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PART 1
HISTORY OF ANTISUBMARINE OPERATIONS
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SUBMARINES IN WORLD WAR I
THE GREAT CAPABILITIES of the submarine as a weapon of war were first revealed during World War I when the U-boat campaign almost proved decisive. Fortunately, the Germans themselves did not fully realize in 1914 how valuable the U-boatt's ability to submerge and escape detection would be for offensive operations against enemy shipping. The small number of U-boats available to the Germans were used at first only to attack naval ships and it was not until 1915 that a concentrated attack was begun on English merchant shipping.During 1915 and 1916 there were on the average only about 15 U-boats at sea at any time. These U-boats were sinking about 200,000 gross tons of shipping a month, while about 1 1/2 U-boats were being sunk each month. This situation was extremely satisfactory to the Germans, as the average life of a U-boat at sea during this period was about 10 months, during which the U-boat would sink about 13,000 gross tons of shipping a month, for a total of 130,000 gross tns of shipping sunk before the U-boat itself was sunk. Encouraged by these sucesses, the Germans in February 1917 started a large scale campaign of unrestricted warfare on merchant shipping in an attempt to blockade England. This attempt almost proved successful as Allied shipping losses rose steadily to a peak in April 1917. Four hundred and forty-four ships of about 900,000 gross tons were sunk by U-boats during that month. The British Fleet was confined to its bases for there was only 8 weeks' supply of fuel oil in England. Various countermeasures had been tried without success and defeat seemed just around the corner unless an antidote to the U-boat could be found. INTRODUCTION OF CONVOYING
Admiral Jellicoe was brought to Admiralty to deal with the situation. The convoy system, twice turned down on account of lack of escort vessels and loss of time to shipping, was introduced in April 1917 and proved immediately successful in reducing the shipping loss rate. The result of all the various British |
countermeasures, of which the convoy system was the most effective, was that by October 1917, 1501 ships in 99 convoys had been brought into port with the loss of only ten ships sunk while in convoy (a loss rate of less than 1 per cent).
LACK OF SATISFACTORY
Although shipping losses had been checked, it should be kept in mind that, from an offensive point of view (i.e. destruction of U-boats), the U-boat had not been definitely beaten in World War I. After the start of unrestricted U-boat warfare in early 1917, the Germans maintained an average of about 40 U-boat at sea at any time. During this period the average number of U-boats being sunk each month was only about seven; the maximum number of U-boats sunk in any month was only 14 in May 1918. Therefore, the average life of a U-boat at sea during the last year of World War I was still about 6 months. Shipping losses, even during the last year of World War I, were still running at the level of about 300,000 gross tons a month, so that at that time each U-boat was still sinking about 45,000 gross tons before it, itself, was sunk.COUNTERMEASURES These figures indicate that other factors besides U-boat losses must have contributed to the mutiny of U-boat crews in 1918, as the rate of U-boat losses had reached far higher levels in World War II without any corresponding crack-up in morale. Another point to be considered is that a larger part of German U-boat losses in the later part of World War I was due to mines, whos effectiveness was greatly increased by the fact that the geographical position of the German U-boat bases necessitated passage through the North Sea. Of the 178 U-boats sunk during the first World War, about 45 per cent were sunk by surface craft, about half of these by depth charges and half by gunfire and ramming. About 30 per cent were sunk by mines, another 10 per cent were torpedoed by submarines, and the other 15 per cent by other causes. It is therefore clear that the Allies had not developed any offensive weapon during World War I which could deal so effectively with the U-boat at sea that further operations would not be profitable. |
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2HISTORY OF ANTISUBMARINE OPERATIONS
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That the Germans themselves still thought the U-boat was an effective weapon at the end of World War I may be seen from the fact that there were about 220 U-boat under construction in November 1918. Admiral Scheer's building program of October 1, 1918 provided for at least 30 U-boats a month beginning in the middle of 1919 and would probably have been fulfilled if hostilities had continued. If the war had not ended in November 1918 the Allies would have had to face a second and more intensive U-boat campaign.
NEED FOR SCIENTIFIC AND
One of the most significant points about antisubmarine warfare which became apparent early in World War I was the necessity of having scientific and technical aid in combatting the U-boat. The essential problem was that of having some means of detecting a submerged U-boat and then having some weapon that would provide a good chance of destroying the U-boat.TECHNICAL ADVICE The first crude attempt to develop an instrument to detect the submerged U-boat resulted in the installations of hydrophones on Allied naval ships in 1915. The hydrophone was simply an instrument for listening to the noise produced by the submarine, and sonic frequencies below 10 kc were used. No range and only a rough bearing were obtained from these early hydrophones and it was impossible to make attacks on U-boats with any degree of precision. The main effect of hydrophones was on U-boat morale, as U-boats found they were being followed after diving instead of being free of their pursuers. The first depth charges to be used in attacking submerged U-boats were also introduced in 1915. However, so few were available that the Germans did not realize they were being used until 1917. In September 1918 the British formed a small committee, consisting largely of scientists, called the Anti-Submarine Division International Committee (the initials spell ASDIC, the name given by the British to their echo-ranging detector). This committee developed a method of transmitting sound of supersonic frequencies under water and then using the echo returning from the submerged submarine to fix its position. Although the Asdic was still in the experimental stage when World War I ended, |
the labots of the committee were not wasted, as effective underwater echo-ranging gear was developed in the 1930s and proved to be quite a surprise to the Germans at the start of World War II. Due to the ability of Asdic to provide both range and bearing, it proved far better than the hydrophones used in World War I. Hydrophones, themselves, were improved by using supersonic frequencies and making them directional, thereby enabling the operator to obtain more accurate bearing.
ORDER OF BATTLE - SEPTEMBER 1939
At the start of World War II, England had only about 220 Asdic-fitted antisubmarine craft consisting of approximately 165 destroyers, 35 patrol craft (i.e., sloops, frigates, corvettes) and 20 trawlers. This total may be compared with the more than 3000 ships (about 450 destroyers, 170 patrol craft and the remainder trawlers and small craft) available to the Allies for antisubmarine warfare in 1918.The British, profiting from their experience in World War I, had learned that the ocean convoy system did more than anything else to reduce shipping losses. They knew that the convoy system works best in open waters where evasion can be employed and that its success depends upon efficient escorts armed with effective offesnsive weapons. They were also aware of the fact that an efficient U-boat tracking system is necessary to practice effective evasion, and a daily U-boat plot based on contacts, DF fixes, and intelligence was used throughout the war. Meanwhile the Germans had done considerable research in developing and improving U-boats. The U-boats available to the Germans at the start of World War II were faster than those used in World War I and were also considerably stronger, being able to dive deeper and to withstand more depth charge punishment. The Germans had also developed an electric torpedo which left no visible wake. However, in September 1939, the Germans seem to have had available only about 60 U-boats, of which 30 were of the small 250-ton type (of limited endurance-suitable for coastal operataions only) and 30 of the larger ocean-going type, 20 of which were of 500 tons and 10 of 750 tons. This small number suggests that Germany, possibly not anticipating that England would enter the war at that early date, had given higher priorities to the building of tanks and aircraft for land warfare than to the building of U-boats. |
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Chapter 1 FIRST PERIOD SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS SEPTEMBER 1939—JUNE 1940 |
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1.1
U-BOAT OFFENSIVE
The first phase of U-boata was greatly influenced by the rapidly changing overall military situation. Germany invaded Poland on September 1, 1939, and England and France declared war on Germany on September 3. Some U-boats had left Germany early in August and when the war began there were about six at sea, ready to start an offensive in the Northeast Atlantic in the Western Approaches to England. According to statements of early prisoners of war, the commanding officers of U-boats had been ordered to observe International Law, which forbade U-boats to sink merchant vessels without having first placed the passengers and crew in a place of safety. At the beginning of September, these instructions seem to have been generally obeyed, with the notable exception of the Athenia, which was torpedoed without warning on September 3. However, this situation did not last long and, toward the end of September, even neutral ships were being torpedoed without warning. Anticipating unrestricted U-boat warfare, the British had prepared plans before the war for the immediate establishment of the convoy system and the first trade convoy sailed on September 6. As the British defenses against the U-boat attacks were based on the needs of protecting primarily the fleet and secondarily merchant shipping, the limited number of antisubmarine vessels available for convoy escort was inadequate to provide direct protection to the convoys. Nevertheless, it was believed that the British antisubmarine measures were sufficiently effective to ensure that no U-boat could betray her presence by attacking a convoy without running a severe danger of subsequent destruction by the escorting craft. The experience during September tended to justify these expectations, as over 900 ships were convoyed ——— a The term U-boat is used to refer to any enemy submarine (German, Italian, Vichy French, or Japanese) with a displacement of 200 tons or more. |
during the month without the loss of a single ship while in convoy. In addition, two U-boats were sunk during the month by British surface aircraft. The Germans apparently had no knowledge of British Asdic and still believed that they could counter underwater detection by reducing internal noises. The lack of knowledge of British Asdic probably accounted for the early U-boat tactics. The U-boats preferred attacking their targets during the daylight, believing themselves relatively invisible because of their powers of submergence, while they could observe the targets through their periscopes. The U-boat attacks were generally made by torpedo from periscope depth, but if the target was an unarmed merchant vessel, the U-boat usually surfaced and attempted to sink the ship by gunfire. During September, while the convoy system was still not fully established, there was a sufficient number of unescorted targets at sea to enable the U-boats to sink 39 ships of 151,000 gross tons. Ten of these ships were sunk by gunfire alone, from surfaced U-boats, and this lead the British to take immediate steps to arm as many merchant ships as possible to defend themselves against such attacks. At the start of the war antisubmarine forces in the Western Approaches were augmented by aircraft carriers, but after HMS Courageous was sunk by U-boat torpedos on September 17, the carriers were withdrawn. However, shore-based aircraft of the Coastal Command helped considerably by flying over 100,000 miles in September, sighting some 50 U-boats or supposed U-boats, and attacking over 30 of them. Although none of the aircraft attacks were very effective, they did cause the U-boats to submerge and thereby reduced their effective operating period. The September U-boat campaign was followed by a lull during the first ten days of October during which, although U-boats were at sea, hardly any ships were attacked. This lull seemed to reflect the political situation at the time, as it was accompanied by Hitler's offer of peace on October 6. U-boat activity flared up again on October 12, and by the end of the |
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6SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS |
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The main value of this flying was in causing the U-boats to submerge, thus preventing them from shadowing or approaching convoys on the surface. It also helped to discourage them from operating close to the shores of England where the flying was heaviest. U-boats at this time were under orders to submerge as soon as they sighted a plane and the British took advantage of this by starting to use, in November 1939, light aircraft of the Moth type to patrol around the coast. These aircraft were known as "scarecrows," carried no bombs, and were used soley to sight and report U-boats, and to make them submerge. These flying hours and sightings also helped considerably in keeping an accurate U-boat plot. 1.2.3 Scientific and Technical Applying the lessons learned in World War I, considerable scientific work was being done during this period to improve antisubmarine attacks. SOme of the typical problems being investigated then were: 1. Development of an Asdic receiver-amplifier with automatic sensitivity control so that both long and short range echoes would be clearly recorded. 2. Theoretical investigation of improved methods of carrying out antisubmarine attacks and of the best type of depth-charge pattern to ensure destruction of the submarine. 3. Assistance to antisubmarine personnel in distinguishing between submarine and non-submarine targets, as a great amount of effort and a large number of depth charges were being expended on wrecks, whales, and other non-submarine targets. 1.2.4 Sinking of U-Boats Surface craft, equipped with Asdic and depth charges, were by far the most potent enemy of the U-boat during this first phase of U-boat warfare. Twenty-one German U-boats are known to have been sunkb as a result of allied action during this 10-month period; 15 were sunk by surface craft, one by the coordinated action of two ships and one plane, one by a plane from a British battleship, two were torpedoed by submarines, and two were mined in ————— b The estimates given here for U-boat sinkings are based on Allied assesments. Incidents assessed A or B are considered to have sunk the U-boat. Justification for this assumption is givin in Appendix I. |
attempting to pass throug the Dover Barrage in October. Two other German U-boats were sunk under unknown circumstances while one is known to have been sunk in the Baltic after being rammed accidentally. In addition to the 24 German U-boats mentioned above, 10 Italian U-boats were sunk in the Mediterranean, Red Sea, and Indian Ocean between June 10, when Italy entered the war, and the end of the month. 1.3 SURVEY OF RESULTS 1.3.1 From the U-boat Point of View The average number of U-boats at sea in the Atlantic during this first phase of the U-boat war was about six. The average number of ships sunk monthly by them was 26 of about 106,000 gross tons, so that about four ships of about 18,000 gross tons were being sunk per U-boat month at sea. However, about two out of the six U-boats at sea were being sunk each month, so that the average life of a U-boat at sea was only about three months. This relative rate of loss of U-boats was extremely high, much higher than at any stage of the First World War, and makes readily understandable the fact that they preferred attacking unescorted ships to attacking convoys, lightly escorted as they were. It also helps to explain why the German U-boats felt it neccessary to change their tactics during the next phase of the U-boat war; this despite the fact that the overall exchange rate (i.e. 13 ships of about 53,000 gross tons sunk for each U-boat sunk) might be considered satisfactory for the U-boats. The rate of loss of U-boats simply was higher than the Germans could afford. The fact can be clearly seen from another approach. The Germans started the war with about 30 ocean-going U-boats (i.e., 500 tons or larger). By the end of June 1940, 18 of these had been sunk while only about 15 new ones had been commissioned, so that the Germans only had about 27 ocean-going U-boats available at the start of the second period of the U-boat war. 1.3.2 From the Allies' Point of View At the end of June 1940 England was left alone in the war against Germany and her ability to carry on the war was dependend on her keeping her sea lanes open. Total shipping losses of the Allied and Neutral |
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SURVEY OF RESULTS 7 |
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nations were about 280,000 gross tons monthly as compared to a building rate of only about 88,000 gross tons monthly, for a total net loss of 1,920,000 gross tons due to all causes during this 10-month period out of a total of about 40,000,000 gross tons of shipping at the start of the war. It appeared that shipping losses were still on the upgrade and the only hope of keeping the rate of net loss down was a large increase in shipbuilding. Of the 280,000 gross tons of shipping lost monthly, about 223,000 gross tons were lost by enemy action, with U-boats accounting for 106,000 gross tons or 48 per cent of the total lost by enemy action. Mines accounted for 58,000 or 26 per cent, aircraft for 27,000 or 13 per cent, surface craft for 14,000 or 6 per cent, and other and unknown causes for the other 7 per cent of the losses. The U-boat appeared definitely to be the main threat to Allied shipping. The convoy system had |
been the main factor in keeping the shipping losses due to U-boats down to a moderate level. Although the number of British Asdic-fitted antisubmarine vessels increased from about 220 at the beginning of the war to about 450 at the end of June 1940, most of the increase took place in trawlers and other small ships. The 450 ships consisted of about 180 destroyers, about 55 patrol craft, and about 215 trawlers and other small craft. However, the number of these ships that could be spared for escort duty was still insufficient to provide adequate protection to the convoys. The British had been fortunate during the first period that the enemy had onlyu a small number of U-boats available and these had operated in alimited area, almost all the sinking of ships occuring in the Northeast Atlantic (east of 20° west longitude and north of 30° north latitude). This had helped to make the escort problem easier during the first period. |
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