Researcher @ Large - OEG Report 51 - ASW in World War Two Part 1: Chapters 1

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Supplemental Table of Contents

3.2 Countermeasures to the U-boat
3.2.1 Convoys
3.2.2 Aircraft
3.2.3 Scientific and Technical
3.2.4 Sinking of U-boats

3.3 Survey of Results
3.3.1 From the U-boat Point of View
3.3.2 From the Allies Point of View

Chapter 3 THIRD PERIOD START OF WOLF PACKS; END-TO-END ESCORT OF CONVOYS APRIL 1941 — DECEMBER 1941
3.1 U-BOAT OFFENSIVE THE FRUITS of the intensified German U-boat construction program, started late in 1939, were beginning to appear as the average number of U-boats at sea in the Atlantic steadily increased from about 18 in April 1941 to about 36 in August 1941. The main features of U-boat tactics during this third period were the increasing use of wolf-pack attacks forced upon the Germans by the evasive routing of British convoys and the scarcity of experienced U-boat commanders. The outstanding successes achieved by escorts in the Northwestern Approaches during March 1941 produced the direct result that, in April, U-boats abandoned the method of close attack on the surface while antisubmarine escorts were in company. The U-boats continued their search for weak spots in the antisubmarine defenses by moving away from the vicinity of England, where air coverage was heavy, and extended their operations further westward where they could attack convoys before the antisubmarine escort had joined. There was also a southward movement of the U-boats with increased activity in the Azores and Freetown Areas. April opened somewhat disastrously with heavy attacks, started before the antisubmarine escort had joined, on Convoy SC 26. About five U-boats participated in these attacks, ten ships were sunk, and, in addition, the armed merchant cruiser ocean escort was damaged by a torpedo hit. One of the attacking U-boats was sunk after the antisubmarine escorts had joined. Towards the end of April, four ships were sunk from Convoy HX 121 as a result of the first submerged daylight attack by a pack of U-boats. The shipping losses to U-boats in April were about the same as in March, with 41 ships of 240,000 gross tons sunk. However, only about 30 per cent of the tonnage sunk by U-boats was in convoy in April as compared with 60 per cent in March. About 13 per cent of the tonntrge sunk by U-boats in April was sunk in	the Azores Area and another 15 per cent in the Freetown Area. The total shipping losses, from all causes, amounted to 682,000 gross tons in April 1941, a higher figure than for any previous month in the war. This was due mainly to the heavy shipping losses to enemy aircraft, about 296,000 gross tons, most of these losses occurring in the Mediterranean in connection with the evacuation from Greece and Crete. As a result of the heavy attack on Convoy SC 26 at about 28° west longitude, the Iceland routing scheme was adopted earlier than was originally intended. Escorts were based on Iceland, making it possible to meet comovs where the escort from England had to leave, and then to escort the convoys out to about 35° west longitude, the escort there picking up an incoming convoy and then turning it over to an escort group from England. Sunderland and Hudson aircraft were also moved to Iceland to provide air coverage for convoys in waters which could not be covered by aircraft based on England. Obviously this considerable increase in the distance over which transatlantic convoys were escorted was only achieved at the expense of weaker individual escorts with each convoy. This was partly compensated by reinforcing the Western Approaches with Asdic-fitted minesweepers. It is equally clear that the use of Iceland necessitated a certain rigidity of routing and tended to make the location of convoys by U-boats and enemy aircraft a simpler business. Against this, the daylight hours in these northern latitudes were rapidly lengthening as the summer months approached and the U-boat danger in daylight was a lesser menace than at night. The upward trend in shipping losses to U-boats continued as 58 ships of 325,000 gross tons were sunk in May. Over half the losses occurred in the Freetown Area, where a group of about six U-boats sank 32 ships of 186,000 gross tons during the month. To meet the increased U-boat activity in that area, additional escorts were added to the Freetown forces and
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U-BOAT OFFENSIVE17
action was taken to divert all shipping from the Freetown Area, except those ships which must of necessity pass through those waters. Attacks on independents continued to increase as only about 20 per cent of the shipping sunk by U-boats in May was in convoy. In addition, the U-boats continued to move further westward and on May 20 located Convoy HX 126 at about 11° west longitude, before the antisubmarine escorts had joined. Eight ships were sunk before the convoy was forced to disperse. This attack forced iho adoption of complete transatlantic escort. It was felt that the considerable weakening in the number of escorts with a convoy must be accepted in order to provide some degree of protection throughout the voyage. Complete transatlantic escort was accomplished by basing escort forces in St. John's, Newfoundland, and escorting in stages from England, using Iceland as a refueling base. The Royal Canadian Navy cooperated in these measures by placing all available destroyers and corvettes at the service of the Newfoundland Escort Force. Canada had about 35 ships fitted for antisubmarine service at that time. The first escorts from St. John's sailed on May 31, 1941, and, as a natural sequence to this development, it was decided by the middle of June to escort the convoys all the way from Halifax. The long-endurance corvettes were to run all the way between Halifax and Iceland, with the destroyers being limited to St. John's. Areas of U-boat activity in June were further afield and wider spread than before, with reports of U-boats near Newfoundland and south of Greenland. Despite the magnitude of the elfort exerted, the shipping losses showed an improvement over May, with 57 ships of 296,000 gross tons being sunk by U-boats in June. Despite the increasing number of U-boats at sea, the losses were kept down by the efficiency of British countermeasurcs as five U-boats were sunk during June by surface craft. Losses in the Freetown Area were greatly reduced and the U-boats had difficulty in locating the transatlantic convoys. When they did finally locate Convoy HX 133 on June 23, the results must have been rather disappointing to the Germans, as only five ships were sunk at the cost of at least two U-boats sunk. This successful defense of this convoy was due in large measure to the fact that, when DF bearings indicated that HX 133 had been sighted by a U-boat, the escort was increased from one destroyer and three corvettes to two	destroyers, one sloop, and ten corvettes. This was accomplished by taking the risk of stripping the escorts from two OB convoys within comparatively easy reach. Fortunately, one of these OB convoys escaped unscathed while the other suffered the loss of only one ship. On June 22, 1941, Germany invaded Russia and this seemed to end the threat of invasion of England for the time being. This released additional air and surface craft to help in the battle against the U-boats. In addition, German aircraft were diverted to the Eastern Front and attacks on shipping by aircraft were greatlv reduced during the last half of 1941. The average number of U-boats at sea continued to increase during July and August, but they had very little success as only about 23 ships of 90,000 gross tons were sunk in each of these months. This meant that the average U-boat at sea in the Atlantic was sinking less than one ship a month, a much lower rate than had been experienced in the past. In an endeavor to make the interception of shipping easier the U-boats withdrew to the eastward towards the end of July and concentrated in the waters west of Ireland and to the east of 25° west longitude. This placed them at a focal point of shipping where they could intercept both the East-West and the North-South convoys. However, the U-boats had no better luck there in August than they had in July. By this time, even the Gibraltar and Freetown convoys had more or less complete end-to-end escort. Towards the end of August, there were indications that the U-boats were resorting to long-range attacks on convoys, probably firing a browning salvo, and also to deliberate attacks on escorts. This policy of attacking escorts might have proved more profitable in the earlier days of the war, when the number of escorts with convoys was much smaller and when the general escort situation was much tighter. In addition to the defensive successes scored in August, this month was marked by one of the outstanding events of the U-boat war, the surrender of U-570 to a Hudson aircraft on August 27, 1941. U-570 left on her first cruise on August 24 and was at sea for only 74 hours before she surrendered. The U-boat came to the surface at 1030 on the 27th, the precise moment the Hudson from Squadron 269 was overhead. The U-boat tried to crash dive but the Hudson was too quick for her, diving from 500 feet to 100 feet, and dropping four depih charges. Captain Rahmlow, believing the U-boat more seriously damaged
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Figure 1. Surrender of U-570 (HMS Graph) to Hudson aircraft S-269 on August 27, 1941. Note German crew crowded into conning tower.
than was actually the case, ordered the crew to put on life jackets and go to the conning tower. The Hudson opened fire and kept the crew from abandoning the U-boat. After a white flag was displayed by the U-boat the Hudson guarded it until relieved by a Catalinn. A trawler arrived at 2250 and U-570 was towed to Iceland. She was subsequently repaired and re-christened HMS Graph, proving an invaluable addition to the British Navy. This was the first U-boat actually captured by the British and proved to be an extremely valuable source of information about the operating characteristics of U-boats. U-boat activity continued at a high level in September and their intensive efforts met with greater success than during the previous two months. Allied shipping losses rose sharply to 51 ships of 205,000 gross tons sunk by U-boats, with 70 per cent of the tonnage sunk being in convoy. This increase was due in a large measure to the severe casualties suffered by	four large convoys as a result of determined and sustained attacks by wolf packs. Two of these convoys were slow SC convoys which were intercepted and heavily attacked south of Greenland, losing 21 ships and one escort. Two U-boats were sunk by escorts during these attacks. The other two convoys, homeward bound from Freetown and Gibraltar, lost 15 ships and one escort to the U-boats. However, in viewing the situation at this time it would be well to compare it with the previous year. In September 1940, when about seven U-boats were at sea, the losses to U-boats were about 300,000 gross tons. In September 1941, when there were about 35 U-boats at sea, the losses to U-boats were only about 200,000 gross tons. In September 1940 the U-boats were attacking convoys with impunity. Rarely was a U-boat sighted during her attack, and even more rarely was she counterattacked. In contrast to this, in September 1941 it was a matter of the keenest
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U-BOAT OFFENSIVE19
disappointment it a convoy were attacked and the enemy escaped unpunished. The limited number of escorts, usually about one destroyer and three or four corvettes, although unable to prevent the U-boats from attacking the convoy, were generally able, with the help of covering aircraft, to shake off the pursuing U-boats by persistent counterattacks. No convoy was attacked for more than three successive nights in September 1941. The effect of Coastal Command aircraft on U-boat operations may be seen by the fact that, of the tonnage sunk in the North Atlantic during September by U-boats, about 75 per cent was lost in the area outside the economical range of Whitley and Wellington aircraft (100 miles). Aircraft made 45 sightings and 39 attacks on U-boats in September, the highest monthly figures recorded to that date. September was also marked by the introduction of HMS Audacity, an auxiliary aircraft carrier, as a convoy escort. One of her fighter aircraft shot down a Focke-Wulf attacking the rescue ship of Convoy OG74. As the radius of U-boat operations in the Atlantic extended further west and U. S. ships were being sunk by U-boats, the U. S. Navy announced on September 15, 1941, that it would provide protection for ships of every flag carrying land-aid supplies between the American continent and the waters adjacent to Iceland, on which a U. S. base had been established in July 1941. On September 16 the first convoy (HX 150) to have U. S. Navy ships as part of its escort sailed from Halifax. The losses in October 1941 dropped to 32 ships of 157,000 gross tons sunk by U-boats. Only one convoy (SC 48) was heavily attacked by U-boats; nine ships and two escorts were sunk and the USS Kearny was torpedoed but arrived at Iceland. The USS Reuben James was sunk by a U-boat torpedo on October 31 while acting as an escort of Convoy HX 156. In a number of other cases the convoys were located by U-boats but the escorts were able to drive them off without suffering serious losses. An interesting feature of the operations during October was the disinclination of the U-boats to pursue their quarry too far northward or eastward, presumably because they did not care to enter the areas swept by Coastal Command aircraft. This is indicated by the fact that of the 26 ships sunk within 800 miles from air bases, 14 were lost in portions more than 600 miles out and 12 in the 400 to 600-mile	zone (covered lightly by Catalinas). No ships were sunk within 100 miles from Coastal Command bases. This meant that the U-boats were being forced ever further westward, with consequent greater wastage of time and U-boats, particularly in winter. In addition, air attacks on transit U-boats in the Bay of Biscay began to increase in frequency and effectiveness, thereby further cutting down the operational time of U-boats. During the first week of November, the scale of the U-boat effort was probably the greatest and the scope of their patrol the widest spread of the whole Atlantic campaign up to that time. When Convoy SC 52 was intercepted shortly after rounding Newfoundland and four ships were sunk on November 3, it was considered prudent not to risk it upon a transocean journey for the greater part of which many U-boats might have maintained continuous harrying attacks. The convoy put back to port and the ships sailed later in Convoy SC 51, which consisted of 71 ships. This decision proved fortunate, as there followed a period of successful evasion which lasted for the remainder of the month. The total losses for November 1941 dropped to 12 ships of 62,000 gross tons, the lowest figure since May 1940. Weather contributed to the reduction in shipping losses but the main factor was skillful evasive routing. Successful evasion meant fewer chances of contacts between escorts and U-boats and therefore less chance for the destruction of U-boats. In the meantime the British offensive in Libya had been launched and the Germans withdrew a large proportion of their U-boats from the Atlantic to the Mediterranean to help the Italian Fleet and temporarily, at least, to use their U-boats for the transport of military supplies to Rommel. Toward the end of November, convoys from Gibraltar were suspended and every available ship was used in an endeavor to close the Straits of Gibraltar to the passage of U-boats and to destroy U-boats attempting passage. Two German U-boats attempting the passage were sunk during the month, one by surface craft and the other by a Dutch submarine. The perceptible slackening in tension in the North Atlantic that started toward the end of November continued throughout December 1941. The losses in the Atlantic continued at a very low level, with only 10 ships of about 50,000 gross tons sunk by U-boats during the month. Howvever, the tendency of the U-boat war to become world wide became apparent,
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as for the first time the losses to U-boats outside the Atlantic became significant. Seven ships of 27,000 gross tons were sunk by U-boats in the Mediterranean, three in the eastern approaches to Gibraltar and four while on passage between Alexandria and Tobruk. However, the enemy paid a heavy price for these seven ships, as five U-boats were sunk in the Mediterranean during December.
Japanese submarines sank nine ships of 42,000 gross tons in the Pacific: during the remainder of December after their attack on Pearl Harbor on December 7, 1941. One Japanese submarine was probably sunk. Over 200,000 gross tons of shipping were lost in the Pacific due to capture by the Japanese or unknown causes. Accordingly, the total shipping losses for the month (from all causes) rose to over 500,000 gross tons.
The outstanding feature of the month's operations in the Atlantic was the prolonged engagement between Convoy HG 76 and a pack of half a dozen U-boats. The escorts sank four U-boats (including U-567, commanded by Endrass, one of the leading U-boat aces) and only two of the merchant ships of the convoy were lost. However, one escort and HMS Audacity, an auxiliary aircraft carrier, were also sunk by the U-boats. As 1941 drew to an end, the number of U-boats in the Atlantic began to rise again, the majority leading westward.

3.2 COUNTERMEASURES TO THE

U-BOAT

3.2.1 Convoys

U-boat activity during the early months of this period forced the adoption of complete end-to-end escort of British convoys. Thanks to the steadily increasing number of escorts becoming available, and to cooperation from the Canadian and United States Navies, this did not result in the anticipated serious weakening in the protection of the convoys. In fact, ships sailing in convoy were safer in this period than they had been in the preceding period, as of the 4100 ships being convoyed monthly only 14 (about 1/3 of 1 per cent) were sunk monthly by U-boats. By the end of 1941, it appeared, as evidenced by ihe battle of Convoy HG 76, that the wolf-pack attacks on convoys could be beaten off without serious losses to the convoy and in turn these attacks could prove costly to the U-boats, provided a sufficient number of escorts were present.

An analysis of 17 of the convoys attacked by the U-boats at about this time gives us some information about ihe "average attacked convoy." This convoy was engaged by 4.2 U-boats, of which 2.6 succeeded in delivering effective attacks. A total of 4.6 ships in the convoy were torpedoed, 1.7 ships being torpedoed in each effective attack. Of the 4.2 U-boats engaging the convoy 3.2 (or 76 per cent) were attacked by the air and surface escorts and 0.65 (or 15 per cent) of them sunk.
There were not many changes made in the convoy system during this period. The minimum speed for HX convoys was raised from 9 knots to 10 knots, with the minimum speed for SC convoys remaining at 7½ knots. In July 1941 a rule was introduced requiring ships of less than 15 knots, crossing the Atlantic, to sail in convoy. The designation of the OB convoys (outward-bound from England) was changed to ON for the north-bound convoy heading for Halifax and to OS for the south-bound convoys heading for Freetown. July 1941 also marked the introduction of CAM ships in convoy. These were merchant ships equipped with a catapult-launched fighter aircraft which was to be used against enemy reconnaissance aircraft. In August 1941, the first convoy to Russia sailed for Archangel. No losses were suffeted on the convoys sailing between England and Russia during 1941.

3.2.2 Aircraft

At the beginning of this period, in April 1941, it was apparent that an improvement in the quality of aircraft attacks was urgently needed. Actual kills by aircraft had been disappointingly few and it was felt that, on the relatively rare occasions when a pilot sights a U-boat, he should-have a reasonably good chance of bringing off a kill. A committee of Coastal Command scientists and naval representatives was therefore formed to review this situation.
Analysis showed that in 35 per cent of the aircraft attacks the U-boat was still visible at the time of release of the depth charges and that in 15 per cent of the attacks the U-boat had disappeared less than 30 seconds previously. The solution adopted was to concentrate on those U-boats which were still on or near the surface and to adjust tactics, depth-bomb settings and spacing, so as to give the aircraft the maximum chance of killing in these conditions, since the probability of hitting the U-boat under these conditions is much greater than it is after the U-boat has been submerged for some time.

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  Instructions were accordingly issued by Coastal Command in June 1941 providing that the depth setting for all depth charges should be 50 feet, the spacing between charges should be 60 feel, and all depth charges were to be released in one stick. The need for more frequent practice and training was also stressed and a standard for bombing accuracy was set up requiring a mean radial error of all bombs dropped of not more than 20 yards.
  This change resulted in a significant improvement in the quality of aircraft attacks on U-boats. Aircraft made about 27 sightings a month during this period and about 18 of these resulted in attacks on U-boats. The proportion of these attacks which resulted in at least some damage to the U-boat rose from about 10 per cent in the earlier periods to about 25 per cent in this period. The lethality of the aircraft attacks did not change much, as only about 2½ per cent of the attacks resulted in the U-boat's being sunk. However, it was realized at the end of this period that even the 50-foot depth setting was loo deep for surfaced U-boats and steps were being taken to modify depth charges to allow a 25-foot depth setting.
  As the U-boats moved further westward, the total number of flying hours by Coastal Command aircraft on antisubmarine duties decreased, but transit U-boats crossing the Bay of Biscay on their way to and from French bases began to receive more attention. In the period September to November 1941, 36 attacks were made on U-boats in this area. This was equivalent to each U-boat's being attacked in the Bay on one out of every three cruises and was a sufficient menace to force them to remain submerged in the Bay during the daytime in December, thereby increasing their transit time. By December 1941, Coastal Command had initiated night antisubmarine patrols with radar-fitted aircraft in the Bay of Biscay.
  Midway in 1941, the British scraped together a squadron of long-range planes, new untried Liberators rejected by U. S. Air Forces. Fitted with Mark II radar, they did a dual job, ranging far out on patrol and giving air cover to convoys asking for help.
  The flying effort during this period was marked bv the increased use of protective sweeps around convoys to put down shadowing U-boats, as distinct from the close convoy coverage provided earlier. It was also marked by the first attempt, in May 1941, by radar-equipped aircraft to hunt a U-boat to exhaustion. Although this attempt did not succeed, much useful information was obtained from this operation.

3.2.3 Scientific and Technical

  A new antisubmarine weapon for use by surface craft was coming into production at the end of 1941. This weapon was "Hedgehog," a multispigot mortar which throws 21 projectiles, fited with contact fuzes so as to produce a pattern in the form of a ring whose center is about 250 yards ahead of the ship. The Hedgehog pattern, theoretically, has a greater chance of killing a submarine than the depth-charge pattern due to the fact that its design incorporates the following three important principles:
  1. Ahead thrown. This permits a reduction in blind time from 45-90 seconds to 15-20 seconds.
  2. Multiple small charge. Three hundred pounds of high explosive (lethal range of 21 feet) has about one half the chance of killing the U-boat as ten 30-lb charges (lethal radius of 6 feet). The lower limit of the charge is the smallest weight which will have a reasonable chance of sinking the U-boat when it explodes on the target.
  3. Contact firing. The contact fuze "sweeps" all depths. This is of particular importance in view of the fact that the antisubmarine gear used at that time did not determine the depth of the U-boat.
  However, against these advantages of Hedgehog must be considered the fact that the large explosion of depth charges has a desirable anti-morale effect and produces considerable shaking up of the U-boat in cases where the depth charges are not lethal. Hedgehog fails to do this since the charges explode only on hitting the U-boat.
  During this period, the results from radar were more promising as a consequence of increased training and knowledge of the gear. A new set, Type 271, was being fitted on British corvettes. This was a shortwave (10 cm) "beatm"-principlc set and constituted a great advance in radar for antisubmarine purposes. The average range at which radar contact was made on a U-boat during this period was about 1000 yards, with the maximum range being 7000 yards.
  Coastal Command was also experimenting, during this period, with a searchlight, carried in a Wellington aircraft, to provide illumination for night attacks on U-boats.

3.2.4 Sinkings of U-boats

The steadily upward trend in the number of U-boats sunk or probably sunk monthly continued throughout this period, reaching a new high for the

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Figure 2. Hedgehog ready to fire.
war in December 1941 when 13 U-boats are believed to have been lost. However, it should be kept in mind that the rate of increase in the number U-boat sunk was much lower than the rate of increase in the number of U-boats at sea, so that the average U-boat at sea had a much smaller probability of being sunk in this period than in either of the two preceding periods. The total number of enemy U-boats sunk during this 9-month period was 44. Thirty of these were lost in the Atlantic (22 German and 8 Italian) and 13 in the Mediterranean (6 German and 7 Italian). One Japanese U-boat was probably sunk in the Pacific during December. Two German U-boats were known to have been lost in-the Baltic as a result of collisions with their own craft. Surface craft continued to be the most effective craft in sinking U-boats, accounting for 20 of the 30 U-boats sunk in the Atlantic. Another two were sunk	as a result of combined attacks by ships and aircraft. Two were lost as a result of aircraft attacks and one was torpedoed by a submarine. The circumstances under which the other live U-boats, known to have been lost in the Atlantic, were sunk are not known. During the last six months of 1911, one out of every three U-boats attacked by surface craft was at least damaged, while one out of every seven attacked was sunk or probably sunk. 3.3 SURVEY OF RESULTS 3.3.1 From the U-boat Point of View The average number of U-boats at sea in the Atlantic during the third period was about 30, three times as many as in the previous period. Their main effort was directed in the form of wolf-pack attacks against the North Atlantic convoys. They did succeed in forcing the Allies to adopt complete end-to-end
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Figure 3. DE firing Hedgehogs on shakedown cruise. Projectiles can be seen above the bow.

escort of their transatlantic convoys but they definitely failed in their main objective of cutting off supplies to England and toward the end of this period there were signs of their shifting to other areas.
Despite this threefold increase in the average number at sea, the U-boats were able to sink only about 34 ships of about 166,OOO gross tons monthly in the Atlantic- during this period, or about 25 per cent less than in the previous period. This meant that, the average U-boat was only sinking a little over one ship of about 5500 gross tons per month at sea, and was therefore only about one-fourth as effective as in the previous period. This drop in efficiency reflects the successful evasive routing of convoys and the very rapid expansion in U-boat personnel. This latter condition resulted in a high proportion of U-boats being sunk on their first cruise. The evasive routing of convoys also resulted in fewer contacts between convoy escorts and U-boats and consequently the average U-boat was relatively much safer during the third period. About 3 ¹/₃ U-boats were sunk monthly in the Atlantic, of the 30 at sea on the average, and consequently the average life of a U-boat at sea was about 9 months, more than twice as long as it had been in the previous period. The average U-boat during this period was sinking ten ships of about 50,000 gross tons before it was sunk itself. Hence, despite the longer lifetime of the average U-boat, the exchange rate was about 40 per cent less than in the previous period, reflecting the decreased effectiveness of U-boats in sinking ships.

The number of ocean-going German U-boats in commission was expanding at a very rapid rate, increasing from about 54 at the beginning of this period to about 200 at the end of 1941. About 174 German U-boats were commissioned during this period while only about 28 were lost. From the point of view of number alone, the U-boais offered a considerable threat and it seemed likely that with increased experiences the effectiveness of these new U-boats would increase.

3.3.2 From the Allies' Point of View

  Total shipping losses of the Allied and neutral nations decreased to 363,000 gross tons monthly during the third period, while the building rate of new shipping increased to about 175,000 gross tons monthly. Consequently, the net monthly loss of shipping was only 188,000 gross tons, about 45 per cent less than during the preceding period.
  Of the 363,000 gross tons of shipping lost monthly, about 323,000 gross tons were lost by enemy action. U-boats accounted for 175,000 gross tons a month, 54 per cent of the total lost by enemy action. Monthly losses to enemy aircraft increased to 75,000 gross tons, while the losses to enemy surface craft dropped to only 17,000 gross tons. Mines were responsible for only l9,000 gross tons a month, while other and unknown causes accounted for 37,000 gross tons a month.
  The total shipping available decreased from about 35,000,000 gross tons at the beginning of this period to about 33,300,000 at the end of 1941, but the upward trend in shipping losses seemed to have been checked. The number of British antisubmarine ships suitable for ocean escort had increased from about 375 at the start of the period to about 500 at the end of 1941. In addition, the entry of the United States into the war added about 175 destroyers to the above numbers, but many of them were committed to action in the Pacific.
  From the defensive point of view the U-boats, which constituted the main threat to Allied shipping, seemed to have been defeated in the Battle of the Atlantic during this third period. They were experiencing difficulty in locating Allied convoys and even when they did locate a convoy, the escorts were generally able to beat off the wolf-pack attacks without serious losses. Shipping losses to U-boats had been kept to a reasonable level, considering the number of

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U-boats at sea, while shipping construction was gradually increasing. However, from an offensive point of view, the U-boats were still relatively safe. Surface craft acting as convoy escorts were the only serious threat to the U-boat, although aircraft were gradually becoming more effective in harassing and damaging<

U boats. It seems, therefore, that the general situation in the Battle of the Atlantic at the end of 1941 was roughly the same as the situation which prevailed at the end of World War I — that is, with shipping losses checked, but with the U-boats relatively safe at sea.

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- On to ASW in World War Two Part 1: Chapter 4 -

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