NIGHT SCOUTING WITH LISTENING DEVICES
By Lieut. Commander Holloway H. Frost, U. S. Navy
All methods of scouting in use to-day rely upon the actual sighting of the enemy ships. This fact makes it practically impossible to scout during night or while in a fog. The chances for a successful search are also considerably reduced by the haze or mist which frequently reduces the visibility to five or even two or three miles. When darkness comes on, when fog is encountered or when the visibility is so decreased by mist that the area between any two ships on the scouting line cannot be covered, not only must the scouting line cease its advance, but it must retire at the assumed speed of the enemy. The fact that we can scout but 12 hours in 24 on the average in itself reduces the efficiency of the search by one-half. In addition to this, heavy fogs are very frequent off the North Atlantic and the Pacific Coasts of the United States, and in the North Atlantic misty weather prevails almost continually. The difficulty of scouting off the North Atlantic Coast was one of the most important facts demonstrated by the fleet maneuvers held in August, 1916; for a period of four days the visibility varied from four to eight miles and efficient scouting operations by the defending force were consequently impossible. .
A typical search by the out and in method is shown in Fig. 1. At daylight a line of six scouts spaced at 40 miles is in position AF. During the daylight period of 12 hours it advances at the speed of 20 knots a distance of 240 miles to a position A'F'. Then, as scouting during darkness with the vessels spaced at 40 miles is impossible, the line must retire at 10 knots, the assumed speed of the enemy, for 12 hours. By daylight a retirement of 120 miles has been completed and the line is in position A’F’. During the entire period of 24 hours only 120 miles have been made good to the front, although 360 miles have been steamed at the average speed of 15 knots. Fuel expenditure is a very important factor in scouting operations and if only 120 miles can be gained by 360 miles of steaming, the scouting force cannot expect to accomplish very much before it must return to its base to replenish its fuel supply.
It is apparent that the greatest advantages would be gained if some method of search could be developed so that scouting could be continued during darkness or fog in the same manner as it is carried on during fine weather in daylight. A possible way of continuing the search would be to use for locating enemy ships the listening devices developed for the anti-submarine warfare.
Rear Admiral E. A. Anderson, U. S. Navy, commanding the American Patrol Detachment, was the first to develop and test out the idea of using listening devices for scouting. As commander of the submarine chaser squadron attached to his command I assisted in the development of this idea. The 12 chasers of the Squadron were exercised continually in listening for surface craft. It was made the normal procedure for chasers proceeding on an exercise cruise to form a scouting line at distances of about 10 miles for the purpose of conducting a night search for the numerous merchant vessels which are continually passing through the Florida Straits. After the scouting line was formed it was the doctrine for chasers to stop for the first 15 minutes of each hour for listening. The K-tube was put over in about 5 minutes; a listening period of about 8 minutes was then allowed; and the tube was hauled aboard in about 2 minutes. If there was reason to believe that vessels were within about 5 miles, the SC and MB tubes, which are fitted in the hull and can be lowered in a few seconds, were also used. For the remaining 45 minutes of the hour the chasers proceeded at 12 knots. A special code was issued for reporting bearings, for informing the squadron commander of the action taken by individual chasers and divisions, and for allowing the squadron and division commanders to give instructions for the scouting line.
In general the actual results obtained in these exercises were not so satisfactory as to prove that an efficient search can be made during darkness with the present design of K-tube. Sometimes bearings accurate to within five degrees were obtained of surface craft distant from 12 to 14 miles; in other cases vessels only 5 miles distant could not be heard.
The operation of listening devices is a very delicate process. Wide variations must be expected in the results obtained. In the first place the skill of the listener is a very important factor and a great amount of experience is necessary if the devices are to be used with real effect. An experienced listener will hear a vessel two or three times farther away than will one who has had only a short training. Also the skill of a listener varies on different days. On one day he will hear a vessel at 8 miles; the next day he will fail to hear the same vessel at 4 miles under the same conditions. The tubes themselves are delicate and no two will have the same degree of sensitiveness. They are also liable to go completely out of commission at any time. The degree of roughness of the sea has a considerable effect upon listening, and even when the sea is calm and when conditions are apparently the same there seem to be important differences in the distances which can be heard, perhaps due to currents or eddies in the water, differences in temperature, the conformation of the bottom, or the depth of water.
Night scouting with listening devices will hardly be successful until it is possible to detect surface craft with a reasonable degree of certainty at a distance of 15 miles even under bad conditions. Possibly there are devices now which will accomplish this when skillfully used; if not, there is no reason why they cannot be developed. Since the signing of the armistice interest in the development of listening devices has waned; they were designed for use against submarines and now that the submarine campaign is over little attention has been given to them. It is true that another submarine campaign on commerce, such as was carried on by the Germans, will probably never be repeated in the future. But still the submarine will play an important part even if it is limited to attacks on men-of-war. This is well shown by what happened when the Grand Fleet was drawn into the submarine trap of 19 August, 1916. At least 14 torpedoes were sighted by British ships. The Falmouth was hit by four torpedoes and the Nottingham by three; both ships being lost. There were no less than 22 reports of submarines being sighted. The British submarine E-23 torpedoed the Westfalen; the ship, however, succeeded in making port.
But while we will always need listening devices for use against submarines, they have a far wider field open to them in scouting, reconnoitering, screening and in making destroyer attacks. In order to stimulate the material development of the listening devices, it is proposed to show the advantages which can be gained by their use in night scouting. In order to show how they may be used when once developed, the detailed methods of night search which may be employed will be set forth.
While there are many forms of scouting operations, in nearly every case some form of the out and in method or of the retiring search patrol are used. We will therefore see how listening devices can be used in these forms of search. For the purpose of discussion let us assume that the listening devices are effective at a distance of 15 miles. It is not pretended that there is any distance at which you can always hear a vessel any more than there is a distance at which you can always see one. But in solving a scouting problem it is necessary to make certain assumptions. In order to be able to assume a listening range of 15 miles, we would want to be certain that the devices would be effective at that range about 8 times in 10 under all weather conditions. Then, when you consider that the chances are very small that an enemy vessel will pass exactly midway between two scouts and that even then two scouts would have to miss it for it to pass through undetected, we could consider the scouting operation 95 per cent efficient and feel that in giving the enemy 1 chance in 20 to get through we would be taking a perfectly legitimate risk. In the sketches of the scouting operations, we invariably assume that the enemy is making a long cruise and is proceeding at 10 knots. In the tables, however, the figures for enemy speeds of 12 and 14 knots are given.
Now let us take up the out and in method. Assume that at sunset a scouting line arrives at the position A'F in Fig. 1. Instead of retiring during the night, we desire to advance the line still farther to the front or at least to maintain it in this position. If the scouts are spaced at 30 miles, they can lie dead in the water during the entire night and by constantly listening for the enemy maintain their position without expenditure of fuel. If they are spaced at more than 30 miles, each scout must cruise back and forth along the scouting line, in a way which will be described later, if the line is to be maintained. If the scouting distance is less than 30 miles, then the scouts can advance the line to the front either by proceeding on constant courses of by steering a zigzag. The efficiency of the night search therefore depends to a very great extent upon the scouting distance. It will be shown later that, while favorable results will be gained if the scouting distance is less than 30 miles, the solution is not so satisfactory for distances over 30 miles. This at first might appear to be a very important disadvantage. In order to clearly define the situation, it will be well to see what scouting distance can be used in out and in scouting in actual war or maneuvers with actual ships.
At the War College it is assumed that single ships can be seen at a distance of 14 miles and that the smoke of formations of more than eight ships can be seen at 25 miles. In clear weather it is probable that single ships can be seen at 14 miles. The assumption that a formation of ships can be discovered at a distance of 25 miles depends upon the fact that a large number of ships creates a great mass of smoke which can be seen long before the ships themselves can be seen. Even with coal burners it seems rather doubtful that formations of ships can be seen at the distance assumed even if the weather is perfect. With oil burners—which will run smokeless in order to prevent detection— it would seem that 20 miles would be about the maximum distance at which the sighting of a formation of ships could be counted on. Therefore, for reasons of visibility alone, 40 miles seems the maximum distance at which scouts should be placed even in perfect weather. When it is considered that, except in tropical waters, there are comparatively few days of perfect weather, 30 miles would seem the usual scouting distance and this would frequently be reduced to 25 or even 20 miles. It will seldom be desirable to use a scouting distance just double the distance of visibility; a commander of a scouting line will always wish to have a factor of safety on his side; ships may break down, especially if run at high speed, radio communication may fail, vessels may be sunk or driven out of position by the enemy, and may lose their assigned position on account of unavoidable errors in navigation. Thus, excepting the length of line covered, every advantage is on the side of the closely spaced scouting line. There is more chance of sighting enemy ships, radio communication is better—thus increasing the rapidity with which reports of contacts can be made to the scouting line commander and the readiness with which he can maneuver the line—vessels attacked by superior forces can receive assistance quicker from adjacent ships, more reports of enemy forces encountered will be made—thus fixing the position of enemy screens from which an estimate of the position of the enemy main body can be made—and a force of ships from the scouting line can be concentrated quicker for an attack upon enemy scouts or a thrust through their line into the probable position of the enemy main body. For all of these reasons the scouting distance should usually be kept below 30 miles. For such distances the search can be continued during the night almost as efficiently as during daylight when our listening devices are effective at a distance of 15 miles.
If the scouting distance is just 30 miles the scouts can maintain their position dead in the water during the night, so that for the total period of 24 hours and advance of 240 miles will be made at an average speed of 10 knots In the usual out and in methods the advance was but 120 miles for an average speed of 15 knots.
Next assume that the scouting distance is 40 miles. In Fig. 2 two ships are at A and B distant 40 miles. After listening for 15 minutes in these positions, they proceed North to positions A' and B' in the remaining 45 minutes of the hour. Then after listening for 15 minutes they proceed South for 45 minutes to their original positions. This procedure is continued during the night and the scouting line is maintained. Assume that the ships are in positions A and B at the instant of getting under way on course North. The most favorable position for an enemy ship attempting to pass through the line undetected is at C, on the edge of the listening circle of the ship at A. In 45 minutes the enemy ship can run at the speed of 10 knots a distance of 7.5 miles to a position C. With C as a center and a radius equal to 15 miles draw an arc DD, cutting the line AB at B'. Therefore the scout which was originally at B must reach B' after a run of 45 minutes if the enemy ship is to be detected. BB'= 12.5 miles = the average speed of the scouts per hour during the night. As the scouts must make this distance in 45 minutes their actual running speed will be 16.6 knots.
In Fig. 3 the scouting distance is 50 miles. The distance BB' in this case is 22.5 miles. The running speed of the scouts is therefore 30 knots. As it will be practically impossible to maintain this speed on the scouting line, this method of maintaining the line will be unsatisfactory when the scouting distance is as great as 50 miles. Such a distance, however, will be exceptional for daylight scouting. In case it is used during daylight it will be necessary to decrease it to about 40 miles at sunset.
The following table shows the average and running speeds of scouts necessary for maintaining the line for various scouting distances and for various assumed speeds for the enemy:
Table 1 | |||
Assumed speed of enemy | Scouting distance | Average speed of scouts | Running speed |
10 | 35 | 7.5 | 10.0 |
10 | 40 | 12.5 | 16.6 |
10 | 45 | 17.5 | 23.3 |
10 | 50 | 22.5 | 30.0 |
12 | 35 | 8.5 | 11.3 |
12 | 40 | 13.5 | 18.0 |
12 | 45 | 18.5 | 24.6 |
12 | 50 | 23.5 | 31.3 |
14 | 35 | 10.5 | 14.0 |
14 | 40 | 15.5 | 20.7 |
14 | 45 | 20.5 | 27.3 |
14 | 50 | 25.5 | 34.0 |
Now let us consider cases where the scouting distance is less than 30 miles and the scouting line can be advanced. In Fig. 4 assume that two scouts are at A and B—distant 20 miles—at the end of a listening period. The most favorable position for an enemy ship attempting to pass through the line is at C. In 45 minutes this vessel can run 7.5 knots at 10 knots speed to the position C. From C draw the arc DD with a radius equal to 15 miles cutting BB' at B'. The ships at A and B can therefore advance 14 miles to A' and B' during the running period of 45 minutes. The average speed of advance is therefore.14 knots per hour and the running speed will be 14 x 4/3= 18.8.
In Fig. 5 the scouting distance is 25 miles, the average speed of advance is 7.5 and the running speed is 10.
The following table shows the average speed of advance and the running speed of the scouts for various speeds of the enemy and various scouting distances:
Table 2 | |||
Assumed speed of enemy | Scouting distance | Average speed of scouts | Running speed |
10 | 20 | 14.0 | 18.6 |
10 | 25 | 7.5 | 10.0 |
12 | 20 | 12.0 | 16.0 |
12 | 25 | 6.0 | 8.0 |
14 | 20 | 10.5 | 14.0 |
14 | 25 | 4.0 | 5.3 |
In advancing the line as shown in Fig. 5 the scouts proceeded on a constant course. If the scouts steer a zigzag, changing course after each listening period, a greater advance to the front can be made. In Fig. 6 three scouts at 20 miles distance are at A, B and C at the end of a listening period. The most favorable positions for enemy ships attempting to pass through the line are D and E. In 45 minutes' steaming at 10 knots the enemy vessel at D can advance to D'. With D' as a center draw an arc FF with a radius equal to 15 miles. This cuts BB'—the course of the scout 300 to the left of the base course—at B'. At the end of the running period the three scouts will be on the line A'B'C. It will be noted that all points on the arc D'E' are within the listening circles drawn from B' and C, so that no enemy vessel could have passed through undetected. The average speed of the scouts per hour (BB') is 21 knots. Their running speed is 28 knots. The average speed of advance—the distance between the line AC and the line A'C—is 18.4 knots. After listening for 15 minutes at A'B'C the scouts proceed on a course 300 to the right of the base course to A'B"C". When a zigzag is steered a greater advance can be made to the front (18.4) than when constant courses are steered (14). However, the average speed of the scouts is 21 against 14 and the running speed is 28 against 18.6, thus showing that it is a far less economical method of search. There will, however, be cases where the rapidity of advance is more important than economy and then the zigzag search will be used.
The following table shows the running speed, the average speed of scouts and the speed of advance of a 300 zigzag for various speeds of the enemy and various scouting distances:
Table 3 | ||||
Assumed speed of enemy | Scouting distance | Average speed of scouts | Running speed | Speed of advance of the line |
10 | 20 | 21 | 28 | 18.4 |
10 | 25 | 18 | 24 | 15.5 |
12 | 20 | 19 | 25.3 | 16.7 |
12 | 25 | 16 | 21.3 | 13.5 |
14 | 20 | 17.5 | 23.3 | 15.4 |
14 | 25 | 13 | 17.3 | 11.5 |
?
In the Retiring Search Patrol the leading scout runs the retiring search for the assumed speed of the enemy; it is followed by other scouts at regular scouting distances. For determining the maximum distances at which scouts may be spaced and still prevent an enemy vessel from passing through undetected, the retiring search curve can be considered as a straight line without introducing an important error. In Fig. 7 a scout at the end of the first listening period is at A. Assuming that its average speed is 15 knots it will be at A' at the end of the second listening period. Assuming a speed of 10 knots for the enemy, C will be the most favorable position for a vessel attempting to break through the line undetected. Here it is on the edge of the listening circle drawn from A at the end of the first listening period; it advances in one hour 10 knots to C on the edge of the listening circle drawn drom A'. At the end of the third listening period it wili be at C. At this time the first scout will be at A", well out of listening range of the enemy vessel. In order to prevent the enemy vessel from passing through undetected the second scout must be at the end of its third listening period within 15 miles of C", or at B", the intersection of an arc drawn from C", with a radius of 15 miles, with the line AB. If the second scout must be at B" during the third listening period, it must be at B' during the second period and at B during the first period. AB, the scouting distance, is 43 miles.
The following table shows the scouting distances for various speeds of the enemy and various average and running speeds of the scouts:
Table 4 | |||
Assumed speed of enemy | Average speed of scouts | Running speed | Scouting distance |
10 | 15 | 20 | 43 |
10 | 18 | 24 | 46 |
10 | 21 | 28 | 49 |
12 | 15 | 20 | 42 |
12 | 18 | 24 | 44 |
12 | 21 | 28 | 46.5 |
14 | 15 | 20 | 37 |
14 | 18 | 24 | 39.5 |
14 | 21 | 28 | 41 |
In Fig. 8 there is shown an example of search by the Retiring Search Patrol Method. At 6 p. m. an enemy force was reported at O. Assuming a maximum speed of ten knots for the enemy it is required to search the sector XOY. At 6 a. m. the next morning four scouts spaced at 40 miles are in position on the arc A1-D1, drawn with a radius of 120 miles from O as a center. At 6 a. m. the retiring search patrol commenced at 15 knots, the scouts reaching position A2-D2 at 6 p. m. During the night the scouts retire at 10 knots, reaching at 6 a. m. the position A 3-D 3. Then the retiring search is taken up again, the scouts arriving in position A4-D4 at 6 p. m. Therefore, after 36 hours of search at an average speed of 13.3 knots the required sector has not been covered.
If listening devices could be used to continue the search during the night, the scouts by running at an average speed of 15 knots with a speed of 20 knots while under way, could have reached the position A5-D5 by 6 a. m., thus completing the search of the required sector. Therefore, it will be readily seen that the continuous search by the Retiring Search Patrol is far more efficient than the usual method in which searching is attempted only during daylight.
But it is not only in searching that listening devices will be of value. They can be used effectively in all night operations and during daylight when the visibility conditions are poor. When an enemy vessel is once heard during a night search a run of 30 minutes in its direction should bring it within the range of the listening tubes installed in the hull. Many of these devices can be used while the vessel is still under way and even if it is necessary to stop an accurate bearing can be taken in less than three minutes. When this close the class of the enemy vessel can be determined. If it is decided to attack, our vessel can be guided into position by means of the listening devices and the attack with guns or torpedoes can be made with the advantage of surprise on our side. If it is decided not to attack, then the enemy vessel can be easily avoided and its position can be reported so that, if considered desirable, superior forces can be concentrated upon it. In this way enemy forces can be reconnoitered during the night and their positions can be reported without ever getting within sighting distance. This will allow destroyers making a night torpedo attack to be guided to their targets and will facilitate the surprise of enemy surface craft by superior forces at daylight.
Listening devices will also be of great value in screening operations. At Key West a great number of practice attacks were made by submarines of the K class on surface craft escorted by submarine chasers; the chasers stopped to listen for three minutes in every twelve. In every case the position of the attacking submarine was accurately determined by listening bearings before it could reach a favorable firing position. In the same way surface craft escorted by vessels equipped with listening devices could be warned of approaching destroyers attacks in sufficient time to avoid them.
Listening devices will undoubtedly play an important part in naval warfare in the future. Therefore, every effort should be devoted to the development of new and improved devices and to the determination by actual practice of the ways in which they may be used to the greatest effect.
?