SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS
SEPTEMBER 1939 - JUNE 1940
1.1 U-BOAT OFFENSIVE
THIS first phase of U-boat1 warfare 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 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 torpedoes 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
month 28 ships of 136,000 gross tons had been sunk by U-boats. In addition Kapitän-leutnant Prien, in command of U-47, penetrated the harbor of Scapa Flow in the middle of October, and sank HMS Royal Oak, a British battleship. This served to direct British attention to the necessity of protecting harbors against U-boats by means of fixed defenses, such as booms, indicator loops, mine fields, and harbor defense Asdics.
During November and December the main effort of the German U-boats seem to have centered upon a mine-laying campaign on the east coast of England, particularly in the Thames Estuary. The mains laid were both the old type of contact mine and a new type of magnetic mine, which at first proved rather difficult to sweep. Monthly losses due directly to U-boats (torpedoes and gunfire) fell to 18 ships of about 65,000 gross tons and were exceeded by the 100,000 gross tons of shipping sunk by mines during each of these months.
U-boat activity began increasing again in the second week of January 1940 and by the end of the month there were as many U-boats at sea as at the start of the war. In February, the U-boat effort was greater than during any previous period and 35 ships of 135,000 gross tons were sunk. The U-boats continued to follow a policy of attacking Allied and neutral ships without warning. They preferred attacking single ships or stragglers from convoys, thus making it difficult for the antisubmarine ships to conduct an effective search and counterattack. The respect the U-boats had been showing for the British convoys is indicated by the fact that only 7 of the 169 ships sunk by U-boats during the first six months of the war were in convoy when sunk, although roughly half the shipping sailed in convoy at this time.
Losses due to mines fell off during January and February as better methods of sweeping the magnetic mines were developed and more ships were degaussed (magnetic field of ship changed to protect it against magnetic mines).
There was a marked lull in U-boat activity throughout March, featured by the complete absence of U-boats from Atlantic waters after about the 12th of the month. Early in April, every available U-boat left Germany to take up patrol positions in the North Sea to help in the impending military operations against Norway. The average number of U-boats at sea reached a peak of about 15 during the second week of April, when Germany invaded Norway. Despite the large concentration of U-boats, the damage done by them was remarkably small. No British capital ship was even attacked by U-boats and only six ships of 31,000 gross tones were sunk by the U-boats during the whole month of April, a new low for the war. In addition, the Germans lost six U-boats during the month, a new high for the war.
There was very little U-boat activity during the first half of May as Germany started her invasion of Holland and Belgium on May 10. It is believed that during May no U-boat proceeded to the Western Approaches until the 21st and only 10 ships of 48,000 gross tons were sunk by U-boats during the month. Shipping losses to U-boats were exceeded for the first time during the war by the 154,000 gross tons sunk by aircraft. These losses were incurred largely in connection with the operation and evacuation of the British Expeditionary Force, which left Dunkerque on May 29.
The Germans announced on May 29 that the U-boat warfare was about to recommence and warned neutrals not to enter the protection of British convoys. This threat was followed by a period of intense U-boat activity as convoys were attacked with greater boldness than in earlier periods, advantage being taken of the paucity of escorts, rendered inevitable by the demands of the military evacuation and the Home Fleet. The losses for June were the highest of the war, with 56 ships of 267,000 gross tons being sunk by U-boats. The German ace, Klt. Prien, contributed his share by sinking ten ships of about 67,000 gross tons during one cruise. By the end of June, France was out of the war and Italy had entered the war with over 100 U-boats, about 60 of which were ocean-going (650 tons and over).
1.2 COUNTERMEASURES TO THE U-BOAT
The convoy system was by far the most effective countermeasure in keeping down shipping losses to U-boats during this first period, just as it had been during World War I. This was still true, even though the number of antisubmarine vessels suitable for ocean escort was insufficient to provide direct protection to the convoys. The British met this problem by keeping their convoy system flexible, changing the number of escorts and the distances for which convoys
were escorted in accordance with U-boat activity. It should be noted that the Germans made this problem more difficult by sending U-boats out in waves, so that peaks of U-boat activity occurred in September 1939 and in February and June of 1940.
Although the first convoys sailed early in September 1939, the convoy system was not fully in force until the beginning of October. The designations of the main convoy routes that were set up then were:
OB Outward bound from England to America and Africa. HX Homeward bound to England from Halifax. SL Homeward bound to England from Sierra Leone
In order to illustrate some of the problems involved in setting up the convoy system a detailed account is presented of the changes made in the HX convoy route during this period. On October 7, 1939, it was decided to discontinue the convoys from Kingston, Jamaica and all ships in the West Atlantic were routed independently to Halifax, taking advantage of U.S. waters as far as possible. Convoys were divided into slow (9- to 12-knot) and fast (12- to 15-knots) convoys, which left Halifax together at about the same time in order to arrive four days apart at the rendezvous point. At this point, located at about 15° west longitude, the convoys were met by one or two destroyers which provided antisubmarine escort to England. The ocean escort, provided between Halifax and the rendezvous point primarily for protection against surface raiders, consisted of a battleship, cruiser, or armed merchant cruiser, and one or two submarines when available.
The first of these convoys, HX 6 and HFX 6, consisted of 62 and 6 ships, respectively. The dividing line was then altered to 11 knots to equalize the number of ships, and during November 1939 the number of ships in these sections averaged 32 and 12. On February 12, 1940, the fast convoys were discontinued, and all HX convoys sailed at 9 knots, at 3- and 5-day intervals. These convoys consisted of ships with speeds between 9 and 15 knots ships of higher speeds sailing independently. At the beginning of April, in order to equalize the size of the convoys, 4-day intervals were started.
Early in May 1940 Bermuda began to be used as an assembly point for vessels from the West Indies and other points in that vicinity, and HX 41 was the first combined Bermuda and Halifax convoy. The sections formed at sea, as arranged, at about 41° north latitude and 43° west longitude, and the Bermuda escort then returned to base. This change enabled about 60 per cent of the ships that formerly sailed from Halifax to cut down their voyage by 500 miles and to avoid the fog off Newfoundland. The average number of ships in these HX convoys had risen to 46 by May 1940.
In addition to the above mentioned convoys, the British also sailed coastal convoys to protect shipping on short trips around the English coast and Scandinavian convoys too and from Norway. The main energies of the French light craft were also devoted to the protection of merchant shipping. They were fitted with Asdic as soon as possible after the opening of the war and provided escorts for purely French convoys, helped escort the Gibraltar convoys for most of the way, and assisted in covering the military cross-Channel convoys.
The extreme value of the British convoy system may best be appreciated by noting that during this period about 2500 ships were being convoyed monthly, while only about 5 of these were being sunk monthly by U-boats (21/2 in escorted convoys, 11/2 in unescorted convoys, and 1 straggler). The rate at which independent merchant vessels were being sunk by U-boats was roughly about four times as high.
Another important countermeasure to the U-boat was the use of aircraft. These had seen very little use against U-boats during World War I and consequently it took some time before the problems of how to use aircraft most efficiently against U-boats were solved. In addition, the aircraft were still armed only with bombs. Consequently the direct contribution of aircraft toward sinking U-boats was negligible during this period.
Nevertheless, aircraft performed a defensive function of greater value in helping to protect shipping. Coastal Command aircraft flew, on the average, about 4500 hours monthly on purely antisubmarine work. About 20 U-boats were sighted monthly and 12 of these were attacked, with about 10 per cent of the attacks resulting in some damage to the U-boat. This effort reached a peak of 9500 hours during June 1940, when about 2800 hours were spent on antisubmarine patrol and 6700 hours on convoy escort duty.
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 solely 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:
- Development of an Asdic receiver-amplifier with automatic sensitivity control so that both long and short range echoes would be clearly recorded.
- 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.
- 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 sunk2 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 attempting to pass through 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 necessary 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 depended on her keeping her sea lanes open. Total shipping losses of the Allied and neutral
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 only a small number of U-boats available and these had operated in a limited area, almost all the sinking of ships occurring 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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