163: The 1940 Sea Change


In June 1940 the war at sea was about to change.



Hello everyone and welcome to History of the Second World War Episode 162 - The Early Naval War Pt. 12 - The 1940 Sea Change. This week a big thank you goes out to Trevor, Ellen, Yitzhak, Eoghan, Alexander, and Doug for choosing to support the podcast by becoming members. You can find out more at historyofthesecondworldwar.com/members. Last episode touched on some of the disappointing results of the U-boat campaigns during the last months of 1939, with those disappointments entirely attributable to the difficulty faced by the Kriegsmarine in just keeping enough U-boats at sea and actively hunting for merchant shipping. From London the situation looked different, especially for the more optimistic British leaders, including those at the Admiralty. By the end of 1939 the British clearly knew that the U-boat attacks had not had the huge effect that they had at their height during the First World War, and they also knew that after the huge numbers of September the overall rate of attacks had decreased in the last months of the year. But they also had to contend with the fact that they did not know exactly how many U-boats had been sunk due to British attacks, but they believed they had a good estimate. Unfortunately for British leaders, the estimates that they had were very optimistic, leading them to believe that they had heavily damaged the U-boat fleet, and that it would be a non-factor at least for some period of time. Then in early 1940 the weather, and then the diversion of U-boat focus to the assistance of the invasion of Norway, seemed to confirm that the U-boat threat to merchant shipping was receding. This episode will discuss the U-boat contributions to the invasion of Norway, an invasion that the next series, starting next episode, will cover in far greater detail. Then the back half of this episode will discuss the actions during the spring of 1940 and more attempts to fix the continuing disappointing performance of the German torpedoes.

The invasion of Norway would be the German Navy’s largest operation of the entire war, with nearly the entire surface fleet committed to some part of the operation. The U-boat fleet would also find itself committed to the operation with two primary purposes. The first was to attack any vessels of the Royal Navy which might be sent to try and interdict the German ships during the invasion. The second was to try and intercept any British troops that might seek to land in Norway to provide land based support for the Norwegian defenders. For reasons that we will dive into deeper in later episodes, the Germans knew that British troops were preparing to move into Norway, which is part of the reason that the German invasion was launched when it was. On March 11th Dönitz received orders to send out the maximum possible U-boat fleet to patrol the seas off of Norway to prevent any British troop movements. For this operation 24 total U-boats would be used during March, with the longer range U-boats patrolling the seas off of Northern Norway while the smaller U-boats focused on the coasts off of southern Norway. What the German leaders did not know is that the planned British operation had actually been delayed, which meant that the only targets that the U-boats would find during this period were a few small merchant ships, with 12 total ships successfully attacked, but only a total of 28,000 tons among the 12 of them. Worryingly 4 U-boats had been lost to secure this measly collection of merchant ships. In early April the U-boats would begin more direct attacks on Norwegian ports in support of the invasion, including a force of 9 U-boats that were sent to the port of Narvik. Narvik was a very important port in northern Norway, and one that would become very important to the German invasion and to the British attempts to thwart the German plans. On April 12th several U-boats would arrive at the port and begin an attack, including Prien’s U-47. He would later report that when he moved into Vaags Fjord he would see “Three large transports, each of 30,000 tons and three more, slightly smaller, escorted by two cruisers.” These were British troop transports that were even at that moment landing troops at Narvik. Near the middle of the night, at nearly point blank range, the U-47 would launch a full load of torpedoes, then countdown the expected travel time, and then nothing happened. Still undetected the tubes were reloaded, and this time the U-boat surfaced to launch another round of torpedoes, and when they were, again nothing happened. Misses were always a possibility, but the setup for the U-47 could not have been better, large stationary ships at near point blank range, also it was clear that Prien, one of the most successful German U-boat commanders of the war knew what he was doing. When he made is report of the what had happened to Dönitz, the German admiral was quite displeased. This was also combined with further failures by other U-boats during this same time, again captained by experienced men and crews with distinguished service records. He would write “To have missed these ships, lying motionless and overlapping each other, would have been quite impossible. Either, therefore, the torpedoes must have been [running] at a far greater depth than that anticipated by the technical personnel, or the pistols had failed to function. And so we found ourselves with a torpedo which refused to function in northern waters either with contact or with magnetic pistols…. To all intents and purposes, then, the U-boats were without a weapon.” Dönitz would then telephone Raeder would lodge further complaints about the state of the German torpedoes that were being supplied by the Torpedo Directorate. Germany’s best U-boat crews were launching attacks against large stationary ships, and even priority targets like battleships and heavy cruisers to absolutely no result. Dönitz refused to believe the excuses that it was simply crew failures. After this telephone call further reports of torpedo failures would trickle in from the U-boats off Norway, which simply fanned the fires of anger among the U-boat captains, crews, and Dönitz. To make the situation even more frustrating during this time 2 of the U-boats stationed off Narvik were attacked and destroyed by British warships. Suffering such losses while being robbed of any successes by faulty equipment made for a very potent mix. Up until this point the Naval Ministry and the torpedo directorate had been able to deflect any blame onto mistakes made by those on the U-boats, but after the events off of Norway they could not so easily dodge criticism. If you total up all of the ships that had been attacked, only to see the torpedoes malfunction the entire campaign could have been completely different. And it wasn’t just merchant ships, but capital ships of the Royal Navy that could have been damaged or even sunk. When further investigations were done, there were problems with both the magnetic pistols that cause magnetic torpedoes to explode while there were also issues with the contact pistols on contact torpedoes. On the magnetic side, they simply were not exploding. Magnetic torpedoes, much like magnetic mines were designed to explode when they encountered the magnetic disturbances of a metal hulled ship. In theory they were designed to explode when they were within a certain proximity of the ship, but they simply did not work. The contact pistols were the backup option, but even they were having problems. The problem with contact pistols were around angle of impact. For the Germans, the contact torpedoes were detonated by a series of levers into the warhead when the torpedo hit the ship. These were mechanical mechanisms that had to be sensitive enough to explode when hitting a ship at an angle, but not so sensitive that they might prematurely explode. The theory was that they would still function correctly if they hit a ship at a 21 degree angle, which is a pretty severe angle, but instead it was more like 50 degrees. If this was none it might be able to be accounted for, but this was combined with the issue that the German torpedoes were running deeper than expected. If you imagine the hull of a ship, as you move deeper the hull becomes more parallel to the surface of the water. This meant that it was very easy for a torpedo that may have had the correct angle of 50 degrees or more suddenly did not have the correct angle because it was running too deep and it would hit the hull as it curved away from the path of the torpedo. This issue would prove to be far easier to address, as soon as it was admitted that there was actually a problem. During late April and early May there were no U-boats put out on patrol due to the combination of the losses during the Norway operation, the normal refit times for submarines after a patrol, and then honestly just the lack of confidence in the tools available to them. Dönitz would write that “Faith in the torpedo had been completely lost, I do not believe that ever in the history of war men have been sent against the enemy with such a useless weapon. These brave, enterprising [U-boat] crews, who had proved their worth during the previous months of the war, had been plunged into a state of dismal depression … a slough of despond.” He also felt that he had to be very hands on with trying to address the issues, also writing “It is monstrous, that I should have to be burdened with lengthy discussions and investigations of the causes of torpedo failures and their remedy. This is the business of the technical directorates and departments. But as long as these authorities are slow to do what is necessary, I am forced to take action myself.” One of the events that helped address these problems was the capture of a British minelayer with torpedoes onboard on May 5th. The torpedoes were equipped with the British version of the contact pistols, which appeared to be far simpler than the German mechanisms, and it was believed that they would far more reliable as well. And so the German engineers did what any reasonable group would do in such a situation, they just copied what the British were doing. There was also work done on trying to address the depth keeping problems that the German torpedoes had always experienced. They believed that they had addressed this to the point that the torpedoes would always stay within half a meter of their assigned depth settings. By mid-May 1940 U-boats who were putting out to sea had on board an improved magnetic and contact pistol design. But the U-37 had several additional instances where the magnetic pistols still failed when they should not have, but it would have very good results with the new and improved contact pistols. What was not known, but was guessed by some within the German navy, was that the British had in some way perfected their ability to demagnetize, ships, which we know they actually had as we discussed in a previous episode. This greatly reduced the effectiveness of any magnetic detonators, so fixing and then putting more reliance on the contact pistols was the correct move. Since they consistently actually exploded the U-37 would set a new record for the number of ships successfully attacked in a single patrol at 10, although they were still unable to beat the tonnage record, missing it by just 700 tons. The experiences of the U-37 would cause Dönitz to ban the further use of the magnetic pistols until further design revisions were made. The fixed torpedoes may have had a greater impact in May, except for the fact that two U-boats were tied down in supply operations in Norway. The Norwegian campaign was still ongoing, and due to the nature of Norwegian geography and the German campaign plan, there were several different groups of German soldiers that were only able to be supplied from the sea, and the only way to move those supplies without risking further German ships was to instead use U-boats. U-boats were not very efficient cargo vessels, but they could get the job done. Hitler wanted more U-boats to be used in this role, but Dönitz was strongly against the diversion of his resources to what he believed to be a task that was in no way related to the primary goal of the U-boats, and so far outside their most impactful role of commerce raiding. The diversion of two of the submarines to these missions, and the need to undo some of the changes made to others which were originally be used in that role, meant that Dönitz’s original plans for a major effort in May 1940 had to be delayed until June 1940.


June 1940 would be a month of major changes for the war at sea, changes that would be solidified by the fall of France later in the month. But even before that occurred, Dönitz and the U-boat fleet already planned on making June a month to remember because it would be the first time since September 1939 that almost the entire available U-boat fleet would be at sea at one time. During at least some portion of June there would be a total of 21 U-boats in the North Atlantic. They also benefitted from the fact that they were going to be armed with torpedoes that were far more likely to actually work, which of course made them far more effective. In total 91 merchant vessels of some kind would be attacked and sank during the month, for a grand total of almost 285,000 tons of shipping. This would make it their best month of the war so far, by a staggering 100,000 tons. Prien and U-47 would also set the new single cruise tonnage record at 51,483, finally beating the record set in the opening weeks of the war. Against these massive numbers, only 3 U-boats were lost which would make the U-boat to tonnage exchange rate very favorable when compared to the war up to that point, and really comparable to some of the good months of the First World War before the introduction of so many countermeasures by the British. Of course this massive effort in June made it difficult to keep up the momentum in July, with fewer U-boats on patrol, but in this instance at least that effort had resulted in such large results in June. Another major change that would occur during this time was the entrance of Italy into the war. Italy’s contribution to the overall course of the commerce war in the Atlantic is a heavily under covered topic, and they would have some real challenges being impactful in the theatre. But at least on paper they were poised to make a massive impact on the war at sea, because they would enter the war with 115 submarines, twice the number of U-boats in the German Navy at that point in the war, and 39 of those Italian submarines were large enough and had enough endurance to operate in the Atlantic. 69 of them were capable of operations in the Mediterranean, which just in and of itself very important to the overall flow of British trade, with the presence of the Italian military units, including their submarines, forcing British shipping to go around Africa instead of using the quicker route through the Mediterranean. The problem was that those large numbers of submarines did not immediately translate into results, and the each combat experiences of the Italian submarines went very poorly. When Italy entered the war 54 total submarines were sent on their first war cruise, a mix of cruises into the Atlantic, Mediterranean, and even into the Indian Oceans. 28 of those 54, so over half, would have to abort their missions within 3 days of leaving port due to some kind of mechanical issue. Of the remaining 26 that were at sea long enough to actually participate in the war, 10 of them would be lost both to enemy action and to unknown causes. This very high loss rate was caused by generally poor designs, quality issues, and then just poor tactics while at sea. Often times Italian submarine captains took risks that were unwarranted and put their submarines in positions were disaster was very likely. All of this was relatively good news for the British, they really did not need another highly competent enemy submarine force, but the disasters of these first war cruises would cause the Italian Navy to decide to change their codes. This would mean that for the first time in years, the British would not be able to read Italian naval communications, this hampered the ability of the Royal Navy to anticipate and prepare for Italian naval actions for a lengthy period of time. The final major change that we will cover in this episode was the addition of French naval bases as U-boat bases. The first base to be used in this way was the French port of Lorient which was in northwestern France. There were three key benefits to being able to use Lorient and other French ports to base U-boats out of. The first was simply a matter of distance. The prime U-boat hunting grounds were to the west of the British Isles, and Lorient was almost a thousand kilometers further west than the primary U-boat base in Germany, Wilhelmshaven. This not only cut down the travel time for the U-boats but also allowed them to stay on station longer, which made it easier to maintain more U-boats on patrol at any given time which had been such a problem for the Germans during the early months of the war. The second major benefit was that it made it more difficult for the British to set up anti-submarine patrols, or at least it made them cover more area. When every U-boat had to exit the North Sea to make it into the Atlantic there were only so many different ways that they could travel, and so British interdiction efforts could be focused on those areas, but once France had fallen it was possible to begin basing U-boats out of ports in the Bay of Biscay, which then also had to be added to the area that was under patrol by British U-boat interdiction efforts. They would do this, and the Bay of Biscay would become one of the primary hunting grounds for British anti-submarine forces as they believed that the best place to sink a U-boat wasn’t after it attacked a convoy, but instead while it was entering or leaving base. The third major benefit was that it was simply faster to move through the Bay of Biscay and into the Atlantic. In the weeks and months after the war started the North Sea became heavily mined, which made it difficult and dangerous to transit through the sea, and it also became heavily patrolled. This meant that U-boats were required to often run submerged during the daylight hours, which greatly hindered their speed. Until very late in the war, U-boats wanted to stay on the surface as much as possible, because their battery endurance was very limited, and their electric motors simply could not give them the speed that their diesels could. But the North Sea was so dangerous and was so well patrolled by the British, both on the sea and in the air, that the only way to travel through it was submerged. All of these benefits for the U-boats would begin to be used as early as July 1940 when the first Submarines would put in to Lorient after their Atlantic cruises were coming to an end. This then required a British response, and it forced them to begin to patrol a wider area including the Bay of Biscay, as I mentioned earlier, but it also forced British anti-submarine forces to be concerned about convoys further to the west. Surface and air escorts now had to pick up convoys further to the west, out to the 17 degrees west longitude, simply because the U-boats could easily move further west. This spread British escorts even thinner than they had been before this change. The combination of new German advantages, the continued increase in priority applied to U-boat construction, and the dissipation of British resources would mean that the Battle of the Atlantic was about to enter a new and far more deadly phase after June 1940. A phase that we will wait to cover at some point in the future, because next episode will be the first in a series of episodes discussing the invasion of Norway.