Breaking the Enigma code
Polish contribution to victory

Andrzej Dabrowa, Ph.D.

   

12. Battle of the Atlantic

While the Battle of Britain was essentially a local defence of the British Isles, the Battle of the Atlantic was a remote fight to maintain shipping routes over vast stretches of the Atlantic Ocean. The Battle of Britain was short and fast paced with relatively few people involved. The German Air Force Enigma code was relatively easy to break and Fighter Command had the advantage of reading all of the orders. The Battle of the Atlantic, on the other hand, was long, slow paced and a large number of people were involved. The German Navy Enigma code was very difficult to break. It took a military action against German shipping to capture a Naval Enigma and code books in order to start decrypting messages.

In 1939 Britain had the biggest merchant fleet in the world of about 3,000 ships, totalling 17 millions tons. In addition, Allied nations such as Norway, France, Holland and Greece contributed most of their merchant fleets. Since England's very existence depended upon these ships delivering food, fuel, materials and arms it was of the utmost importance to move them safely. At the beginning of the war Germany had 57 U-boats. After a lull in 1941, the German U-boat fleet increased to 100 at the beginning of 1942, to 146 at the beginning of 1943 and peaked at 196 during 1943. Since the Allies could only provide limited convoy protection, the shipping losses were climbing very rapidly. By the end of the war the total Allied shipping losses were about 15 millions tons, almost 100% of the 1939 tonnage! With that high shipping loss Britain found it difficult to maintain manufacturing replacements over a long period of time so it became imperative to reduce the rate of loss. The limited number of escort vessels could protect convoys only over a limited extent of the Atlantic and replacement of every merchant ship meant one Naval vessel less. The only other well proven countermeasure was to break the German Navy Enigma code. In spite of all out effort at Bletchley Park and urging by Prime Minister Churchill, the German Navy Enigma code could not be broken except for a few lucky strikes. The British Admiralty needed a continuous flow of information about U-boat orders and locations to be able to organise an effective counter offensive.

The Germans used up to 12 networks and the "Triton" network used Enigma with seven rotors for communicating between on-station U-boats and headquarters. Faced with that, the Admiralty decided to initiate a series of well-planned actions against German shipping with the specific aim of capturing German Naval Enigmas and code books. These attacks had to be a total surprise and extremely fast so that the crews did not have time to destroy documents and dump Enigma machines overboard. Furthermore, there had to be absolutely no possibility of the Germany Admiralty suspecting that the code has been compromised! In the first operation in April 1941 the German trawler "Krebs" was captured with Enigma spare rotors and some additional printed material. This gave Bletchley Park their first insight into Naval Enigma operation. Unfortunately, the captured documents and equipment enabled them to decrypt only the ciphers for the previous February.

The second opportunity occurred in May 1941 when information was obtained that the weather ship "Munchen" had sailed to a station in the Atlantic. Using direction finding the British located the ship and a raiding party was able to recover the Short Weather Cipher Book (a map of the Atlantic Ocean with a superimposed rectangular grid that was used to define actual ships' positions in arbitrary grid numbers) and rotor settings (key) for the month of June 1941. Although this did not contribute to an Enigma solution, it vastly reduced the search area in which U-boats were expected to sail and to operate on station.

The third operation in May 1941 was a great success. U-110 was forced to surface and the crew abandoned it in great haste without dumping the Enigma and the key book overboard. Capture of the Naval Enigma and the key books for Short Signal Officer Traffic enabled Bletchley Park to reduce decryption time from days to hours. For Naval purposes this was as good as an instant solution. The next successful dedicated raid was in June 1941. On the basis of previous decrypted messages the Admiralty estimated that the weather ship "Lauenburg" would be on station for the months of June and July and therefore would be carrying Enigma keys for those two months. The weather ship was damaged in the resulting encounter with three Naval ships. A boarding party secured the July Enigma key, two sets of plug board sheets, rotor settings and other pertinent papers. Again, this allowed Bletchley Park to further refine decrypting methods and reduced the decrypting time to three hours.

The next action occurred in the Mediterranean in October 1942 as a result of a U-boat sighting by a patrol aircraft. Four destroyers with a highly trained and disciplined boarding team were dispatched to apprehend the U-boat and recover Enigma-related items. After being damaged by depth charges U-559 surfaced to be met by a barrage of machine gun fire. The boarding party boarded the submarine and found an Enigma machine and a large number of documents. The Short Signal Book (giving location grids on a map) enabled Bletchley Park to find the positions of 15 U-boats and as a result Allied shipping losses for January and February 1943 were reduced by 50% compared with the previous two months! The last action undertaken to capture Enigma material was by an American Naval Task Force. The capture of U-505 in June 1944 provided keys for the grid positions of U-boats.

Breaking the very difficult Naval Enigma code enabled the Allies to locate German U-boats positions, to move convoys around them and to attack them with U-boat destroyers and, as a result, Allied shipping losses dropped rapidly in 1943. By 1943, increased convoy escort, better antisubmarine warfare methods and substantially increased air coverage reduced the effectiveness of the German U-boats. U-boat losses that started with 30 in 1940 climbed steadily until they reached a peak of 300 in 1944. Germany could not make up these losses and, for practical purposes, it was the end of the German Navy. In spite of the enormous Allied merchant marine losses the German Navy was never able to achieve clear superiority over the Allies.

(c) 2003 A R Dabrowa