80 boats , only 86 ships were sunk all over the world.
The failure of this summer campaign convinced the submarine command that the only region a profitable exchange could be looked for was the North Atlantic. There was confidence that new weapons would allow the U-Boats to triumph again. They had a new acoustic torpedo , improved search receivers and a new quadruple mount 20mm anti aircraft gun. The last half of September saw the U-Boats headed north again to their formerly happy hunting grounds. By October they were there in strength but the result was dismal failure. The exchange rate in October was 1 ship sunk for every 7 U-Boats lost. In November they sank no ships at all even though over 30 U-Boats were concentrated there. This continued throughout the winter. In March 1944 the last wolf-pack was disbanded. The events of June 1944 forced the U-Boats to operate near the channel , the Atlantic convoys were safe.
Up to August 1940 the Germans could read just about all British Naval Signals. Then the RN changed theCiphers and Codes. The Germans then broke into these.
From June 1941 the German effort was directed at Naval Cypher Number 3 , this was used by the British , Americans and Canadians. The Germans broke into it in December 1941 and by February 1942 they had reconstructed the code and until mid December 1942 they were reading up to 80% of the messages. Then there was a key change that set them back but by February 1943 they were again able to read the messages.
From Feb-Jun 1943 they read signals 10 to 20 hours in advance of the actions in them. They could also decode the daily Admiralty U-Boat disposition signal nearly every day.
In June 1943 Naval Cypher No5 replaced No 3 in the Atlantic. By Jan 1944 the Germans had lost all their high level sources of information. They tried more breaking but decided unless they captured a code book , luckily for us they did not manage to do that.
The reason for the British code changes were suspicions that their codes were not secure. This was confirmed when they started breaking Enigma again in 1943 but it took a while to get all the systems upgraded.
Early in the battle while some signals intelligence was available , the U-Boats could find and attack the convoys but as this intelligence dried up , they were forced to look in pre-determined areas where they guessed convoys would be. As the Allies improved their anti submarine operations , it became ever more dangerous for the U-Boats to operate. Even their new weapons did not save the U-Boats from defeat.
The signals intelligence about the convoys decreased dramatically between mid 1942 to March 1944. Initially up to 70% of convoys were known to the Germans from this method sinking to a meagre 5% by March 1944. Not all known convoys were contacted and attacked because of submarine command’s obsession with using the maximum number of U-Boats to make an attack.
Signals Intelligence was stopped in June 1943 by a change in the Allied cipher. By 16 September 1943 , the Germans had succeeded in breaking a part of the new cipher , however they were only able to read messages about stragglers and early rendezvous points. On 12 December 1943 even this was rendered useless when the Allies started using reference points instead of locations.
In part 1 there was reference to refuelling of U-Boats , this allowed a boat to remain on station for much longer than the up to 16 days without it. They tried refuelling with surface ships but these were attacked by the Allies so the Germans built submarines to be used for refuelling. However , as well as running the gauntlet in the Bay of Biscay these had to use radio to make the resupply rendezvous and these refuellers were a particular target for the Allies. More of this in Part 3.
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