Antisubmarine warfare in World War II

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Preface

T his volume on antisubmarine warfare [ASW] represents a compromise between two major aims, to produce a unified summary of the events and problems of the antisubmarine war on the one hand, and to illustrate the scientific evaluation of naval operations on the other. The approach is fundamentally historical on both accounts, however, since the illustrations of scientific evaluation are taken from various analyses and studies made in connection with antisubmarine warfare during World War II. Great care should therefore be exercised in making predictions concerning the future of ASW from it. There is no guarantee that the antisubmarine measures successful in the past will continue to be adequate in the future.

A clear understanding of the events of World War II, their reasons and consequences, is necessary, however, as background for any decisions which are to be made in the postwar period. It is hoped that this volume may serve to some extent as a convenient reference and source of factual material. One overall conclusion is clearly evident from it: the introduction of new weapons, gear, and tactics has led to a continual interplay of measures and countermeasures in which no other conclusion retains its validity for very long. If this lesson alone is learned from it, the volume will have served a useful purpose.

The general organization corresponds closely to the dual aim described above, with two parts quite different in character. Part I is a historical summary of the progress of enemy submarine operations and Allied antisubmarine operations during World War II. No attempt has been made, however, to give a complete chronology of all events. The point of view is statistical, and every effort is made to describe the progress of World War II in quantitative and objective terms. The data are interpreted in terms of the ever-changing tactical and strategical situation. Accordingly, the historical summary is divided into seven chronological periods, as indicated in the Table of Contents. This division is necessary because of the radical changes in the nature of the U-boat war due to changes in U-boat tactics and the introduction of new weapons and countermeasures.

The periods were chosen in such a way that U-boat strategy and tactics were fairly homogeneous in each. The tactics of individual U-boats varied considerably, however, from the typical characteristics which were common to the operations of most of the U-boats during the periods in question. In particular the division points between periods are somewhat indefinite and represent the approximate dates at which a majority of the U-boats had made a major change in their methods of operation.

The data for a given period are further divided into three main sections.

1 . A chronological narrative of the most important activities and accomplishments of the U-boats and Allied antisubmarine forces.

2. A more detailed account of the main Allied countermeasures to the U-boats, subdivided into

a. Convoys

b. Aircraft

c. Scientific and technical

d. Sinkings of U-boats.

3. A survey of the results achieved during the period, from both the U-boats’ and the Allies’ point of view. This section attempts to explain the reasons for the new tactics which characterize the new period.

Part II of the volume is a more detailed analysis of certain of the major problems of ASW, in particular those which were the subject of operations research studies. The emphasis is on the evaluation of tactics and materiel both by theoretical analyses and by special studies of operational data. Although the principles of ASW derived from such evaluation are strictly applicable only to the situation which obtained during World War II, the methods of evaluation are of more general interest.

The U-boat war spread over all the oceans of the world, but the main battle was fought in the Atlantic. Consequently U-boat activity in other regions has not been discussed as completely as the Battle of the Atlantic. A standard subdivision into areas, which has been used throughout the text, is given in the frontispiece.

There are, in fact, many aspects of the war which are omitted from the discussion. The operations of midget submarines and small battle craft are generally excluded, since they are not considered U-boats. The importance of the training of personnel and developments along this line are not considered. The activities of Naval Intelligence in obtaining information upon which operations are planned are not deix

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PREFACE scribed in any detail. The indirect effects of factors such as strategic bombing are largely neglected. The net result is to limit the discussion fairly closely to Navy antisubmarine operations, though Royal Air Force Coastal Command aircraft and those of the United States Army Air Forces are included when flying antisubmarine missions. The distinction is not a hard and fast one.

The sources of material used are so widely scattered through correspondence and informal memoranda that it has not been practical to quote references to them. Much of the basic information is available in periodicals such as the United States Fleet Anti-Submarine Bulletin, British Admiralty Monthly Anti- Submarine Review, and RAF Coastal Command Re- \ iew, which have been given wide distribution. Other reports, such as Operations Research Group Memoranda and Research Reports and British reports originating with Director Naval Operations Research

[DNOR] and the Operations Research Section of Coastal Command [ORS/CC] have been given limited distribution and are less accessible, fn addition, numerous letters, notes, informal memos, and even oral conversations go to make up the background of this volume. One of its chief aims has been to set down in writing a fair sample of this store of material, whose previous status verged on that of folklore.

No effort has been made, therefore, to assign credit for the work discussed. It originates with various members of British and United States operations research groups, military services, and civilian war agencies. We have tried to collect available information and tell a reasonably unified story, not of the accomplishments of a particular group, but of the progress of a special type of naval warfare.

C. M. Sternhell A. M. Thorndike ' Editors

C

Part I History Of Antisubmarine Operations

CHAPTER page

1 First Period— Submerged Daylight Attacks on Independents, September 193 9 -June 1940 3

2 Second Period— Night Surfaced Attacks on Convoys, July

3 Third Period-Start of Wolf Packs; End-to-End Escort of

Convoys, April 1941-December 1941 16

4 Fourth Period— Heavy Sinkings on East Coast of United

States, January 1942-September 1942 25

5 Fifth Period — Large Wolf Packs Battle North Atlantic

Convoys, October 1942 -June 1943 34

6 Sixth Period— Aircraft Defeat U-Boats’ Attempted Come- Back and Force Adoption of Maximum Submergence,

7 Seventh Period — Schnorchel U-Boats Operate in British

Home Waters, June 1944-End of War 64

Part II Antisubmarine Measures And Their Effectiveness

13 Offensive Search I39

14 Employment of Search Radar in Relation to Enemy

Countermeasures I53

Epilogue 177

Appendix I 181

Index 191

Ocean Areas Referred to in the Text

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KEY WEST.

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AZA

Azores Area

CSW

Caribbean Sea Frontier — West

NSA

North Sea Area

BCA

Biscay Channel Area

ESF

Eastern Sea Frontier

NTE

Northern Transit Area — East

BDA

Bermuda Area

FRA

Freetown Area

N FW

Nojrthern Transit Area — WTst

BLA

Baltic Area

GMA

(iibraltar Morocco Area

NWA

Northwest Atlantic Area

BTA

Barents Sea Area

GSF

Gulf Sea Frontier

PSF

Panama Sea Frontier

BZA

Brazilian Area

IND

Indian Ocean

SEA

Southeast Atlantic Area

CCZ

CSE

Canadian Coastal Zone

Caribbean Sea Frontier — East

MED

NEA

Mediterranean Red Sea Area Northeast AUantic Area

SWA

Southwest Atlantic Area c

L

PART I

HISTORY OF ANTISUBMARINE OPERATIONS

SUBMARINES IN WORLD WAR I

T he great capabilities of the submarine as a weapon of war were first revealed during World War I when the U-boat campaign almost proved decisive. Fortunately, the Germans themselves did not fully realize in 1914 how valuable the U-boat's ability to submerge and escape detection would be for offensive operations against enemy shipping. The small number of U-boats available to the Germans were used at first only to attack naval ships and it was not until 1915 that a concerted attack was begun on English merchant shipping.

During 1915 and 1916 there were on the average only about 15 U-boats at sea at any time. These Uboats were sinking about 200,000 gross tons of shipping a month, while about U-boats were being sunk each month. This situation was extremely satisfactory to the Germans, as the average life of a Uboat at sea during this period was about 10 months, during which the U-boat would sink about 13,000 gross tons of shipping a month, for a total of 130,000 gross tons of shipping sunk before the U-boat itself was sunk.

Encouraged by these successes, the Germans in February 1917 started a large scale campaign of unrestricted warfare on merchant shipping in an attempt to blockade England. This attempt almost proved successful as Allied shipping losses rose steadily to a peak in April 1917. Four hundred and forty-four ships of about 900,000 gross tons were sunk by U-boats during that month. The British Fleet was confined to its bases for there was only 8 weeks’ supply of fuel oil in England. Various countermeasures had been tried without success and defeat seemed just around the corner unless an antidote to the U-boat could be found.

INTRODUCTION OF CONVOYING

Admiral Jellicoe was brought to Admiralty to deal with the situation. The convoy system, twice turned down on account of lack of escort vessels and loss of time to shipping, was introduced in April 1917 and proved immediately successful in reducing the shipping loss rate. The result of all the various British countermeasures, of which the convoy system was the most effective, was that by October 1917, 1501 ships in 99 convoys had been brought into port with the loss of only ten ships sunk while in convoy (a loss rate of less than 1 per cent).

LACK OF SATISFACTORY COUNTERMEASURES

Although shipping losses had been checked, it should be kept in mind that, from an offensive point of view (i.e., destruction of U-boats), the U-boat had not been definitely beaten in World War I. After the start of unrestricted U-boat warfare early in 1917, the Germans maintained an average of about 40 U-boats at sea at any time. During this period the average number of U-boats being sunk each month was only about seven; the maximum number of U-boats sunk in any month was only 14 in May 1918. Therefore, the average life of a U-boat at sea during the last year of World War I was still about 6 months. Shipping losses, even during the last year of World War I, were still running at the level of about 300,000 gross tons a month, so that at that time each U-boat was still sinking about 45,000 gross tons before it, itself, was sunk.

These figures indicate that other factors besides U-boat losses must have contributed to the mutiny of U-boat crews in 1918, as the rate of U-boat losses has reached far higher levels in World War II without any corresponding crack-up in morale. Another point to be considered is that a large part of German U-boat losses in the latter part of World War I was due to mines, whose effectiveness was greatly increased by the fact that the geographical position of the German U-boat bases necessitated passage through the North Sea. Of the 178 U-boats sunk during the first World War, about 45 per cent were sunk by surface craft, about half of these by depth charges and half by gunfire and ramming. About 30 per cent were sunk by mines, another 10 per cent were torpedoed by submarines, and the other 15 per cent by other causes. It is therefore clear that the Allies had not developed any offensive weapon during World War I which could deal so effectively with the U-boat at sea that further operations would not be profitable.

HISTORY OF ANTISUBMARINE OPERATIONS

That the Germans themselves still thought the U-boat was an effective weapon at the end of World A\^ar I may be seen from the fact that there were about 220 U-boats under construction in November 1918. Admiral Scheer’s building program of October 1, 1918 provided for at least 30 U-boats a month beginning the middle of 1919 and would probably have been fulfilled if hostilities had continued. If the war had not ended in November 1918, the Allies would have had to face a second and more intensive U-boat campaign.

NEED FOR SCIENTIFIC AND TECHNICAL ADVICE

One of the most significant points about antisubmarine warfare which became apparent early in World War 1 was the necessity of having scientific and technical aid in combatting the U-boat. The essential problem was that of having some means of detecting a submerged U-boat and then of having some weapon that would provide a good chance of destroying the U-boat.

The first crude attempt to develop an instrument to detect the submerged U-boat resulted in the installation of hydrophones on Allied naval ships in 1915. The hydrophone was simply an instrument for listening to the noise produced by the submarine, and sonic frequencies below 10 kc were used. No range and only a rough bearing were obtained from these early hydrophones and it was impossible to make attacks on U-boats with any degree of precision. The main effect of hydrophones was on U-boat morale, as U-boats found that they were being followed after diving instead of being free of their pursuers.

The first depth charges to be used in attacking submerged U-boats were also introduced in 1915. However, so few were available that the Germans did not realize they were being used until 1917.

In September 1918 the British formed a small committee, consisting largely of scientists, called the Anti-Submarine Division International Committee (the initials spell ASDIC, the name given by the British to their echo-ranging detector). This committee developed a method of transmitting sound of supersonic frequencies under water and then using the echo returning from the submerged submarine to fix its position. Although the Asdic set was still in the experimental stage when World War I ended.

the labors of the committee were not wasted, as effective underwater echo-ranging gear was developed in the 1930’s and proved to be quite a surprise to the Germans at the start of World War II. Due to the ability of Asdic to provide both range and bearing, it proved far better than the hydrophones used in World War I. Hydrophones, themselves, were also improved by using supersonic frequencies and making them directional, thereby enabling the operator to obtain more accurate bearing.

ORDER OF BATTLE -SEPTEMBER 1939

At the start of World War II, England had only about 220 Asdic-fitted antisubmarine craft consisting of approximately 165 destroyers, 35 patrol craft {i.e., sloops, frigates, corvettes) and 20 trawlers. This total may be compared with the more than 3000 ships (about 450 destroyers, 170 patrol craft and the remainder trawlers and small craft) available to the Allies for antisubmarine warfare in 1918.

The British, profiting from their experience in World War I, had learned that the ocean convoy system did more than anything else to reduce shipping losses. They knew that the convoy system works best in open water where evasion can be employed and that its success depends upon efficient escorts armed with effective offensive weapons. They were also aware of the fact that an efficient U-boat tracking system is necessary to practice effective evasion, and a daily U-boat plot based on contacts, DF fixes, and intelligence was used throughout the war.

Meanwhile the Germans had done considerable research in developing and improving their U-boats. The U-boats available to the Germans at the start of World War II were faster than those used in World War I and were also considerably stronger, being able to dive deeper and to withstand more depth-charge punishment. The Germans had also developed an electric torpedo which left no visible wake.

However, in September 1939, the Germans seem to have had available only about 60 U-boats, of which 30 were of the small 250-ton type (of limited endurance-suitable for coastal operations only) and 30 of the larger ocean-going type, 20 of which were of 500 tons and 10 of 750 tons. This small number suggests that Germany, possibly not anticipating that England would enter the war at that early date, had given higher priorities to the building of tanks and aircraft for land warfare than to the building of U-boats.

Chapter 1 FIRST PERIOD

SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS SEPTEMBER 1939 -JUNE 1940

T his first phase of U-boat^ warfare was greatly influenced by the rapidly changing overall military situation. Germany invaded Poland on September 1, 1939, and England and France declared war on Germany on September 3. Some U-boats had left Germany early in August and when the war began there were about six at sea, ready to start an offensive in the Northeast Atlantic in the Western Approaches to England.

According to statements of early prisoners of war, the commanding officers of U-boats had been ordered to observe International Law, which forbade U-boats to sink merchant vessels without having first placed the passengers and crew in a place of safety. At the beginning of September, these instructions seem to have been generally obeyed, with the notable exception of the Athenia, which was torpedoed without warning on September 3. However, this situation did not last long and, toward the end of September, even neutral ships were being torpedoed without warning.

Anticipating unrestricted U-boat warfare, the British had prepared plans before the war for the immediate establishment of the convoy system and the first trade convoy sailed on September 6. As the British defenses against U-boat attacks were based on the needs of protecting primarily the fleet and secondarily merchant shipping, the limited number of antisubmarine vessels available for convoy escort was inadequate to provide direct protection to the convoys. Nevertheless, it was believed that the British antisubmarine measures were sufficiently effective to ensure that no U-boat could betray her presence by attacking a convoy without running a severe danger of subsequent destruction by the escorting craft.

The experience during September tended to justify these expectations, as over 900 ships were con-

a The term U-boat is used to refer to any enemy submarine (German, Italian, Vichy French, or Japanese) with a displacement of 200 tons or more.

voyed during the month without the loss of a single ship while in convoy. In addition, two U-boats were sunk during the month by British surface craft. The Germans apparently had no knowledge of British Asdic and still believed that they could counter underwater detection by reducing internal noises.

The lack of knowledge of British Asdic probably accounted for the early U-boat tactics. The U-boats preferred attacking their targets during the daylight, believing themselves relatively invisible because of their powers of submergence, while they could observe the targets through their periscopes. The U-boat attacks were generally made by torpedo from periscope depth, but if the target was an unarmed merchant vessel, the U-boat usually surfaced and attempted to sink the ship by gunfire.

During September, while the convoy system was still not fully established, there was a sufficient number of unescorted targets at sea to enable the U-boats to sink 39 ships of 151,000 gross tons. Ten of these ships were sunk by gunfire alone, from surfaced U-boats, and this led the British to take immediate steps to arm as many merchant ships as possible to defend themselves against such attacks.

At the start of the war antisubmarine forces in the Western Approaches were augmented by aircraft carriers, but after HMS Courageous was sunk by U-boat torpedoes on September 17, the carriers were withdrawn. However, shore-based aircraft of the Goastal Command helped considerably by flying over 100,000 miles in September, sighting some 50 U-boats or supposed U-boats, and attacking over 30 of them. Although none of the aircraft attacks were very effective, they did cause the U-boats to submerge and thereby reduced their effective operating period.

The September U-boat campaign was followed by a lull during the first ten days of October during which, although U-boats were at sea, hardly any ships were attacked. This lull seemed to reflect the political situation at the time, as it was accompanied by Hitler’s offer of peace on October 6. U-boat activity flared up again on October 12, and by the end of the

SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS month 28 ships of 136,000 gross tons had been sunk by U-boats. In addition Kapitan-leutnant Prien, in command of U-47, penetrated the harbor of Scapa Flow in the middle of October, and sank HMS Royal Oak, a British battleship. This served to direct British attention to the necessity of protecting harbors against U-boats by means of fixed defenses, such as booms, indicator loops, mine fields, and harbor defense Asdics.

During November and December the main effort of the German U-boats seems to have centered upon a mine-laying campaign on the East Coast of England, particularly in the Thames Estuary. The mines laid were both the old type of contact mine and a new type of magnetic mine, which at first proved rather difficult to sweep. Monthly losses due directly to U-boats (torpedoes and gunfire) fell to 18 ships of about 65,000 gross tons and were exceeded by the 100,000 gross tons of shipping sunk by mines during each of these months.

U-boat activity began increasing again in the second week of January 1940 and by the end of the month there were as many U-boats at sea as at the start of the war. In February, the U-boat effort was greater than during any previous period and 35 ships of 153,000 gross tons were sunk. The U-boats continued to follow a policy of attacking Allied %nd neutral ships without warning. They preferred attacking single ships or stragglers from convoys, thus making it difficult for the antisubmarine ships to conduct an effective search and counterattack. The respect the U-boats had been showing for the Briti^ convoys is indicated by the fact that only 7 of the 169 ships sunk by U-boats during the first six months of the war were in convoy when sunk, although roughly half the shipping sailed in convoy at this time.

Losses due to mines fell off during January and February as better methods of sweeping the magnetic mines were developed and more ships were degaussed (magnetic field of ship changed to protect it against magnetic mines).

There was a marked lull in U-boat activity throughout March, featured by the complete absence of U-boats from Atlantic waters after about the 12th of the month. Early in April, every available U-boat left Germany to take up patrol positions in the North Sea to help in the impending military operations against Norway. The average number of U-boats at sea reached a peak of about 15 during the second week of April, when Germany invaded Norway. Despite the large concentration of U-boats, the damage done by them was remarkably small. No British capital ship was even attacked by U-boats and only six ships of 31,000 gross tons were sunk by the U-boats during the whole month of April, a new low for the war. In addition, the Germans lost six U-boats during the month, a new high for the war.

There was very little U-boat activity during the first half of May as Germany started her invasion of Holland and Belgium on May 10. It is believed that during May no U-boat proceeded to the Western Approaches until the 21st and only 10 ships of 48,000 gross tons were sunk by U-boats during the month. Shipping losses to U-boats were exceeded for the first time during the war by the 154,000 gross tons sunk by aircraft. These losses were incurred largely in connection with the operation and evacuation of the British Expeditionary Force, which left Dunkerque on May 29.

The Germans announced on May 29 that U-boat warfare was about to recommence and warned neutrals not to enter the protection of British convoys. This threat was followed by a period of intense U-boat activity as convoys were attacked with greater boldness than in earlier periods, advantage being taken of the paucity of escorts, rendered inevitable by the demands of the military evacuation and the Home Fleet. The losses for June were the highest of the war, with 56 ships of 267,000 gross tons being sunk by U-boats. The German ace. Kit. Prien, contributed his share by sinking ten ships of about 67,000 gross tons during one cruise. By the end of June, France was out of the war and Italy had entered the war with over 100 U-boats, about 60 of which were ocean-going (650 tons and over).

The 'convoy system was by far the most effective countermeasure in keeping down shipping losses to U-boats during this first period, just as it had been during World War I. This was still true, even though the number of antisubmarine vessels suitable for ocean escort was insufficient to provide direct protection to the convoys. The British met this problem by keeping their convoy system flexible, changing the number of escorts and the distances for which convoys were escorted in accordance with U-boat activity. It should be noted that the Germans made this problem more difficult by sending the U-boats out in waves, so that peaks of U-boat activity occurred in September 1939 and in February and June of 1940.

Although the first convoys sailed early in September 1939, the convoy system was not fully in force until the beginning of October. The designations of the main convoy routes that were set up then were:

OB Outward bound from England to America and Africa.

HX Homeward-bound to England from Halifax.

SL Homeward bound to England from Sierra Leone.

In order to illustrate some of the problems involved in setting up the convoy system a detailed account is presented of the changes made in the HX convoy route during this period. On October 7, 1939, it was decided to discontinue the convoys from Kingston, Jamaica, and all ships in the West Atlantic were routed independently to Halifax, taking advantage of U. S. waters as far as possible. Convoys were divided into slow (9- to 12-knot) and fast (12- to 15-knot) convoys, which left Halifax at about the same time in order to arrive four days apart at the rendezvous point. At this point, located at about 15° west longitude, the convoys were met by one or two destroyers which provided antisubmarine escort to England. The ocean escort, provided between Halifax and the rendezvous point primarily for protection against surface raiders, consisted of a battleship, cruiser, or armed merchant cruiser, and one or two submarines when available.

The first of these convoys, HX 6 and HXF 6, consisted of 62 and 6 ships, respectively. The dividing line was then altered to 1 1 knots to equalize the number of ships, and during November 1939 the number of ships in these sections averaged 32 and 12. On February 12, 1940, the fast convoys were discontinued, and all HX convoys sailed at 9 knots, at 3- and 5-day intervals. These convoys consisted of ships with speeds between 9 and 15 knots, ships of higher speeds sailing independently. At the beginning of April, in order to equalize the size of the convoys, 4-day intervals were started.

Early in May 1940 Bermuda began to be used as an assembly point for vessels from the West Indies and other points in that vicinity, and HX 41 was the first combined Bermuda and Halifax convoy. The sections formed at sea, as arranged, at about 41° north latitude and 43° west longitude, and the Bermuda escort then returned to base. This change enabled about 60 per cent of the ships that formerly sailed from Halifax to cut down their voyage by 500 miles and to avoid the fog off Newfoundland. The average number of ships in these HX convoys had risen to 46 by May 1940.

In addition to the above-mentioned convoys, the British also sailed coastal convoys to protect shipping on short trips around the English coast and Scandinavian convoys to and from Norway. The main energies of the French light craft were also devoted to the protection of merchant shipping. They were fitted with Asdic as soon as possible after the opening of the war and provided escorts for purely French convoys, helped escort the Gibraltar convoys for most of the way, and assisted in covering the military cross-Channel convoys.

The extreme value of the British convoy system may best be appreciated by noting that during this period about 2500 ships were being convoyed monthly, while only about 5 of these were being sunk monthly by U-boats (2i/^ in escorted convoys, II /2 in unescorted convoys, and 1 straggler). The rate at which independent merchant vessels were being sunk b)4U-boats was roughly about four times as high.

Another important countermeasure to the U-boat ^as the use of aircraft. These had seen very little use against U-boats during World War I and consequently it took some time before the problems of how to use aircraft most efficiently against U-boats were solved. In addition, the aircraft were still armed only with bombs. Consequently the direct contribution of aircraft toward sinking U-boats was negligible during this period.

Nevertheless, aircraft performed a defensive function of great value in helping to protect shipping. Coastal Command aircraft flew, on the average, about 4500 hours monthly on purely antisubmarine work. About 20 U-boats were sighted monthly and 12 of these were attacked, with about 10 per cent of the attacks resulting in some damage to the U-boat. This effort reached a peak of 9500 hours during June 1940, when about 2800 hours were spent on antisubmarine patrol and 6700 hours on convoy escort duty.

SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS

The main value of this flying was in causing the U-boats to submerge, thus preventing them from shadowing or approaching convoys on the surface. It also helped to discourage them from operating close to the shores of England where the flying was heaviest. U-boats at this time were under orders to submerge as soon as they sighted a })lane and the British took advantage of this by starting to use, in November 1939, light aircraft of the Moth type to patrol around the coast. These aircraft were known as “scarecrows,” carried no bombs, and were used solely to sight and report U-boats, and to make them submerge. These flying hours and sightings also helped considerably in keeping an accurate U-boat plot.