CONVOYING AND ESCORTING OF SHIPPING
The primary advantage of a convoy system is the concentration of defense that it allows. Since it is impossible to provide an escort for every individual ship at sea, ships must sail in groups so that
CONVOYING AND ESCORTING OF SHIPPING
The primary advantage of a convoy system is the concentration of defense that it allows. Since it is impossible to provide an escort for every individual ship at sea, ships must sail in groups so that each group may be adequately escorted. A secondary advantage is the reduction in number of units at sea for the submarine to contact, since the convoy becomes the unit instead of the individual ship. It is convenient to analyze the gain achieved from convoying in the same three steps as were discussed in the last chapter: contacts made by the enemy, his ability to make an approach, and his chance of sinking ships once firing position is attained.
10.1 THE GAIN IN SAFETY BY CONVOYING
10.1.1 Reduction in Number of Contacts Made by Submarines
The institution of the convoy system results in a considerable reduction in the number of contacts made by submarines. The number of sightings made depends on the number of units present to be sighted. Clearly, 100 independent ships sailing in a given area represent 100 opportunities for sighting, while the same number of ships in two convoys of 50 ships each offer only two opportunities for sighting. Each sighting, however, gives 50 targets and is accordingly of increased value, as will appear from subsequent sections. This effect tends to neutralize the benefit derived from grouping, but only to a small extent. In addition, an increased range of detection tends to increase the number of convoys that are contacted. Although the range of detection on a 50-ship convoy is by no means 50 times that on a single ship, it is appreciably greater, and some consideration of the increase is necessary.
FIGURE 1. Submarine Sweep width on convoy.
Visual sightings, if made on masts or superstructure, will be made at about the same range on convoys as on independent vessels. A large convoy covers a sufficiently large area to increase the sweep rate about 50 per cent, as shown in Figure 1, but this is not a serious increase. Submarines often sight ships by smoke, however, which may be seen at distances up to 40 miles. A single ship may make smoke only a small fraction of the time, but usually there is at least one ship in a convoy which is smoking, and smoke becomes very much more important. If b is the fraction of time that each ship smokes, R1 the range on a ship itself, R2 the range on the smoke, and n the number of ships in convoy, then
For convoys of as many as 50 ships, (1 - b)n is so small that the sighting range is approximately R2, the range of detecting smoke. For a plausible set of values (R1 = 4 miles; R2 = 24 miles; b = 0.10), equation (1) gives the following average ranges.
Single ship 6 miles 8-ship convoy 11 miles 64-ship convoy 23 miles
The submarine's radar will generally detect a convoy at a longer range than a single ship, because the convoy presents a larger reflecting target. Since, however,
the intensity of the radar echo decreases very rapidly with increasing range to the target (normally as 1/r4 or faster), the increase in range is not very great. Operational data giving ranges of submarine radar on large convoys are not available. U. S. submarines have obtained ranges about 35 per cent longer on convoys of about 3 to 4 ships than on single ships. Aircraft radar ranges on large Atlantic convoys have been only 10 to 25 per cent greater than on single ships. It would not be expected, therefore, that grouping of ships in convoy would cause a serious increase in the range of radar detection.
The range of sonar detection will be appreciably increased. The intensity of sound produced by a convoy of 77 ships is about n times that of a single ship. On this basis it has been estimated that the range on a 50-ship convoy under favorable listening conditions would be about three times that on a single ship. Under less favorable conditions the ratio would be smaller.
In order to estimate the overall change in range of detection, it is necessary to determine the relative importance of the different methods. In general, visual detection ranks first, because of long range and ease of identification, radar second, and sonar third. During a period near the end of World War II, for instance, United States submarines made about 800 contacts by visual detection, 300 by radar, and 50 by sonar. German U-boats have not had as effective search radar as United States Submarines, so that the importance of radar in U-boat operations has been much less. These figures would depend a great deal on submarine tactics: a submarine which spent most of the time submerged would make relatively more sonar contacts. It is thus evident that the contact range increases with an increased number of ships but is by no means proportional to it. A fairly reasonable approximation would be to consider the range as proportional to n1/3. Then the expected number of contacts made will depend on two factors: this increased range and the decrease in the number of units at sea to be detected.
Nc = expected number of contacts on convoys;
Ni = expected number of contacts on same ships sailing independently.
A change from n = 1 to n= 64 will reduce the number of contacts made on a given number of ships at sea by a factor of 16, according to eq