NIGHT SURFACED ATTACKS ON CONVOYS
JULY 1940-MARCH 1941
2.1 U-BOAT OFFENSIVE
THE SECOND PHASE of the U-boat war was marked by a complete change in enemy tactics. The Germans, having discovered as a result of their high rate of loss that the U-boats were quite vulnerable to Asdic when submerged, decided to make use of the hours of darkness to regain their relative invisibility. At night, trimmed down on the surface, a U-boat offers a very small target to the human eye and is also rather difficult to detect by Asdic. The surfaced U-boat has the advantage of high speed and maneuverability and therefore has good chances of avoiding detection by the escorts. Acting on this principle and encouraged by the results achieved at night during the first period by a few of the more successful U-boat captains, the U-boats started, in July 1940, the general practice of attacking convoys at night from a surfaced position and then using their high surface speed to escape. Occasional daylight attacks were still made on ships sailing independently, and on stragglers from convoys.
Accompanying this change in enemy's tactics came the occupation of the French ports and their establishment as U-boat bases, marked by the first visit of a U-boat to Lorient on July 22. The use of French bases served to cut down the transit time of the U-boats and enabled them to extend their area of operations further westward in the Atlantic. From his air bases in France, the enemy was also able to send out long-range reconnaissance aircraft to pick up convoys in the Atlantic.
In addition, after the fall of France there developed the threat of a sea-borne invasion of England. This confined a large number of destroyers to the East and South coasts of England and consequently, at the number of ships available for convoy escort was necessarily limited, the U-boats were encouraged to attack convoys more frequently. Aircraft were also diverted from antisubmarine patrols over the Western Approaches to England, where the U-boats were operating, to anti-invasion patrols to the eastward.
Some Italian U-boats had also started operation in the Atlantic in August 1940. These Italian U-boats used Bordeaux as a base and followed the same methods as German U-boats, presumably working directly under German orders. Their operational areas were usually southward of the ones used by the German U-boats.
Increased U-boat activity, which had commenced in June 1940, continued throughout July and August with over 200,000 gross tons of shipping being sunk in each of those months. Up to the middle of July, the most active area was still the Western Approaches between the latitudes of 48° north and 51° north. After the threat of air attack from French bases had led to the rerouting of British convoys around the north of England, the U-boats lost no time in shifting their area of activity to the Northwestern Approaches to meet the increased traffic there. This activity was marked by increased attacks on convoys while antisubmarine escorts were actually present, but these attacks were generally on large convoys which, owing to the shortage of escort vessels, were guarded by only about two Asdic-fitted ships.
On August 15 Germany proclaimed a complete blockade of the British Isles and called upon neutral governments to forbid their ships to sail through the Anglo-German war zone. U-boat activity was considerably intensified after that date and the shipping losses continued to increase, with about 300,000 gross tons being sunk by U-boats in September, and a new high for the war was reached in October when 62 ships of 346,000 gross tons were sunk by U-boats. The scene of greatest activity during these months was still the Northwestern Approaches, with night attacks on convoys being the most favored method of attack by the U-boat. Of the 59 ships attacked in this area in September, 40 were in convoy; 71 per cent of the total were night attacks. The concentration of aggressive operations into the period of, and immediately following, the full moon was especially notable during October when 31 ships were attacked on October 18 and 19.
It should be kept in mind that during these months of heavy losses the average number of U-boats at sea was still only about six. This means that ten ships of about 60,000 gross tons were sunk by the average U-boat at sea during October 1940, probably an all-time high in operating efficiency for submarines. In addition to inflicting these heavy losses, the U-boats were almost invariably escaping unscathed, as, for example, in October when only one U-boat was sunk in the Atlantic. These were the days when the star German commanders (U-boat aces) such as Prien and Kretschmer were operating. These aces had survived the hazards of operating during the first period and had profited from the experience gained then. The U-boats making these night attacks on convoys were operating individually and usually only one or two U-boats would be involved in the attack. Despite this, some of the convoys suffered rather heavy losses, as, for example, HX 79, which lost 12 ships to two U-boats in one night in October.
The normal procedure for U-boats attacking convoys at this time seems to have been as follows: The U-boat gained contact with the convoy during the day, either as a result of reports from long-range German reconnaissance aircraft, reports from other U-boats, or by sighting smoke, and their proceeded to shadow the convoy at visibility distance on the bow or beam. When darkness had fallen, the U-boat, trimmed down on the surface, closed the convoy, and endeavored to reach a position broad on its bow. She kept very careful watch for the escorts and endeavored to pass astern of those stationed on the bow of the convoy. The approach was pressed home as close as the U-boat captain dared, and it is possible that, in some cases, a firing range of about 600 yards was achieved. Having reached a firing position on the beam of the convoy, most U-boats increased to full speed, fired a salvo of four torpedoes, turned away still at full speed, firing stern tubes if possible, and retired as rapidly as possible on the surface in the direction considered safest. If their retreat was unseen, the\, might reload their torpedo tubes on the surface and attack again in the same manner later in the night.
The serious damage inflicted on British convoys by these new U-boat tactics caused a considerable number of changes to be made in the convoys. The spacing of the convoy columns was opened up to reduce the chance of more than one ship's being hit by a salvo. Escorts were stationed farther away from the convoy and new plans were developed for searching for the U-boat with illuminants following attack. To improve the tactical efficiency of the escorts, these ships were formed into groups and as far as possible ships of one group were to work together. Admiralty took over the responsibility for the routing of all ocean-going convoys, thus enabling emergency changes to be made without delay. In addition, great efforts were made to equip all convoy escorts with radar, which would enable them to locate U-boats on the surface at night outside visibility distance and possibly before they could attack the convoy.
By November 1940 Lorient had become the principal U-boat base and during this month one German U-boat had left this port and gone as far south as Freetown, sinking four ships in three days. November was also marked by several heavy air attacks on the ports of Lorient and Bordeaux, which were considered to have inflicted severe damage on both U-boats and their bases.
The first known successful counterattack against the German method of night attack on convoys occurred on November 21 after two ships of convoy OB 244 had been torpedoed. The British corvette HMS Rhododendron, stationed astern as a rescue ship, sighted an object momentarily at a range of 1500 yards about an hour after the torpedoing. Three minutes later Asdic contact was gained and then depth charges were dropped with the result that considerable metallic wreckage and oil were blown to the surface.
This successful counterattack, plus the loss of two other U-boats, might account in part for the reduced number of attacks on escorted convoys in November and December 1940. Heavy winter weather in the North Atlantic was probably also a factor in accounting for the decrease in shipping losses to U-boats, as only 150,000 gross tons were sunk in November and 200,000 gross tons in December.
Early in December a westerly movement of the U-boats became noticeable, with most of them stationed as far out as 20° west longitude. This may have been due in part to Coastal Command flying and in part to an attempt to intercept incoming convoys before the antisubmarine escort joined. However, this actually increased the enemy's difficulty in locating convoy traffic.
As a counter to the fact that U-boats in the Northwestern Approaches had been attacking British convoys in longitudes 20° to 25° west, which is an area
beyond the point which could be leached by the escorts, new evasive routing measures were adopted in December. It was decided to make use of dispersion to the maximum extent that the endurance of merchant ships permitted, and the routes of the convoys were spread between 63½° and 57° north latitude. The cycles of convoys were also opened out, with the object of reducing the strain on escorting forces.
This thorough diversion of convoy routes scents to have been the main factor in the reduction of shipping losses, just as it had been in World War 1. No attacks were made on escorted convoys from December 19-10 until January 29, 1941, and the shipping losses to U-boats in January dropped to 21 ships of 127,000 gross tons, the lowest figure since the Germans announced their intensified U-boat campaign in May 1940. This occurred despite the fact that the average number of U-boats at sea in the Atlantic had increased to about 12. Most of the ships lost during these two months were not in convoy, since the U-boats had difficulty finding convoys and resorted to the much easier task of picking off stragglers or ships sailing independently.
The month of February 1941 opened with a continuation of the comparative lull in U-boat activity. Evasive routing had frustrated the normal German "hit and run" method of night attack on the convoys. However, it became clear in February that this had provoked intensified enemy offensive measures, in the form of greater cooperation between aircraft and U-boats, and special searching patrols. The days of wolf-pack attacks were foreshadowed as the U-boats started operating in groups of three to five, each U-boat being given a limited patrol area within the wider area covered by the group. The first U-boat to gain contact shadowed the convoy while others were ordered to concentrate in a position to attack. The shadower usually emitted radio signals to home other U-boats or aircraft to the attack. Similarly, aircraft were able to home U-boats to a convoy.
Cooperation between U-boats, aircraft, and surface craft is well illustrated by the attack on HG 53, consisting of 21 ships escorted by one sloop and one destroyer. The convoy was attacked by a U-boat at 0435 on February 9, two ships being sunk. The U-boat continued to shadow the convoy and probably homed six Focke-Wulf aircraft to it during the afternoon of the 9th. Five ships were bombed and sunk while one plane was shot down. The U-boat continued to shadow the convoy and again attacked on the 10th, sinking one ship. After this, she maintained touch with the convoy, reporting its position. Her reports were evidently intended for a German "Hipper" class cruiser. While closing HG 53, this cruiser came upon the unescorted slow portion of SL 64 and directed her attack against this easy target, sinking seven ships.
Three other convoys were attacked by U-boats in the last week of February, and as the month drew to an end, with the losses mounting to 36 ships of 189,000 gross tons, it was evident that the expected spring offensive of the U-boats had commenced. The average number of U-boats at sea in the Atlantic rose to 16 in March and these included some of Germany's most skillful U-boat captains. Their tactics included a repetition of the concentrated night attacks upon convoys, and six convoys were attacked during the month. The upward trend of shipping lost by U-boat action reported in February was maintained during March with the total losses reaching 10 ships of 239,000 gross tons. These losses were considerably less than those recorded during September and October 1940, the last previous period of intense U-boat activity, and were not considered unduly alarming considering the fact that the number of U-boats at sea in March 1941 was more than twice as great as in the earlier period.
More encouraging was the evidence of the increased efficiency of antisubmarine escorts and of the fact that U-boats which attacked adequately escorted convoys could be dealt with effectively. This evidence was clearly demonstrated by the loss to Germany, during March, of her three outstanding U-boat aces (Prien, Kretschmer, and Schepke), the top three U-boat captains in terms of tonnage sunk, each having more than 200,000 gross tons of shipping to his credit.
Prien, commander of U-47, was the first to be lost as a result of his attack on Convoy OB 293 when he sank one ship shortly after midnight on March 8, 1941. HMS Wolverine, one of the escorts, sighted smoke about 20 minutes alter the attack on the convoy and subsequently made contact with the U-boat. The U-boat was attacked for over five hours, during which time there occurred a remarkable chase of the U-boat on the surface for over an hour, before it was finally considered sunk. There were no survivors but
prisoners of war from U-boats sunk subsequently have supplied information from which it is believed that the Wolverine's attacks were made on U-47, commanded by Prien. Berlin subsequently admitted the loss of Prien. U-100 (Schepke) and U-99 (Kretschmer) were both sunk as a result of the attack on Convoy HX 112, during which five ships were sunk. U-100 located the convoy, which had been reported earlier by U-99, on the evening of March 16. Later she sighted a destroyer astern and dived. At 0137 on the 17th, HMS Walker obtained Asdic contact and attacked with depth charges. Three further depth-charge attacks were carried out by HMS Vanoc and HMS Walker before contact was lost at 0250. Meanwhile U-100, which had been considerably damaged by the depth charging, had surfaced. While the escorts were preparing to take station for an organized search, Vanoc's radar operator reported a contact 1000 yards away and U-100 was subsequently sighted on the surface and rammed by HMS Vanoc. Schepke, who was caught and crushed between the stove-in side of the bridge and the periscope, was carried down with the sinking U-boat.
While HMS Vanoc was picking up survivors of U-100, HMS Walker obtained Asdic contact. Although it was considered unlikely that another U-boat would remain so close, the Asdic operator was so convinced that he had a submarine contact that HMS Walker fired six depth charges. This attack was extremely accurate and brought U-99 to the surface almost at once. Both destroyers opened fire and U-99 was abandoned shortly afterward, with Kretschmer and other survivors taken as prisoners. Kretschmer had been quite successful up to that point as he claimed a record total of 86,000 tons of shipping sunk on this last cruise, while his total sinkings had reached 338,000 gross tons, more than any other U-boat captain.
There were immediate indications that the enemy was severely shaken by the results of his attacks on adequately escorted convoys. The only subsequent attack during the month was made far west before the antisubmarine escort had joined, when three ships were sunk from HX 115 at 22° west longitude. Beside the effect on the tactics and operations of U-boats, the loss of three of Germany's most skillful U-boat commanders must have had a profound effect on U-boat morale.
2.2 COUNTERMEASURES TO THE U-BOAT
The high percentage of hits obtained by U-boats in night attacks made it necessary in November 1940 to increase the distance apart of convoy columns from about 600 yards to about 1000 yards. The distance between ships in the same column was about 400 yards. In December 1940 the distance between columns during the daytime was reduced back to 600 yards to increase protection against air attacks.
To counter the heavy losses suffered as a result of the night attacks by surfaced U-boats in September and October 1940 the escorts were stationed in positions down each wing at a greater distance from the convoy than before. In the event of an attack, they were instructed to proceed outward from the convoy for a distance of 10 miles at full speed, firing star shell to illuminate the area where the U-boat might be, in an attempt to sight her or force her to submerge, thereby improving the chances of Asdic detection. If contact was made, two escorts were to hunt the U-boat, while the remainder were to rejoin the convoy.
Later, when radar-equipped escorts became available, they were stationed one on each beam of the convoy, about 4 miles from it in order to avoid back echoes from the convoy on the radar set. The beam escorts were to steam on the same and opposite courses as the convoy, zigzagging as requisite for self-protection. Another method of sweeping, which was under trial in order to effect an economy in fuel, was for the escort to start a slow turn of 360° when in a position abeam of the leading ships, thus sweeping outwards and astern at about 1° per second, and on completion assuming station abeam of the rear ships. The remainder of the escorts were disposed as before, but were instructed to bear in mind that, at night, the rear wing positions were the most important and that, in the event of a U-boat attack, star shell searches were to be made in the rear of the convoy also.
By the beginning of this period, in July 1940, the convoy system was fully established and most of the subsequent changes were made necessary by enemy activity. This may again be illustrated by continuing the history of the transatlantic HX convoys, the main line of supply to England. Besides the serious U-boat
threat, enemy air and surface craft activity also had their effects on these convoys.The first HX convoy to be routed around the north of England in July 1940 made a rendezvous with the local antisubmarine escort at about 17° west longitude. Slow convoys to include ships between 7½ and 9 knots were organized in August to assemble at Sydney, Nova Scotia, and were designated SC. This reduced the HX convoys to a reasonable size of about 45 ships. The HX convoys sailed on a 4-day cycle, while the SC convoys were on an 8-day cycle. In December, Sydney was abandoned as a convoy assembly port, but the SC convoys were to continue to sail from Halifax.
In order to extend the antisubmarine escort further west, the convoy intervals were lengthened, with HX convoys sailing at alternate 6- and 4-day intervals, while the SC convoys sailed at 10-day intervals. In addition, Loch Ewe, in the northern part of Scotland, was started as an assembly port for ships on the east coast of England, and destroyers serving as antisubmarine escorts were able to refuel there and operate further west.
In February 1941, two HX convoys were routed on a southern course but heavy air attacks resulted in these convoys being rerouted to the north. Following an attack by a surface raider and the sighting of two German battle cruisers in the Atlantic, it was decided to give close battleship cover to all Halifax convoys. This threat of surface raiders also led to the discontinuance of the Bermuda section of HX convoys and ships were routed independently to the Halifax assembly.
British convoys were much harder hit during this second period than in the previous period. The number of ships convoyed monthly increased to about 3600, with 26 of these being sunk monthly by U-boats (13 in escorted convoys, 3 in unescorted convoys, and 10 stragglers). This meant that the total loss rate to U-boats was about 0.7 per cent, more than three times as high as in the earlier period.
Moreover, the HX and SC convoys sailing across the Atlantic to England were much harder hit than other convoys. Of the 360 ships sailing monthly in these convoys (only 10 per cent of the total convoyed shipping), about 14 ships were sunk monthly by U-boats (over 50 per cent of the total losses of convoyed shipping). The loss rate to U-boats on these vital convoys was about 4 per cent, more than five times as high as for all convoys.
Despite these high losses in convoys, the chances of being sunk by a U-boat were still higher for independently routed ships. Comparable figures are available for shipping passing through the Northwestern Approaches during the 6-month period from September 1940 through February 1941. About 80 per cent of the total losses to U-boats occurred in that area. Of the 1180 ships sailing through this area monthly in convoy, about 29 were sunk monthly by U-boats for a loss rate of about 2/2 per cent. Of the 70 independently routed ships sailing through this area monthly, about three were sunk monthly by U-boats for a loss rate of about 4 per cent. In making this comparison, one should keep in mind that the ships sailing independently were generally capable of making a speed of at least 13 knots, much higher than the average speed which ships sailing in convoys were capable of making. This means that if all shipping had sailed independently the loss rate would probably have been much higher than the 4 per cent experienced by the select group of ships that sailed independently.
During the second period, increased use was being made of aircraft as a counter to U-boats. Though suffering from many limitations for antisubmarine operations, the airplane possesses obvious advantages denied to surface craft (e.g., speed, cheapness, large field of vision, and economy of personnel and matériel). A U-boat on the surface can rely on the fact that she will almost certainly sight an enemy ship before she, herself, is seen. However, she must always keep a vigilant lookout against being surprised by a plane sweeping down out of the clouds.
In an attempt to improve the lethality of aircraft attacks, Coastal Command tried using naval depth charges modified for air use. Sunderland aircraft started carrying both depth charges and bombs in July 1940. The first success of this new weapon came on August 16 when a U-boat was severely damaged as a result of a depth-charge attack. The Sunderland plane, carrying two depth charges and four 250 pound antisubmarine bombs, first dropped a single depth charge set to explode at a depth of 100 feet about 20 yards ahead of the conning tower of the submerging U-boat. The U-boat was forced to the surface and two minutes later the second depth charge, set for 150 feet, was dropped about 20 feet
ahead of the conning tower. The U-boat was again blown to the surface and was then observed to sink sideways. On the third attack the stick of four bombs was dropped on the submerged U-boat. Air and oil came to the surface. In view of the initial successes of depth charges, steps were immediately taken to modify other Coastal Command aircraft in order to enable them also to carry depth charges. It was expected that the lethal value of aircraft attacks on U-boats would be considerably increased by this change.
The night attacks on convoys led, in September 1940, to the fitting of radar to the aircraft of Coastal Command and the Fleet Air Arm. This was supposed to be especially valuable for detecting U-boats on the surface at night and it was hoped that this would make it possible to operate aircraft at night for convoy escort work. It was also intended to provide the maximum air escort for the three hours before darkness falls, as this is the period in which U-boats could be found in shadowing positions preparatory to the night attack.
After the evasive routing of shipping had led to the start of wolf-pack tactics in February 1941, the shadowing U-boat became the main problem. Having contacted a convoy, the U-boat took great care not to reveal her presence by attacking in daylight, but shadowed the convoy at some distance. There was, therefore, only a small chance of the limited number of escorts discovering these U-boats and this task fell to the escorting aircraft. In view of this it was decided to reinforce the number of Coastal Command aircraft available for escort duty in the Northwestern Approaches. Consideration was also given to the problem of evolving the best type of aircraft patrol, round the convoy, to prevent the U-boat from shadowing it.
Despite the curtailment of routine antisubmarine patrols in favor of anti-invasion patrols during this period, the average number of hours flown monthly by Coastal Command aircraft on antisubmarine duties increased by about 1000 hours over the previous period to reach 6300 hours, 5100 hours on convoy escort and 1200 hours on patrol. The number of flying hours on antisubmarine work dropped to about 4000 during the winter months of December 1940 and January 1941, due to longer hours of darkness and poorer weather. By March 1941 it was again up to about 8000 hours. This increased amount of flying was less productive than during the first period, as the number of sightings made monthly dropped to about 14 and the attacks to about 8. This decrease in the number of sightings, despite the increased number of U-boats at sea, was due mainly to the movement of the U-boats further westward, out of range of much of the flying. Again, about 10 per cent of the attacks resulted in some damage to the U-boat but the lethality of the attacks improved, as two (about 22 per cent) of the attacks resulted in the probable sinking of a U-boat.
2.2.3 Scientific and Technical
Considerable research was done during this period on improving Asdic sets with one of the chief goals being tie development of practical depth-determining gear. Very little progress was made on this difficult problem and the only immediate solution was the use of larger depth-charge patterns to counterbalance the large effect of the unknown factor of depth.
Another scientific development during this period was the extensive use of high frequency - direction finding [HF/DF] towards the end of 1940, after Germany had acquired the French bases and the U-boats had started widespread operations in the Atlantic. The principle of HF/DF was that a shore station could determine the bearing of any U-boat making a radio transmission, and it was hoped that the point of interception of the bearings from several shore stations would determine the transmitting U-boat's position. However, as more HF/DF shore stations became available around the Atlantic shores and as U-boats started to operate in numbers on the Atlantic trade routes, it became clear that shore-based HF/DF could only provide a rough indication of the general area in which the U-boat was and, at best, it could only provide a warning for a threatened convoy and so assist convoy routing. The Germans appreciated this and felt that shipborne direction-finding was restricted to medium frequencies. They therefore used high-frequency communications extensively once contact had been made with the convoy. As a result it was realized that HF/DF on convoy escorts themselves might do a great deal more; it might even enable the escorts to find U-boats before they could launch their attacks. The immediate requirement was an HF/DF outfit for ships which was quick and easy to operate.
However, the main scientific achievement during this period was the introduction of radar sets on
both ships and aircraft. Radar worked on the principle of transmitting short pulses of very high-frequency radio waves and then receiving the echoes from objects, like a U-boat on the surface. The echoes would enable the range and bearing of the object to be determined even at night and in conditions of poor visibility.
We have seen that the heavy shipping losses suffered at the start of this period as a result of night attacks on convoys had made radar an urgent necessity. As a stop-gap, the first radar sets fitted in destroyers were of a Royal Air Force design known as air-surface vessel [ASV] or in the British Navy as radio direction-finding [RDF] Type 286 M. The fitting of these sets on ships was started about November 1940 and by April 1941 radar had been fitted on about 40 destroyers of the Western Approaches Command. It was hoped that radar would enable the escorts to detect the presence of any U-boat on the surface within a radius of some two or three miles.
Type 286 M had a fixed aerial and received echoes from a target over an arc covering about 50 degrees on each side of the bow and also over a similar arc astern at considerably shorter ranges (back echoes). The wavelength of these early sets was relatively long, over a meter, and consequently the aerial had to be very high above the surface of the sea before any considerable range could be obtained on small objects. This limited the effectiveness against U-boats of early radar sets on ships, but not on aircraft, as a plane flying at 2500 feet could expect to detect a U-boat on the surface at a range of about 15 miles.
Radar could be used in antisubmarine warfare for several subsidiary purposes, besides the main one of detecting U-boats. It could give warning of the approach of aircraft; it could be used in low visibility to make contact with single merchant ships or convoys; to pick up navigation buoys; to keep station on a convoy at night; or for making landfall.
By January 1941 it appeared that, as an antisubmarine device, radar on surface ships had been a disappointment. Escorts had considerable trouble owing to confusing "back echoes" from the convoy. As a temporary measure it was hoped to alleviate this trouble by reducing the range scale from ten miles to five miles. Work was also being done on a newly designed aerial, screened to cut out back echoes. At the end of this period, in March 1941, new types using shorter wavelengths and directional aerials were tinder trial and an improved radar set of naval design, Type 290, was in production and was to replace Type 286.
2.2.4 Sinking of U-boats
Surface craft continued to be the most effective craft in attacking and sinking U-boats during this period, making about 25 attacks a month. Of the 23 U-boats sunk or probably sunk in the Atlantic, surface craft could be credited with 13, or about 53 per cent. Submarines proved highly effective early in this period, patrolling close to the French bases, and torpedoing five U-boats in September 1940 and another in December. These submarine attacks made it necessary for the U-boats to enter and leave their bases submerged. Two U-boats were probably sunk as a result of aircraft attacks, one was sunk as a result of a combined attack by a ship and plane, and another was mined.
In addition, one German U-boat was known to have been sunk under unknown circumstances while 11 Italian U-boats were sunk in the Mediterranean, with surface craft again accounting for six, or 55 per cent of them. Italian U-boats operating outside the Atlantic had very little success against Allied shipping, as only five ships of 28,000 gross tons were sunk in the Mediterranean and Indian Oceans during the first two years of the war, from September 1939 to September 1941.
2.3 SURVEY OF RESULTS
2.3.1 From the U-boat's Point of View
The new U-boat tactics adopted during this second period had accomplished their primary objective of reducing the high rate of loss of U-boats. The average number of U-boats at sea in the Atlantic during this period rose to about 10, while only about 2½ of these were lost monthly. The average life of a U-boat at sea had increased by about 33 per cent, from 3 months to 4 months.
In addition, the efficiency of U-boats in sinking ships increased slightly, as the average U-boat sank four ships of about 22,000 gross tons per month at sea. The combined effect of these two factors improved the overall exchange rate to 16 ships of about 88,000 gross tons sunk for each U-boat sunk or probably sunk, an extremely profitable transaction for the U-boats.
From a quantitative point of view, the position of the German U-boats had improved considerably during this period as the increased U-boat building program, which the Germans had started shortly after England entered the war, began to take effect. As a result of commissioning about 45 new ocean-going U-boats while losing only about 18 of the large ones (500 tons or larger) Germany had about 51 ocean-going U-boats available at the end of this period, about twice as many as it had at the start of the period.
Thus the Germans would be able to send many more U-boats out to sea during the third period than they had sent during the early periods of the war. However, they had lost many of their ablest and most experienced captains and crews, and these were not as easily replaced as the U-boats themselves. The necessity of sending out relatively inexperienced U-boat captains was probably a factor influencing the Germans to decide, in February 1941, to operate their U-boats in groups, so that several inexperienced captains could operate together with a more experienced one.
2.3.2 From the Allies' Point of View
Total shipping losses of the Allied and neutral nations were about 456,000 gross tons a month during the second period, more than 60 per cent higher than during the first period. Meanwhile the building rate had increased only slightly to about 114,000 gross tons a month, making the net loss of shipping about 342,000 gross tons a month. Total shipping available had decreased from about 38,000,000 gross tons at the start of the second period to about 35,000,000 gross tons at the end of the second period.
Of the 456,000 gross tons of shipping lost monthly, about 404,000 gross tons were lost by enemy action. U-boats accounted for 42 ships of 224,000 gross tons a month (55 per cent of the total tonnage lost by enemy action), more than twice the monthly tonnage sunk by U-boats during the first period. Monthly shipping losses due to enemy surface craft jumped to 87,000 gross tons (22 per cent) and those due to enemy aircraft increased to 61,000 gross tons (15 per cent). Monthly losses due to mines dropped from second place in the first period to only 27,000 gross tons (7 per cent), with other and unknown causes accounting for the other 1 per cent of the total losses due to enemy action.
There is no doubt that the U-boats had inflicted a serious defeat on the Allies in the Battle of the Atlantic during the second period, but the situation was beginning to look more promising toward the end of this period. One favorable element was the increasing number of antisubmarine ships and aircraft becoming available for convoy escorts as the threat of the invasion of England was decreasing. The number of antisubmarine ships suitable for ocean escort (i.e., destroyers and patrol craft such as sloops, frigates, corvettes) had increased from about 235 at the start of this period to about 375 (includes 240 destroyers) at the end of the period. Important factors in this increase were the coming into service of the new corvette and also the transfer of the 50 old Town class destroyers from the United States to England in September 1940. These destroyers were equipped with U. S. echo-ranging gear, called sonar, which was similar in principle to the British Asdic.
In addition, an increasing number of ships and planes were being equipped with radar in order to combat the U-boat's night activity. Officers and crews had increased experience and training in antisubmarine warfare and had shown in March 1941 that they could inflict heavy losses on U-boats attacking adequately escorted convoys. It appeared as if the main problem during the third period would be that of meeting the westward movement of the U-boats by extending antisubmarine escort westward, without weakening the escort strength.
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