LARGE WOLF PACKS BATTLE NORTH ATLANTIC CONVOYS
OCTOBER 1942 - JUNE 1943
5.1 U-BOAT OFFENSIVE
DESPITE the fact that the U-boats had been driven from the coastal areas of the United States by the beginning of this period, the shipping losses to U-boats continued to run at the same high level as during the previous period. The Atlantic continued to be the chief area of U-boat activity with about 85 per cent of the shipping losses to U-boats occurring there. There were over 100 U-boats at sea in the Atlantic during most of this period. This enabled the enemy to conduct an intensive U-boat campaign in the Northwest Atlantic Area against the strategically important North Atlantic convoys and still maintain a number of subsidiary campaigns in other widespread areas (such as the Caribbean, Brazilian, Freetown, and Southeast Atlantic Areas) in an attempt to force the Allies to disperse their forces.
The total shipping losses to U-boats continued to be rather heavy in October 1942 as 93 ships of 614,000 gross tons were sunk. The intensity of the U-boat campaign against the transatlantic convoys increased with the number of U-boats in the Northwest Atlantic Area rising from 14 in September to 22 in October. These U-boats sank 24 ships with the bulk of the losses occurring in the "gap," the region outside the range of shore-based aircraft. Experience indicated that a convoy, not protected by aircraft, may be so disorganized by a concentrated attack and the resultant breaks in formation for rescue work and other adjustment that the escorts may become comparatively ineffective for either protection or offense. However, the presence, even for a few hours, of one or two aircraft has again and again prevented a concentrated attack from developing.
This was well illustrated during October. On the 4th, Convoy HX 209 was in some peril when about 300 miles south of Iceland. U. S. Navy Catalinas provided close escort for 151/2 hours and protective sweeps were laid on by seven aircraft from Iceland and three Fortresses from England. In the course of these operations the aircraft made nine sightings and carried out six attacks; the attack on the convoy did not develop. On the other hand, Convoy SC 104 was heavily attacked on the nights of the 13th and 14th when out of the range of aircraft and eight ships were torpedoed. On the following day there was a change for the better. The weather improved and air cover arrived enabling the surface escort to go over to the offensive and sink two U-boats.
Another development during October was the start of the expected U-boat operations in the Southeast Atlantic Area, near Capetown. This new "soft spot" was exploited by two groups, each of about six U-boats, which reached the Capetown area and eventually passed into the southern part of the Indian Ocean, where Japanese U-boats had previously achieved considerable success. These groups sank 25 ships in the Southeast Atlantic Area during October. All of these ships were independently routed and some carried rather valuable cargoes.
However, some of the heavy shipping losses suffered in October and November may be charged to the success of Operation Torch, the landings in North Africa early in November 1942. A great deal of merchant shipping and many escorts were naturally diverted from their normal use in October, when the foundations were firmly laid for the achievement of the safe arrival of the first military convoys. In addition, the enemy disposed an abnormally large proportion of his Atlantic U-boat force east of the Azores and Madeira. This suggests that, in a broad sense, the German command had appreciated the likelihood of an Allied attack on Africa. One consequence directly resulting from this re-disposition of U-boat forces was that Convoy SL 125 was severely mauled, losing 12 ships during a 4-day pursuit by a pack of about six U-boats. However, the pursuit was abruptly dropped before it drew the U-boats away from the area southwest of Portugal; it may have been regarded as of greater importance that they should stay there and reconnoiter than that they should further pursue an ordinary trade convoy.
Despite the fact that convoys of the expeditionary
forces had to pass through concentrations of 30 to 40 U-boats before reaching Gibraltar, no U-boat successes were achieved against these convoys until after the assault troops had landed. The number of U-boats in the Western Mediterranean increased from about 10 on November 8 to about 20 on November 11. The losses to U-boats in Operation Torch amounted to only about 84,000 gross tons of shipping and six naval vessels. The total shipping losses from all enemy causes in this operation amounted to about 134,000 gross tons, against which can be offset a gain to the Allies of about 181,000 gross tons of serviceable tonnage acquired in French ports. In addition to the defensive success achieved against the U-boats in Operation Torch, the Allies also scored a notable offensive success, sinking 15 U-boats in the Mediterranean during November. A large factor in these victories over the U-boats was the heavy air cover provided, since aircraft from Gibraltar made 110 sightings and 64 attacks on the large concentration of U-boats.
In contrast to the successful landings in North Africa, world-wide shipping losses reached their highest point of the war in November 1942, as 862,000 gross tons of shipping were lost from all causes. U-boats also sank the greatest tonnage of the war in November, accounting for 116 ships of 712,000 gross tons. The tonnage sunk during October and November was swelled by the loss of a number of large, fast, independently routed ships which, on account of their speed, had been regarded as fairly safe from U-boat attack. Losses in the Northwest Atlantic continued at their high level as 26 ships of 144,000-,gross tons were sunk there by U-boats. Losses in the Southeast Atlantic fell off slightly to 23 ships of 127,000 gross tons but shipping losses in the Caribbean and Brazilian Areas mounted again as the U-boats sank 34 ships of 210,000 gross tons there. These heavy losses in widespread areas occurred despite the intensive effort made by the U-boats in opposing the Allied landings in North Africa.
The shipping losses to U-boats in December 1942 were 62 ships of 344,000 gross tons, only about half the record total of November. This steep decline was probably due to the cumulative effect of various contributing factors, such as the concentration of U-boats in the Atlantic approaches to the Western Mediterranean and a consequent withdrawal elsewhere (particularly from the West and South Atlantic), the heavy bombing by Allied aircraft of the Biscayan U-boat bases, and some effective work by surface and air escorts on the North Atlantic routes, as well as adverse weather and some luck in routing. The enemy attributed the smaller sinkings to bad weather.
Only one convoy, ONS 154, suffered heavy losses in the North Atlantic. This convoy was given a southerly route and was attacked by a pack of about 20 U-boats as it drew out of the range of air cover. These U-boats followed the convoy for four days and sank 14 ships. This again demonstrated the immense importance of efficiently combining surface and air escort and indicated that surface escorts are physically incapable of warding off concerted attacks by a pack of U-boats which outnumbers them by more than a 2-to-1 ratio. In selecting convoy routes at this time, the advantage of the northerly route within the range of air cover from Iceland had to be balanced against the increased time of the trip due to greater distance and worse weather. In addition the bad weather reduced the efficiency of the escorts.
The shipping losses to U-boats in January 1943 dropped to only 35 ships of 203,000 gross tons, less than in any month of 1942. Bad weather was the primary factor responsible for this reduction and it was recognized that the gravity of the U-boat threat was much more serious than indicated by the low shipping losses. The fate of a tanker convoy bound from Trinidad to Gibraltar seemed a truer index of the strength of a U-boat pack operating in favorable conditions at the beginning of 1943. Convoy TM 1, made up of nine tankers and four escorts, lost seven tankers as a result of U-boat attacks, all well outside the range of shore-based aircraft. This was the highest proportion of ships sunk from any convoy by U-boats during the war.
Of the 100 or more U-boats at sea in the Atlantic on any one day, over a third were operating in the Northwest Atlantic Area. The U-boats were concentrating against the North Atlantic convoys which carried cargoes of immediate importance for military operations. Grand Admiral Doenitz had become supreme commander of the German Navy and it was becoming evident that the critical phase of the U-boat war in the Atlantic was approaching.
To meet to some extent the need for shore-based air cover for Atlantic convoys, the Allies planned to introduce Very Long Range [VLR] aircraft with an endurance of 2000 to 2500 miles. Some flying was also done from Greenland although weather conditions
were a severe handicap. Continuous shore-based air cover was available between Gibraltar and Freetown.
In February 1943 the U-boat effort in the North Atlantic increased in intensity. The world-wide shipping losses to U-boats increased to 63 ships of 359,000 gross tons, with about half these losses occurring in the Northwest Atlantic Area. Two of the North Atlantic convoys were heavily attacked by packs of about 20 U-boats. Convoy SC 118 lost 12 ships to U-boat attacks early in the month, seven of them during a period of three hours. However, at least two U-boats were sunk and six damaged during this action. Convoy ON 166 suffered the loss of 14 ships over a period of five days as a result of U-boat attacks outside the range of shore-based aircraft. The enemy was concentrating his forces in an effort to interrupt the flow of supplies from America to Great Britain and the U-boats were displaying increased boldness in attempting to achieve this objective. However, about 20 VLR aircraft had become operational in February 1943.
In March 1943 the world-wide shipping losses to U-boats mounted to 107 ships of 627,000 gross tons. The tempo of the U-boat offensive against the transatlantic convoys increased as the number of U-boats in the Northwest Atlantic Area rose to about 50. These U-boats sank 38 ships in that area, a high for the war, and an additional 15 ships were sunk in the adjacent Northeast Atlantic Area. The enemy's resources were such that he was also able to maintain offensives in the South Atlantic and in the Caribbean and Azores Areas. About two-thirds of the tonnage sunk by U-boats during March was in convoy when sunk and the figure of over 400,000 gross tons of shipping sunk in convoy by U-boats during the month was a record for the war. The main reason for the large increase in tonnage sunk in convoy was an increase in the number of convoys attacked and not a weakening in the protective value of the escorts.
The intensity of the U-boat effort in the North Atlantic during March is clearly indicated by the attacks on four consecutive eastbound transatlantic convoys. The U-boats generally outnumbered the escorts by about two to one and were able to torpedo 30 ships from these four convoys, in addition to eight stragglers. Convoy SC 121 was badly scattered by heavy weather and lost five ships in convoy and seven stragglers. Convoy HX 228 lost four ships to the U-boats but the escorts were able to sink two U-boats in one of the dramatic incidents of the U-boat war.
After an attack on the convoy, HMS Harvester sighted a U-boat and rammed it at about 26 knots. The U-boat hung under the stern and could not be dislodged for about ten minutes. The FFS Aconit finished off the damaged U-boat. In spite of considerable damage, HMS Harvester was able to proceed on one engine for several hours until she became completely disabled and was torpedoed. The FFS Aconit then sank the U-boat which had destroyed HMS Harvester.
Convoys HX 229 and SC 122 were routed closely together and ran into a concentration of about 40 U-boats, the largest pack employed up to that time, losing 20 ships to the U-boats. On the night of March 16-17, 13 ships were sunk when the convoys were about 850 miles from the nearest air bases. The following day the convoys received some air cover from VLR aircraft, with one plane sighting six U-boats. In all, 54 sorties were flown in defense of these two convoys resulting in 32 sightings and 21 attacks. The U-boats were finally forced to disengage on March 20 and this date may well be considered as the turning point in their offensive power as sinkings of merchant vessels were at a much lower rate thereafter.
Although the shipping losses in March were very heavy, there were signs that the situation in the North Atlantic was improving. Advances had been made towards the provision of continuous air cover for transatlantic convoys by the increased use of VLR aircraft from Newfoundland, Iceland, and England and by the introduction of escort carriers and Merchant Aircraft Carriers [MAC] ships. MAC ships were merchant ships whose decks were converted so that they could carry four Swordfish aircraft which could take off and land. The USS Bogue, the first U. S. escort carrier, started operations as an escort for the North Atlantic convoys during March. At the Atlantic Convoy Conference held in Washington in March, it was agreed that, starting in May, the United Kingdom and Canada should be responsible for the security of convoys across the North Atlantic, the United States providing certain air and sea forces to help them. It was also decided to set up a unified air command in Newfoundland corresponding to the Coastal Command system. The United States was to base a substantial force of VLR aircraft in this region, which, together with the VLR Liberators of the RCAF, were to close the gap in the North Atlantic.
Although the number of U-boats at sea continued at the same high level in April 1943, the shipping loss
was greatly reduced with the U-boats sinking 56 ships of 328,000 gross tons. The shipping losses to U-boats in the Northwest Atlantic Area in April were only 18 ships of 108,000 gross tons, less than half the March record. Ten ships were sunk by U-boats in the Freetown Area and Japanese U-boats sank six ships in the Southwest Pacific, off the east coast of Australia.
Accompanying the decrease in shipping losses there was a considerable increase in the number of U-boats sunk, 1% being sunk in the Atlantic for a record high to that date. April also marked the first sinking of a U-boat in which planes from an escort carrier took part. A plane from HMS Biter found and attacked a U-boat which HMS Pathfinder, one of the escorts, sank.
A more satisfactory criterion that the U-boat offensive strength had passed its peak was the fact that, for the first time, U-boats failed to press home attacks on convoys when favorably situated to do so. In none of the attacks on transatlantic convoys did the enemy succeed in obtaining anything like the upper hand. All of the U-boats' efforts tended to avoidance of detection and, once it was apparent that they had failed in this endeavor, they seldom pressed home their attacks. There were five Support Groups operating in the North Atlantic during April, two of them having their own escort carrier, and the number of VLR aircraft available had risen to over 30. More important, there was a noticeably enhanced standard of group training, better use of HF/DF was made, and cooperating between surface and air components of the escorts was greatly improved.
May 1943 seems to have been the crucial month in the Battle of the Atlantic. The number of U-boats at sea in the Atlantic reached a peak of about 120, with about half of these U-boats concentrated in the Northwest Atlantic Area. The decisive battle took place at the beginning of the month between Convoy ONS 5 and a pack of about 40 U-boats. On May 4, when HF/DF activity indicated that the U-boats had made contact, the convoy consisted of about 30 ships and four stragglers. The average number of escorts present during the battle was about eight. Despite the bad weather, aircraft of the Royal Canadian Air Force provided air cover during the afternoon of the 4th and carried out two promising attacks, one of which is considered to have sunk the U-boat. The U-boats started attacking shortly after midnight and 12 ships were sunk on May 5, about half during the night and half during the next day. Due to bad weather, air cover was provided for only an hour on the morning of the 5th. By midnight the weather had become calm and foggy and from then onwards the escorts had the upper hand. In the course of the night they frustrated about 24 attacks by the U-boats without suffering any losses and, in addition, inflicted heavy losses and much damage on the enemy. At least five U-boats are considered to have been sunk by the escorts during the battle. The U-boats broke off the action in the course of the day and did not renew it. In no succeeding convoy operation did the enemy display the same determination.
Several other convoys were threatened in the first half of the month but no serious losses were suffered and the toll of U-boats mounted steadily. After May 17, no ship was lost in the Atlantic north of 45° north latitude. The world-wide shipping losses to U-boats in May 1943 decreased to 50 ships of 265,000 gross tons. Only 14 ships were sunk in the Northwest Atlantic Area while diversionary U-boats sank 15 ships in the Freetown and Southeast Atlantic Areas.
The most notable feature of the operations during May was the success of our offensive against the U-boats. The number of U-boats sunk during the month reached a record high of 44, almost twice the previous high, with 38 of the sinkings occurring in the Atlantic. A record number of 15 were sunk in the Northwest Atlantic Area and the number sunk in the Bay of Biscay also reached a new high of 11. Shore-based aircraft accounted for over half the U-boats sunk during the month while carrier-based aircraft participated in three of the attacks in which the U-boat was considered to have been sunk.
By the end of May it was clear that the U-boats had been decisively beaten as the number at sea in the Atlantic dropped to about 85, with no small proportion of the reduction due to sinkings of U-boats. The U-boats could not afford to suffer the heavy losses experienced in May and they were forced to withdraw from the battle against the vital North Atlantic convoys. This was an admission of defeat in itself. It should be noted that superior leadership and tactics, quick initial action, and well-coordinated attack and defense played as much of a part in the defeat of the U-boats as did concentration of forces at the decisive points and weapon superiority.
By June 1943 the U-boats had retired from the main battlefield of the North Atlantic convoy routes and a considerable number had moved into the
Azores Area, outside the range of shore-based aircraft. As a result, June was an interim phase for the U-boats and they were able to sink only 20 ships of 96,000 gross tons during the month, the smallest amount of shipping sunk by U-boats since November 1941. Not a single ship was sunk in the Northwest Atlantic Area during June and only seven ships were sunk by U-boats in the whole Atlantic. Seven ships were sunk in the Indian Ocean and six in the Mediterranean.
The offensive against the U-boats continued throughout June, but as the opportunities to attack U-boats were rarer due to their re-disposition, only 19 were sunk during the month. Aircraft continued to play an outstanding part, sinking ten U-boats, two by carrier-based aircraft from the USS Bogue in regions where the U-boats thought they would be safe from shore-based aircraft. The 2nd Escort Group led by HMS Starling sank three U-boats during the month and clearly demonstrated that a well-trained escort group could deal effectively with a U-boat. On June 2, this escort group made contact with U-202. Twelve depth-charge attacks were made without positive evidence of success, so it was decided to hunt the U-boat until its batteries were exhausted and it had to surface. One ship maintained contact while the remainder formed a square patrol. Despite every maneuver and artifice, including the release of 19 SBT's, the U-boat was compelled to surface after contact had been firmly held for 141/2 hours. She was immediately attacked by gunfire and the crew abandoned ship. On June 24, the 2nd Escort Group, acting as a striking force in cooperation with aircraft engaged in the Bay offensive, destroyed two more U-boats in the space of 9 hours.
As the period closed, it was apparent that the U-boats had been decisively defeated in the battle against the North Atlantic convoys. The Allies had meanwhile taken the initiative and joined battle in the approaches to the Bay of Biscay, through which all U-boats must pass to and from their bases.
5.2 COUNTERMEASURES TO THE U-BOAT
There were only a few minor changes in the convoy system during this period. The landings in North Africa in November 1942 resulted in the temporary suspension of sailings of the Sierra Leone and Gibraltar convoys for several months due to lack of escorts. Shortly after these landings new convoys were started between the United States and the Mediterranean (designated UG and GU), to supply Allied forces in North Africa. The U. S. Coastal Convoy System was extended to Brazil in December 1942 following increased U-boat activity in that area.
There were, however, a number of extensive changes in the routing of independent shipping. At the beginning of this period, the Allies were forced, by a concentration of U-boats between Natal and Dakar, to discontinue the route to the Red Sea and India via the Atlantic and the Cape of Good Hope. Shipping was instead routed via the Panama Canal and Cape Horn to Capetown. However, after the U-boats had started operations in force around Capetown, the bulk of the Indian Ocean traffic was routed transpacific from Balboa south of New Zealand to Fremantle for onward routing to destination. This route through the South Pacific proved entirely safe. There have been losses in the Indian Ocean due to raiders and U-boats but, in general, this transpacific route was so successful that it was maintained until the Mediterranean was considered open in July 1943. It can be seen that there was a considerable gain in effective shipping due to the use of the short Mediterranean route instead of the long circuitous route across the Pacific.
During this period, the main efforts of the U-boats were concentrated against convoyed shipping, in particular against the North Atlantic Trade Convoys (ON, ONS, HX, and SC). The proportion of tonnage sunk by U-boats, which was in convoy when sunk, increased from about 30 per cent during the last three months of 1942 to 67 per cent in March 1943 and then dropped back to about 25 per cent in June 1943 after the U-boats had withdrawn from the North Atlantic convoy routes. About 720 ships sailed monthly in the North Atlantic Trade Convoys and about 21 of these ships were sunk monthly by U-boats, so that the loss rate was about 3 per cent per crossing. This loss rate reached a peak of about 5 per cent in March 1943. By June 1943 the U-boats had been forced to withdraw and 850 ships arrived safely in these convoys without the loss of a single ship to the U-boats.
The strategy of the U-boats in their battle against the North Atlantic convoys was to maintain patrols in positions designed to find convoys at a time when
they were about to leave the protection of air cover, and it was in this gap that the U-boats scored their greatest success. There were three main U-boat formations for attacks on convoys, namely (1) Patrol line, (2) Reconnaissance sweep, and (3) Attack formation. In the first, U-boats up to perhaps 25 in number are spread about 20 miles apart on a given line of bearing across the convoy routes. Each U-boat patrols at slow speed on either side of and at right angles to the line, not going further than half an hour's run from it. If the U-boat sights a convoy she must report it to Admiral, U-boats, who passes the signal to the other U-boats in the line. This Patrol line formation is a pool from which groups of U-boats may be detached for any given duty, such as an attack on a convoy or a Reconnaissance sweep of an area through which a convoy is expected to pass.
For the Reconnaissance sweep formation, Admiral, U-boats, signals the limits of the area to be swept, the time at which the sweep is to start, and the U-boats which arc to take part. The U-boats ordered to the area, the limits of which may be 150 miles apart, proceed at a speed of about 10 knots on parallel courses and spread about 25 miles apart. When one of the U-boats has established the position of the convoy, a group of U-boats are ordered by Admiral, U-boats, to take up an attack formation. These U-boats may be formed up into a semicircle around the line of approach of the convoy. Once the order to attack is given, there is no such thing as coordinated action between the U-boats, though they may keep in touch with each other by radio. Each U-boat attacks at its discretion. The U-boats continue their attacks until ordered by Admiral, U-boats, to break off.
An analysis of attacks by U-boats on the North Atlantic convoys at about the beginning of this period indicated that about 70 per cent of the attempted attacks were frustrated by the escorts. About two ships were torpedoed in each successful attack. In only about half the successful attacks did the U-boat escape detection. Radar was responsible for more than half the cases in which the U-boat was detected.
The primary function of aircraft in the defense of convoys was the prevention of the gathering of U-boat packs capable of making destructive attacks. This function is accomplished in two ways. First, as a pack is gathering on a trailed and reported convoy, protective sweeps far ahead and to the flanks of the convoy enable aircraft to make killing and damaging attacks or at least to force U-boats to remain submerged at slow speed sufficiently long to delay the formation of an effective pack. Second, through keeping the trailing U-boats, on which the others are homing, submerged, aircraft can break the enemy's contact, enabling the convoy to escape by a change of course.
What the U. S. Navy needed in late 1942 was more long-range bomber coverage. The U. S. Army was anxious to help and set up the 1st Anti-Submarine Army Air Command in October 1942. This was an expansion of the 1st Bomber Command. Two antisubmarine squadrons were sent to England in December 1942. Early in February 1943 they operated in the Bay of Biscay and in March 1943 the two squadrons were ordered to Port Lyautey, Morocco, where they were to help the U. S. Navy protect the Mediterranean Approaches. By the end of this period two other U. S. Army squadrons had been sent to England while two U. S. Navy squadrons were operating from Iceland under Coastal Command.
When the U-boats started using GSR at the beginning of this period in October 1942 the need for microwave radar became more urgent. As a stop-gap 14 DMS-1000's were crash-built at a U. S. laboratory and rushed into British service. By the end of 1942 the U. S. Navy's AN/APS-2 [ASG] radar (S-band) was coming into service. The U. S. Army equivalent was the SCR-517.
The first effect of the introduction of GSR was a sharp reduction in the effectiveness of the aircraft offensive in the Bay of Biscay. The U-boats could detect radar-fitted aircraft ranges which allowed them ample time to dive and the number of sightings and attacks on U-boats dropped sharply. This lull lasted through January 1943. Only four sightings per 1000 hours on patrol were made during this four-month period, as compared to nine during the previous four-month period, when the number of U-boat transits was actually smaller. The search receiver for Mark II ASV enabled the U-boats to make safe night passages across the Bay and only 13 per cent of the transits were sighted. No U-boat is considered to have been sunk by any of the attacks made in the Bay during this period.
This situation was changed by the introduction of Mark III ASV (S-band) on Allied aircraft in February
and March 1943. This set operated on a much shorter wavelength (10 cm) and could not be detected by the R-600 GSR. The proportion of aircraft approaches undetected by U-boats rose and consequently the sightings and attacks began a steady increase. This period of increased productivity in the Bay offensive lasted through July 1943. About 5500 hours were flown monthly during this 6-month period with about 61 sightings and 33 attacks made monthly. About five U-boats were sunk monthly as a result of this offensive. The average number of sightings per 1000 hours on patrol jumped to 11 and about 60 per cent of all transits were sighted during this period.
The German High Command had no idea as to what caused this huge increase in sightings and attacks and after running into several blind alleys in attempting to solve the problem, they were forced to make a number of changes in U-boat tactics. The first step was the strengthening of the antiaircraft armament of U-boats. This development first became apparent in April 1943 when on a number of occasions U-boats, sighted by aircraft, stayed on the surface and fought back using their antiaircraft guns. Although a number of aircraft were lost as a result of this measure, it did provide aircraft with a much larger proportion of surfaced targets and increased their chances of sinking a U-boat. Four U-boats that stayed up and fought back were sunk in April and nine such U-boats were sunk in May.
During the spring of 1943, the night-flying Wellingtons, equipped with Leigh Lights and Mark III ASV, made night surfacing in the Bay so hazardous for the U-boats that they changed their policy to one of surfacing in the daytime to charge batteries and renew their air supply. This, in turn, produced the period of greatest productivity in the campaign during May, June, and July 1943, with the result that during the last two of these months every U-boat transit was sighted once on the average. May 1943 was a record month for Coastal Command aircraft with the squadrons in the United Kingdom, Iceland, and Gibraltar accounting for 213 sightings and 136 attacks, 17 of which are considered to have been lethal. One of the U-boats sunk in May was the victim of the first rocket attack against U-boats. This attack was made by a Swordfish aircraft.
The next indication of a change in U-boat tactics was the sighting of five U-boats making the transit of the Bay in company on June 12. From then onwards most of the U-boats sighted were in packs of from three to five. Although this change should have no effect on the number of U-boats sighted, the number attacked will be less as each aircraft can normally only attack one U-boat. This effect appeared in June when there were 60 sightings in the Bay but only 28 attacks. The other advantages of this change, for the U-boats, are that they will have a better chance of seeing aircraft, they will have much stronger antiaircraft fire support, and it will be easier to provide fighter protection for them. Admiralty reacted immediately to this measure by sending surface craft hunting groups into the Bay to cooperate with aircraft and to follow up aircraft attacks. If the surface craft arrive at the scene of the attack quickly enough, several U-boats will be pinned down and the search should be easier. This countermeasure resulted in two of the U-boat kills made by the Second Escort Group in June. In addition, Mosquito fighters were sent into the Bay in June to operate against German aircraft covering the U-boats.
Aircraft finally came into their own as an offensive power during this period. The 25-foot depth setting was in general use and, especially during the later months, planes were able to attack U-boats still on the surface. About 60 aircraft attacks were made monthly on U-boats during this period, with about 25 per cent of the attacks resulting in at least some damage to the U-boat, while over 10 per cent of the attacks proved lethal. In addition aircraft were used to bomb U-boat bases and lay mines near them. These operations dislocated the servicing facilities at the bases and increased the turn-around time of U-boats operating from these bases.
5.2.3 Scientific and Technical
The main scientific battle during this period continued to center about radar. As we have seen, the Metox R-600 GSR was able to detect Allied meter-wave radar. The Allied countermeasure to GSR was the introduction of short-wave (S-band 10-cm) radar about February 1943. This was made possible by the British invention of the strapped magnetron in the spring of 1940 which enabled sufficient power to be produced to make use of the shorter wavelengths practical. The Metox GSR was particularly unsuited to detecting these short wavelength radar transmissions and the number of aircraft attacks on U-boats increased sharply. The Germans had no idea of what
was causing the trouble and, suspecting supersonic modulation, they fitted GSR sets with a visual tuning indicator of the Magic Eye type. As this did not prove to be of any help they explored other blind alleys, suspecting infrared detection or intermittent operation of radar sets, without finding any solution during this period. The best the Germans could do was the fitting of a permanent GSR aerial, which did not have to be dismounted before diving, on some U-boats. Meanwhile it was evident in May and June 1943 that there was a progressive lessening of confidence in GSR on German U-boats. Radar was fitted on some U-boats, apparently primarily as an offensive weapon against shipping in low visibility. However, there was a great reluctance to use radar for fear of being detected by a hypothetical Allied search receiver. Radar decoy balloons were also used by some U-boats in an attempt to produce a large number of false targets.
A similar lack of appreciation of the situation was apparent in the U-boat's attitude toward HF/DF. As the new technique of using shipborne HF/DF was learned and applied, the successes became more frequent, and by November 1942 HF/DF was accepted as an essential part of the equipment of escort craft. The U-boats overrated the accuracy of shore-based HF/DF, but for a long time they underrated, indeed ignored, the danger of shipborne HF/DF. This was reflected in their communications, which seemed to be conducted on the principle that radio silence was to be strictly kept until contact was made with the convoy, but completely relaxed once contact was made. In other words, they were afraid of revealing their dispositions but saw no danger from the transmissions of individual U-boats once the battle was joined. During the attacks on Convoy SC 118 in February 1943, for example, the U-boats concerned made 108 transmissions during a period of 72 hours.
During this period several ships were saved by AND, the torpedoes either being stopped by the nets or exploding in them. There were six successful Hedgehog attacks during this period and it appeared that the numerous early difficulties with this weapon had been overcome and it was beginning to show some signs of its theoretical lethality.
It had become apparent in 1942 that some device was needed to enable attacking ships to maintain contact with deep U-boats to shorter distances. To meet this need the Q attachment was developed. It consisted of a small special sonar projector of high frequency, attached to the main projector below it and tilted down about 15°. Trials during this period indicated that good echoes could be obtained from targets within an angle of 45° from the horizontal.
The expendable radio sono-buoy was developed during this period to enable aircraft to maintain contact with a submerged U-boat. It could be dropped from an aircraft and contained a hydrophone which would listen to U-boat noises and a radio transmitter which would transmit the noises received to the plane. The service life of a sono-buoy was about four hours.
5.2.4 Sinkings of U-boats
The number of U-boats sunk or probably sunk during this critical period averaged about 19 a month, hitting a peak of 44 in May 1943. The total number sunk during this period was 168, more than twice the number sunk in any previous period. Of these, 128 were German, 18 Italian, 18 Japanese, and four were Vichy French.
There were 112 U-boats sunk in the Atlantic during this period, 43 of these in the Northwest Atlantic Area and 25 in the Bay of Biscay. There were 37 U-boats sunk in the Mediterranean, 15 during November 1942, the month of the North African landings. The 18 Japanese U-boats were all sunk in the Pacific.
This was the first period of the war in which aircraft were the leading killers of U-boats, sinking 76 U-boats (45 per cent of the total) alone and another 10 (6 per cent) in cooperation with surface craft. Ten of these 86 successful aircraft attacks involved carrier-based aircraft. Ships accounted for 59 U-boats (35 per cent) and submarines for 17 (10 per cent).
The quality of surface craft attacks continued to improve. About 60 attacks were made monthly on U-boats in the Atlantic and Mediterranean and about 25 per cent of these surface craft attacks resulted in at least some damage to the U-boats, while 10 per cent of these attacks resulted in the sinking of the U-boat.
5.3 SURVEY OF RESULTS
5.3.1 From the U-boat's Point of View
This was the crucial period of the war for the U-boats, the one in which they made their supreme effort and were decisively defeated. The world-wide shipping losses to U-boats were about 30 per cent
lower than during the preceding period, as about 67 ships of 394,000 gross tons were sunk monthly by U-boats, with about 85 per cent of these losses occurring in the Atlantic. The number of U-boats sunk throughout the world was about 19 a month, more than twice as many as during the previous period. The world-wide exchange rate was, therefore, only about 3½ ships of 21,000 gross tons sunk by the average U-boat before it, itself, was sunk. This exchange rate was only one-third as large as it had been during the previous period.
Activity in the Mediterranean was greatly increased as 44 ships were sunk by U-boats and 37 U-boats were sunk. U-boat operations in the Mediterranean were, in many cases, auxiliary to the military operations in North Africa. In the Indian Ocean, German and Japanese U-boats sank 25 ships, half the total in the previous period, without suffering any losses. Japanese U-boats sank 19 ships in the Pacific and suffered the loss of 18 U-boats, an extremely unprofitable return.
In the Atlantic, the scene of the main battle, the average number of U-boats at sea during this period was 104, higher than in any other period of the war. These U-boats sank about 57 ships of 344,000 gross tons monthly, about one-third of these sinkings occurring in the Northwest Atlantic Area. This meant that the efficiency of U-boats reached a new low for the war as the average U-boat sank only about one-half ship of 3200 gross tons per month at sea, only about one-third the results achieved in the previous period. This indicated that the U-boats' campaign of wolf-pack attacks against the North Atlantic convoys had failed to maintain the average U-boat effectiveness at its previous high level. The U-boats were very rarely able to completely overwhelm the escorts of the convoys and consequently the use of large wolf packs of over 20 U-boats proved relatively inefficient. In addition, when the U-boats had to operate against convoys which were given air coverage, their wolf-pack operations broke down completely as they depended upon surfaced U-boats to keep contact with the convoy and to home the other U-boats.
In addition to the reduction in its efficiency, the average U-boat in the Atlantic found life more hazardous during this period than in the preceding period. This marked the first time in the war that the trend of increasing safety for the U-boat was reversed. About 121/2 U-boats were sunk monthly of the 104 U-boats at sea in the Atlantic, so that the average life of a U-boat at sea in the Atlantic during this period was about 82 months, about one-third less than in the previous period. The main factors causing this shorter average life for U-boats were the increased effectiveness of Allied aircraft and the fact that the U-boats were exposing themselves to increased attacks by surface escorts by concentrating against convoyed shipping.
These figures indicate that the average U-boat in the Atlantic during this period was sinking only about 41/2 ships of 28,000 gross tons before it itself was sunk. This exchange rate was the lowest of the war to that date and only one-fourth as high as the record exchange rate achieved in the previous period. This low exchange rate plus the fact that, in the Atlantic, the number of U-boats sunk in May 1943 was almost as large as the number of ships sunk by U-boats, must have convinced the Germans that they could not continue the battle against the North Atlantic convoys and they disengaged. The average number of U-boats at sea in the Northwest Atlantic Area dropped from 60 in May to 20 in June 1943.
The expansion of the German U-boat fleet was definitely slowed down during this period. Bombing of construction yards had kept new construction at about the same level as in the previous period, with about 20 new U-boats being commissioned monthly. About 128 German U-boats are considered to have been sunk during this period and a number of others decommissioned. Consequently, the available number of German U-boats increased from about 350 at the beginning of this period to about 400 at the end of the period. This huge fleet still constituted a considerable potential threat, but many experienced officers and crews had been lost during this period, and the heavy losses suffered by the U-boats must have had an adverse effect on enemy morale.
5.3.2 From the Allies' Point of View
Total shipping losses of the Allied and neutral nations were about 491,000 gross tons a month during this period, about 30 per cent less than during the preceding period. Construction of new merchant shipping reached a new high of about 1,026,000 gross tons a month, more than twice the total monthly losses. Consequently, the net monthly gain of shipping was about 535,000 gross tons. This was the first period of the war in which new construction of shipping
exceeded the total losses. The total shipping available increased by almost 5,000,000 gross tons during this period to a level of about 36,500,000 gross tons. It appeared that the shipping crisis in the war had definitely passed, as it seemed extremely unlikely that the shipping losses would ever exceed the million gross tons being constructed monthly. The main problem facing the Allies at the end of this period was to keep the shipping losses down to a minimum so that supplies could be built up quickly in England for the invasion of the continent.
Of the 491,000 gross tons of shipping lost monthly, about 436,000 gross tons were lost as a result of enemy action. U-boats accounted for 394,000 gross tons a month, about 90 per cent of the total lost by enemy action. Losses to enemy aircraft, surface craft, and mines were all considerably lower during this period than in the preceding one.
The number of ships suitable for ocean escort, available to the Allies, had increased from about 745 at the beginning of this period to about 950 at the end of June 1943. This included about 50 new destroyer escorts [DE] as well as about 25 new escort carriers [CVE]. This increased number of escorts and the new VLR aircraft that had become available placed the Allies, at the end of this period, in the position of having defeated the greatest efforts the U-boats could make. The Allies now had the opportunity of taking the offensive against the U-boats to prevent them from ever regaining the initiative.
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