ANTISUBMARINE MEASURES AND THEIR EFFECTIVENESS
From the historical summary of operations against submarines during World War II presented in Part I, many conclusions concerning the proper strategy and tactics of antisubmarine warfare [ASW] can be drawn. The most important of these will be discussed in the following chapters and substantiated by quantitative data from Operations Research Group Studies.
Most conclusions have to do with specific problems of tactics, for example, weapons for attacks, proper tactics for search, or methods of protecting convoys. These are all part of a general picture which involves the overall purpose of antisubmarine warfare and the various methods available for accomplishing that purpose.
It is already evident from Part I that the aim of ASW is not simply the destruction of unfriendly submarines. If it were possible to sink all enemy submarines, the mission of ASW would, of course, be accomplished, but the forces available have never been sufficient to do this, for a number of reasons. The submarine is a small and elusive object in a large ocean and consequently very hard to find. When found, it is a tough and inaccessible object to attack. As long as the enemy is able to build and launch submarines, lie can keep some of them at sea, and even the highly effective Allied antisubmarine effort in World War II did not greatly diminish the size of the German U-boat fleet but merely checked its growth.
Antisubmarine warfare must be thought of as a part of a complex overall military strategy whose final aim is to eliminate the enemy's ability to wage war. This must ultimately be (lone by seizing or destroying the military or economic war machine-administration, transportation, men, equipment, or production facilities-by striking at the heart of the enemy, not merely nibbling piecemeal at his periphery, or by convincing him of an ability to do so. When possible it is most efficient to ignore the periphery and proceed directly to the ultimate objective. This is the philosophy underlying the blitzkrieg and war of encirclement.
The aim of ASW is to ensure the use of the oceans necessary for military operations intended to bring about the defeat of the enemy. It is an auxiliary operation, necessary, but not sufficient, for overall success. In this sense, then, the actual aim of ASW is negative, to prevent enemy submarines from accomplishing their aim.
the first Step in this analysis of the subject is to outline the value of submarine operations to the enemy. The objective of ASW is to reduce this value to a minimum, and its general strategy is planned accordingly.
The peculiar value of the submarine among naval craft is its ability to operate in enemy-held waters. Even when surfaced, a submarine is a small target for visual or radar detection. When submerged, it is completely concealed except for detection by underwater sound, whose ranges are short and unreliable.5 As a result, the submarine can operate in regions forbidden to surface craft because of enemy patrol. In these regions aircraft operations are often impossible because the regions are beyond the range of aircraft. Thus the submarine is the primary, and often the only, craft for carrying out operations at a long distance from base in the areas of the enemy's main strength.
Numerous types of operation may be involved. The aim of some may be to gather information that can be gained only by an excursion into enemy territory. In this class are routine scouting and weather reporting missions and also those involving the landing of agents. Some are transport missions to supply isolated units which cannot be reached by other means. The main mission of the submarine, however, is offensive-to attack the enemy's ships, both combatant and merchant. Past experience has shown that the most valuable submarine activity has been that of attacking merchant shipping, and antisubmarine warfare is of urgent concern on this account.
Man being a terrestrial animal, the oceans are valuable to him only as a means of transportation from one piece of land to another or as a barrier between
them. Control of the ocean secures this use to the controller and denies it to his enemy. While an antishipping offensive by submarine does not give control of the ocean, it serves to deny free use to the enemy, and in this negative sense the submarine force may accomplish quite complete control. To the extent that this is done and the enemy prevented or hindered from transporting necessary cargoes, the submarine offensive makes a major contribution to the progress of the war.
In the submarine's antishipping offensive three things must be accomplished: achieving contacts on ships, approach to within torpedo range, and final attack. The first of these is essentially a search problem of the kind discussed in Volume 2B, Search and Screening, with submarine as searching craft. Detection may be made by visual, radar, or sonar means. Visual detection has had the largest range and sonar the shortest, as a rule, though the ranges depend on conditions. Visual detection, for instance, will be ineffective on a dark night, and sonar ineffective in poor sound conditions.
Since the submarine's speed is low, it is not a very efficient searching craft and must operate in regions of high shipping density to make a large number of contacts. To select the high-density areas a knowledge of the expected positions of enemy ships is required, which may be based on intelligence information or gained locally by coordination between submarines. The "wolf-pack" tactics of German U-boats accomplished this by homing many submarines to each contact. Once a single U-boat made contact, the information was used to permit others to achieve contact as well.
When contact has been made, the submarine must approach to within torpedo range before an attack can be launched. If the submarine's speed is greater than that of the target, the approach is not very difficult, and practically all ships contacted may be attacked. However, if the submarine is forced to operate submerged (or if the target is very fast), approach is only possible from positions ahead of the target, and slight errors in the approach may result in failure to achieve the proper position.
Once firing position is reached, torpedoes are fired. Their chance of success depends on the accuracy of the torpedo fire and on the physical characteristics of the torpedo used.
The primary aim of ASW is to reduce the effectiveness with which the submarines carry out these steps, and the success of an antisubmarine effort is to be assessed in these terms. Antisubmarine warfare is not an end in itself, but merely a means of ensuring that the ability to use the ocean for transportation is maintained at the best level possible.
Some antisubmarine measures are specifically intended to hinder the submarine in carrying out a particular phase of its antishipping operations. Evasive routing of convoys to avoid known submarine positions is useful solely in reducing the submarine's chance of contacting the convoys. If all the ships could always dodge the submarines, the latter's' effectiveness would be much reduced. Such a procedure would slow the ships down appreciably, however, so that the submarine effort would not be entirely wasted. On the other hand, most measures designed to combat submarines serve a multiple purpose. Maintaining aircraft patrol in the vicinity of a convoy not only may lead to sightings of submarines, thus permitting evasion by the convoy, but these aircraft also force submarines to dive, thus hindering them in their efforts to track a convoy and approach it. Finally, an occasional aircraft may be fortunate enough to attack a submarine and sink it. The overall value of any measure must be based on all the ways in which it serves to frustrate the submarine.
Most obvious among the measures designed to reduce the submarine's rate of contacting ships is evasive routing. If the submarine positions are fairly well known, they can be avoided and the density of shipping in their vicinity greatly reduced. Convoying is another measure which, in effect, reduces the submarine contact rate. If there are n ships in each convoy, on the average, the submarine contacts only about l/n as many convoys as it would ships. Since it may be made as large as 50 to 100, the resulting gain is considerable. Patrol by aircraft also serves to reduce the contact rate, as indicated earlier, by forcing the submarine to spend a large fraction of its time submerged. This reduces its detection range and interferes with the formation of wolf packs. In a less direct way, both surface and aircraft patrols and hunter killer operations have the same effect. A concentration of offensive operations in areas of high density of shipping serves to force the submarines out of these areas and to make them operate where their chances of contacting ships are not so good. The apparent aim
of such an offensive is to sink U-boats, but its chief effect may be to force them to adopt less profitable tactics, and this effect continues to be valuable even if sinkings oŁ U-boats be reduced to a negligible level.
The methods of hindering submarine approach consist of various forms of escort, both for convoys and single ships. Aerial escort tends to force the submarines to submerge and thus restricts their mobility, making it impossible for them to trail the convoy or to overtake it from the flanks or rear and reach a position ahead from which attack can be made. Even if the submarine is ahead of the convoy when she contacts it, a submerged approach is more difficult than one carried out partially on the surface and is more likely to result in errors.
If the submarine boldly elects to stay on the surface for approach to the convoy, it is very likely that it will be detected and attacked in the process, which experience is almost certain to eliminate any possibility of successful offensive action on its part. Correspondingly, surface craft escorts interfere with the later stages of the approach. Having to try to avoid them complicates the submarine's problem, but, if it fails to do so, it is likely to be detected and counterattacked before reaching torpedo-firing position. The submarine's problem can be made still more difficult by use of the highest possible speed of ship and by zigzag, when possible.
Attempts to interfere with the submarine's final attack, after it has reached firing position, have not been very successful in the past. To some extent the mere presence of surface escorts may constitute such interference. Zigzagging reduces the accuracy of torpedo fire somewhat. A maneuverable ship may turn to avoid the torpedo if the ship has sufficient warning, and special torpedo detectors may be devised to aid in doing so. Anti-torpedo nets and other devices can be streamed to intercept the torpedo. All these are examples of devices and tactics designed to reduce the effectiveness of the submarine torpedo fire.
Supplementing these essentially defensive methods of interfering with the submarine's accomplishment of its objective is the offensive phase of ASW. The most certain way of preventing a Submarine from sinking ships is to sink it first. in this sense, sinking a submarine is equivalent to saving as many ships as it would normally sink during the remainder of its operational lifetime. If, for example, the average submarine makes a total of ten operational patrols and sinks two ships in each, sinking the submarine saves an average of ten ships, because the submarine is likely to be about half through its normal life when sunk.6 In addition to this direct diminution of the submarine fleet, the sinkings tend to reduce the state of training and experience of the submarine crews. If, for example, the rate of sinking can be kept high enough to give the submarine (or crew member) an expected life at sea of only four patrols, there will be very few men available with the experience of, say, ten patrols. The resulting dearth of experienced personnel is a handicap very difficult to assess, but unquestionably of considerable practical importance. Only a fraction of the value of offensive operations can be represented by the numerical decrement achieved in the enemy's submarine fleet.
There is normally an appreciable effect upon morale as well. The effectiveness of submarine operations cannot be divorced from the skill and determination oŁ the submarine force's personnel. By selectively eliminating experienced men, the high rate of loss reduces not only their overall skill but also their determination. The submarine's chance of being sunk if it endeavors to attack a certain convoy, for example, cannot but bear a greater importance to its crew than to the theoretical strategist. The submarine crews will accordingly avoid operations which involve high losses to themselves, even though theoretically profitable, unless their psychology includes a definite suicidal tendency.
From this brief outline it is evident that ASW involves a great many different aspects. The following chapters will discuss a number of specific problems in order to illustrate the general principles involved. The subjects for discussion have been chosen on a dual basis: first, for their importance in antisubmarine strategy and tactics, and, second, for their interest as examples of the methods of Operations Research.
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ANTISUBMARINE MEASURES AND THEIR EFFECTIVENESS
Chapter Page 9 Safety of Independent Shipping 93 10 Convoying and Escort Shipping 100 11 Attacks by Surface Craft 113 12 Attacks by Aircraft 127 13 Offensive Search 139 14 Employment of Search Radar in Relation to Enemy Countermeasures 153 15 Countermeasures to the German Acoustic Torpedo 153 Epilogue 177 Appendix I 181 Glossary 183
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