155: Danger Above and Below


From above soared the bombers from below floated the mines.



Hello everyone and welcome to History of the Second World War Episode 155 - The Early Naval War Part 5 - Danger Above and Below. This week a big thank you goes out to Blade for the Donation and to John, Jeremy, and Abir for becoming members. You can find out more about becoming a member over at historyofthesecondworldwar.com/members. This episode is going to focus on two topics that I don’t think get enough focus when it comes to the early war at sea during the Second World war, mine warfare and the early German air attacks against the Royal Navy. In the case of both of these topics I think they get generally overlooked in favor of flashier topics like the actions of German surface raiders and the German U-boats. But don’t worry, both of those topics will be covered in good detail during the next 7 episodes, but for now this episode is going to be the time to shine for mines and the naval air war. The actions, responses, and responses to those responses around both mines and the Luftwaffe would shape the course of the naval war in important ways, especially during the early period of the war when the Royal Navy was finding its footing and the German U-boat production capacity was still ramping up.

Over the course of the war 10s of thousands of mines would be laid in the North Sea by both sides as they sought to control the naval traffic of the other. In the early months of the war most of these were pre-planned and merely had to be executed, and as soon as war was declared that execution would begin. For the British that meant both defensive and offensive mine belts. The defensive belts focused first on the English channel, with two main belts of mines laid on either entrance into the channel to control German access, especially U-boat access. In both cases over 3,000 mines were used. Generally these mines were a mixture of magnetically activated mines and contact mines, which had to make contact with an enemy ship to explode. The magnetic mines were very good early in the war, but every nation would make efforts to de-magnitize their ships before and during the war. This process was called degaussing and is still in use by modern warships to reduce their magnetic presence, to prevent magnetic mines from being as effective. The original process of degaussing required electromagnetic coils to be installed around the ship, and by running a current through those coils the magnetic effects of the ship could be eliminated. The problem was that these coils were expensive, and so they could not be installed on every ship, and so a cheaper method called deperming was invented which in a large electrical cable was moved over the outside of the ship to alter its magnetic signature. This worked well, with the downside being that this process would have to be periodically repeated due to the effects of the Earth’s magnetic field serving to undo the deperming over time. But this was felt to be a very reasonable trade off, and schedules were developed for ships to ensure that they would be depermed, or wiped, at the correct intervals. Offensive mine laying efforts focused on controlling German actions closer to the German coast with mines laid in the routes that the Kriegsmarine was expected to travel when exiting the ports of Northern Germany and making their way to the North Sea. On the side of the Germans offensive mine laying was seen as a critical component of their war on British trade, and a major reason for this was the volume of British trade that transited the easily mineable coastal areas around the British isles. This trade really could not be moved around the island nation any other way because the structure of the transportation systems that crisscrossed the island were designed and built around the assumption that in many cases the easiest way for something to transit between say London and Scotland was actually to move most of the way on a coastal merchant ship. To lay these minefields the Germans had three delivery methods: u-boats, aircraft, and destroyers. All of these would be used in the minelaying efforts, but maybe the most interesting of them was the destroyers. Destroyers were ideal mine delivery systems because of the number of mines that they could carry, with a German destroyer able to carry and disperse 60 mines at one time. But they had to properly time their actions so ensure that the times that they were most vulnerable to enemy action, when they were laying mines close to the enemy’s coast, was occurring at night. This became much easier to accomplish when the nights grew longer during the late autumn and winter months. Around the middle of October the hours of darkness finally extended long enough that the first German destroyer minelaying operation to lay mines off of the Humber river estuary . To accomplish this 6 German destroyers timed it so that they exited the defensive German minefields and moved into the North Sea just after nightfall, and then made a high speed dash to the British coast. Then when the mines were in position they quickly made their way back to German port, where they were met by German cruisers to provide escort back to port. There were a few instances where the German cruisers actually just accompanied the destroyers on their entire journey, although that practice would end after both the Nürnberg and Leipzig were torpedoed during one minelaying mission by a British submarine. About a month later another mining mission would be launched and the results were even more immediate. This was due to the fact that the the German mine laying run was planned to take place on the night of November 12th and into the morning of the 13th. And there was a British mine laying operation scheduled for the night before, which would have been the 11th into the 12th, but the three British ships, two destroyers and a cruiser were delayed due to fog. This meant that they did not get started until the evening of the 12th, just hours after the Germans had entered the north sea but the British ships were again delayed due to heavy fog. The German ships would be lucky enough to lay their mines and escape, and then just hours later the British cruiser Adventure hit one of those mines, although it would make it to port for repairs that would take almost a year, all the way to August 1940. Remarkably that one single mine laying operation on November 12th would be attributed with the destruction of 35,000 tons of British shipping. Between October and February 11 German mine laying operations would be completed, with around 2,000 total mines laid during that time. These mine fields, even with the British making concerted efforts to clear them over time, would cause the loss of 55 merchant ships, 3 destroyers and 6 smaller vessels. These minefields were not the only ones created by Germany. There would be some instances of merchant ships being used as well, generally also involving a disguise so that they appeared to be a ship from a neutral country hauling a load of cargo. U-boats and aircraft were also used, but in both cases they were a bit more limited in the size of the mines that could be deployed, with some of the larger types of German mines simply being too large to be transported via U-boat or aircraft. Around a quarter of all effective mine attacks during 1939 can be attributed to U-Boat laid mines

When thousands and thousands of mines being laid around the North Sea, both the nations involved knew that they had to put effort into sweeping the mines almost constantly to minimize the impact of new minefields laid by the enemy. Most nations used very similar methods for minesweeping at this time, and so the British system called Oropesa would not have been out of place for other navies. In this system there were two large wires of cables which were deployed on either side of the ship. The ends of the cables were attached to floats which would keep the wire at the correct depth to allow it to achieve its primary purpose: cutting the cables that held the mines in place. There were also instances where a more active cutting of cables was required, and this capability was introduced early in the war . Once the cables were cut the mines generally floated to the surface, where they could be dealt with as simply as shooting them with rifles. It was, overall, a simple process but one that was quite dangerous while at the same time being monotonous . To try and minimize the danger to the ships involved minesweepers generally worked in at least pair, and often even larger groups. During the first sweep of an area the ships would use what was known as a G formation where the first ship would put out its sweep and then the ship behind would be behind but in echelon so that the sweeps of the ship a head of it allowed the second ship to sail in what were theoretically mine free lanes, or at least lanes that had already been swept. This also meant that the lead ship was in the greatest danger, but that was for the most part unavoidable. If it was felt that the area was at low risk of mines being present a different formation could be used, in this formation instead of sailing in echelon the ships would maximize their sweep so that there was no overlap. This was more dangerous, as all ships were unprotected, but when the risk was low the larger area covered was seen as a worthwhile trade off, the wider formation was also often used for double checking channels that had just been swept. There were limits to how much area minesweepers could cover, and it was well understood that the task of sweeping all German mines everywhere was simply not going to happen. And so instead the British would develop swept channels around the coasts which were constantly patrolled by minesweepers to try and minimize the damage caused. Generally this meant that minesweepers would sweep the channels every single day before any traffic was allowed through. The downside to the swept channels system was that it also gave the Germans quite good information of exactly where they should put their mines, but this was felt to be an unavoidable problem.


Due to the confined nature of the North Sea, both the Luftwaffe and Royal Air Force understood that aircraft could play an important role in the naval war. Even from the very start of the war this impact would be felt. At the simplest level, aircraft were really good for scouting and reconnaissance, and the British hoped to use this to their advantage in one of their key missions: prevent German shipping from reaching Germany. The primary group used to accomplish this would be the aircraft under Coastal Command which would spend around 9,000 hours over the North Sea in just the first few months of the war. Weather was certainly a major problem during these patrols, but there was also the possibility of running into German reconnaissance flights . If a German ship was spotted, the aircraft of Coastal Command could not actually attack them, as they did not have any anti-ship weapons or training. This meant that when a ship needed to be attacked the information was transmitted to Bomber command, which would then dispatch bombing aircraft to attack the German vessels. This ran into the problem of the pilots of Bomber command having little experience navigating over open water, which was a major problem during the early war years. These were problems, and in some ways the British were simply fortunate that the Luftwaffe would be experiencing similar problems.

The plans for the Luftwaffe in the North Sea were not really that much different than what the British were planning. The aircraft to be used would be controlled by the Luftwaffe and would be split into two different groups. The first group would be made of reconnaissance aircraft, most of which were either float planes or flying boats. These aircraft, around 154 of them, would stage reconnaissance flights with the goal of finding and tracking British naval movements. If a target was identified the second group of aircraft would be brought in, which were generally just bombing squadrons of the Luftwaffe with perhaps some extra training for naval attack. The primary bombers used for this work were the He-111s and Ju-88s. One aircraft that was not used very often was the Ju-87 Stuka, even though other nations would prove that dive bombing was one of the most effective ways to attack enemy ships while they were at sea. The problem for the Stuka is that it had very limited range, which made it very difficult to use in a naval role because British ships often did not get close enough to any land based airfields. There had been plans to have several Stuka squadrons based on the Graf Zeppelin aircraft carrier, but the carrier was never completed and so the role to be played by the Stuka in the war at sea was quite limited. During the early months of the war Göring would push his squadrons to be more aggressive in their reconnaissance and attacks due to the lack of success that they were having while at the same time the early U-boat campaigns would have some real successes, especially with an attack by U-47 and its commander Gunther Prien in Scapa flow which resulted in the sinking of the British Battleship Royal Oak. The Luftwaffe were presented with an even more enticing opportunity when reports were received that the Hood was entering into Rosyth on October 16th, which was within range of German bombers. What we know now is that it was not the Hood but instead Repulse, and the ship was immediately going into dock which meant it could not be attacked anyway. This was due to orders that were still in effect for German bombers that they were to very strictly avoid collateral and civilian damage during these early weeks and months of the war. Obviously that would change in 1940, but in October 1939 the orders were still in force. But what the Luftwaffe believed was that the Hood was heading into Rosyth and there might be a chance to attack it, so a dozen Ju-88s were dispatched to launch the attack. The strike was commanded by Hauptmann Pohle, and they would arrive over the port at 2:30PM. They would catch the British air defenses completely by surprise, allowing Pohle and the other pilots to carefully evaluate the situation and pick their target. They could not find Hood, because it was not there, and they did see Repulse, but in dock and unable to be attacked. For lack of other options they determined that the best options for the attack were two British cruisers, which would be the Southampton and Edinburgh. Pohle would launch his attack, diving in on the Southampton and actually managing to put his 1,000 pound bomb on the cruisers, but it failed to explode. Instead it simply went through three decks an then out the side of the ship. This would be the only bomb that would hit either of the cruisers, and the fact that it failed to explode was disappointing but shockingly common during the war. One of the trends of the early war years were things that should have severely damaged ships not doing so due to the fact that they did not explode, with many bombs and torpedoes in all theaters suffering from the same problem. Soon after the attacks were launched Spitfires of 602 Squadron arrived on the scene and drove off the Ju-88s, claiming the destruction of two of them, including that of Pohle who would survive and be taken prisoner.

The general failure of the attack on Rosyth was a very disappointing result for the Luftwaffe, with barely any damage from the raid and the lose of two bombers. Soon afterwards they would launch another raid, this time against Scapa Flow. Scapa Flow was the British Home Fleet’s wartime base, where it had been harbored for the entirety of the First World War, and where it planned to spend the Second World War as well. Before the advent of aircraft Scapa Flow had been a safe area, with the perfect position to control the entrances to the North Sea due to its position in the Orkney islands off the northern coast of Scotland. But now it was found to be very vulnerable to the attacks of German bombers, with several Ju-88s launched a raid which would attack the Iron Duke, an old battleship which had been turned into an administrative centre after having most of its guns removed. Two bombs would hit the ship, which would be pushed onto shore by a nearby tug, which is all that prevented the ship from sinking. Even though the attack had not damaged any ship of real value, although 1 person had been killed and 25 wounded, the attack was a rude awakening to the Royal Navy of the vulnerability of ships in Scapa Flow to air attacks. The anti-aircraft defenses in the area simply were not strong enough, and air cover was mostly non-existent. This caused the Home Fleet to be moved out of Scapa Flow and into Loch Ewe, on the western shores of Scotland. Loch Ewe was safe from German bombers, but made it more difficult for the Royal Navy to quickly respond to events in the North Sea simply because it was further away. The Home Fleet would remain based at Loch Ewe until march 1940, when Admiral Forbes brought them back to Scapa Flow. But as soon as this return became known to the Germans they once again prepared to launch a series of air attacks, this was particularly important for the Kriegsmarine at that moment because the operations against Norway were planned for April. The first raid would be launched on March 16th with 16 He-111s and 18 Ju-88s, with the combined 34 aircraft being much larger than any previous Scapa flow raid. However, the only ship that was hit was the cruiser Norfolk which was hit by a bomb that penetrated into the bowels of the ship and exploded near the shell room for Y turret which resulted in flooding and then purposeful flooding of magazines for safety reasons. Even with this success, no other major attacks were immediately launched against Scapa Flow and it would not be until April 8th that another raid would be flown. On that day, and then on the two days that followed raids would be launched in direct support of the invasion of Norway, but in all cases the Home Fleet was already at sea and there was little left to bomb. The final raid on April 10th would be met by increased British fighter support, which would result in 7 German bombers being shot down, which put a pause on Scapa Flow attacks, if only because the Home Fleet would largely be at sea for the next several weeks during the Norwegian campaign.