Commander Richard D. White, Commander, Oriziba, to Rear Admiral Albert T. Gleaves, Commander, Cruiser and Transport Force
U. S. S. ORIZABA
CARE OF POSTMASTER NEW YORK
30 JUNE 1918.
From: Commanding Officer.
To: Commander Cruiser & Transport Forces.
SUBJECT: Zigzagging – Question of desirability of.
1. Since the extension of submarine activity to the Western side of the Atlantic, and the resulting possibility of meeting a submarine almost anywhere on the run between United States and European ports, the question as to what areas the practice of zigzagging should be extended to comes up for decision. I have, during the past year, made considerable study of the question of submarine tactics and my investigations have led me to some curious deductions regarding the effectiveness of zigzagging in general. I beg to submit some of those deductions to the Commander Cruiser and Transport Forces.
2. In order to arrive at any conclusion it is necessary to make some assumption, and I have made the following. Some of them are self evident. I believe them all to be based on fact.
(a) I have assumed the speed of the ship subject to attack as 18 knots, that being the best maintained speed the ship I command can make. This is naturally the case I am most concerned in.
(b) I have assumed the speed of the submarine to be 10 knots, that being generally accepted as the best submerged speed of the enemy type. I consider only the submerged speed because it is generally accepted that a submarine in attacking a well armed ship will run submerged from the time the target is visible until it succeeds (or fails) in attaining a position from which the torpedo can be fired with possibility of success. I assume that it will submerge immediately upon sighting the object of attack if not already submerged, because a more efficient lookout system is kept on board transports than can be kept on board a submarine, and any exposure on the part of the submarine renders it likely to be sighted and the attack frustrated, not to mention the possibility of its being itself destroyed. It will expose the periscope only often enough to take necessary observations.
(c) I have assumed that in clear weather the maximum range of visibility between the target ship and the submarine to be sixteen miles. This is based on the height of the mast of this ship above water, namely, 60 feet, and the height of the lookout on the submarine, 25 feet. As a matter of fact the height of this ship’s masts is 83 feet but something must be above the horizon before anything can be seen. I have allowed three feet for that.
3. Assuming then the speed of a ship which a submarine wishes to attack to be 18 knots and the speed of the submarine while proceeding to attack to be 10 knots and the distance at which a submarine may sight the ship to be attacked as 16 miles, there results a certain definite area about the target ship in which the submarine must find herself before beginning the attack in order to succeed; otherwise the attack is mathematically sure to result in failure. Referring to Figure 1 in which the target ship is represented at 0. This area is bounded by the radial line A0 and B0 and the arc AB. The lines A0 and B0 are drawn at an angle of 34 degrees to the left and right respectively of the course of the target ship. The arc AB is drawn with a radius 16 miles, the assumed distance of visibility. A submarine finding itself anywhere within this area under the conditions assumed is in a position to make a successful attack for the following reasons:
(a) If she is within the angle A0B but outside (ahead of) the area enclosed by the arc AB, she is beyond the radius of visibility, she does not see the target ship, and presumably is ignorant of its presence.
(b) If she is anywhere outside the angle A0B, she will not be able to attain a position from which the torpedo may be fired with a possibility of success, because of the superior speed of the target ship. As a matter of fact, the boundary of the dangerous area takes a curved form in the immediate vicinity of the target ship and extends well past the beam on either side, this being due to the superior speed of the torpedo while running, but this is outside the question under discussion.
4. The target ship as she steams along always carries ahead of her this dangerous area. It forms a zone or swath or lane. In the case assumed this zone is 17.8 miles wide. Any submarine intercepted by this path as it sweeps along is placed instantly within the area from which an attack may be successfully delivered. Naturally were the visibility less, the zone would decrease in width until with complete invisibility its width would vanish and the ship could be said to be immune from attack. Conversely, any reduction in the speed of the target ship causes the width of the zone to increase. A decrease of 1 1/2 knots increases the width by 1.4 miles, approximately 8%. This increase varies with the decrease in speed, but continues until at any speed under 10 knots it reaches its maximum, namely, twice the range of visibility, 32 miles. Any further decrease does not increase the width of the danger zone, but it does make the attack more likely of success.
5. It will be seen from the foregoing statements that even after a submarine has sighted an 18 knot ship, the chance of its being able to attain a position from which a torpedo may be fired with a possibility of success is not over good, viz: 17.8 to 32. However, the method of procedure on the part of the submarine most likely to bring success is comparatively simple, and is the same in every instance. All it is necessary to do is to bring the target ship abeam, then stand on this compass course at best steady speed, taking bearings from time to time with the periscope. If the submarine holds its bearing or gains on it, it is in a way to attain success. If it loses bearing it is destined to fail of attaining the desired position.
6. Not only is this the simplest and surest way of attaining position in the assumed case. It is practically the only way. Anyone who has maneuvered with a slow ship in reference to a faster one realizes how easy it is to lose position and how impossible it is to regain it once lost. In the large majority of cases the submarine is inferior in speed to the ship attacked from the time it dives. It therefore cannot afford to take any chance of losing bearing. The only way to assure using every bit of its own speed to maintain bearing is to proceed as above described. Of course if the conditions are such that the object cannot be attained it will be lost by this method; but it would have been lost by any other method and if there had been any chance whatever of attaining it, this method would have done so, for it utilizes all the speed at its disposal to that end. Any variation therefrom fails to utilize to the fullest extent that speed.
7. If the position when the attack is begun is such that bearing is gained, the submarine is then free to maneuver with some freedom as long as it stays within the effective area already described. Once outside that area, however, all chances are lost. It is therefore not believed that submarines vary greatly from the foregoing procedure simply because of this ever present danger of losing position and therefore losing all chances of success in attack. It is definitely known that whenever possible they try to gain on the bearing till they get directly ahead of the target ship, as indicated by masts being in line, before any liberties whatever are taken with the chance of losing the bearing.
8. The principle involved in the foregoing statement is best explained by referring to Figure II. Lay off the line AB with a length, using any desired scale, representing 18 knots. Draw a line from A through the points C, D, and E, the line AE being at 34 degrees from AB. Draw perpendiculars from these lines through B. The line AB represents the speed of the target ship. The lines AC, AD and AE represent the bearings of the submarine at the points C, D and E, respectively. The element of the target ship’s speed that tends to change the bearing is measured by the perpendicular through the point B on the line of bearing in question. That of the submarine’s speed is represented similarly. In the cases in question it is the entire speed, 10 knots, because it is assumed that she will place the target ship abeam and run her course at right angles thereto, so the course itself will be the perpendiculars to the line of bearing. Measuring 10 knots on each of the lines BC, BD and BE from their point of interception with the line of bearing towards the point B it will be seen that the distance on BC will extend to beyond B, while that on BD will not extend to B, and that on BE will extend exactly to B. In short, the line AE is the critical angle, 34 degrees, explained in paragraph 3. If the submarine be inside this line, as at C, when he takes up the attack as above described he will do better than keep his bearing, and will have some leeway of maneuvering later on. If he is outside of this line, as at D, he will lose bearing and will never be able to attain the desired position. If he is on the line AE he will just be able to maintain his bearing, and should get in. Incidentally it may be said that if the submarine is in line AE he will, in the case in question, come on at the rate of 15 knots per hour, the distance AE measured on the scale employed. If on the line AC at a greater rate, depending on the position.
9. The above conclusions are based on the assumption that the target ship stands on her course during the time of attack – does not zigzag. What happens in case the target ship is zigzagging? In this case we need only consider the movement of the target ship with reference to the base course for, no matter what particular leg the ship is on when the attack is begun, the calculation can be made with reference to the base course just as in navigational computation, we consider only a fixed percentage of the actual speed along the base course, disregarding the small changes to the right or left. In the case of zigzag plan 3 a ship steaming at 18 knots loses, according to the traverse tables, one knot per hour. It may be assumed that she loses also 1/2 knot per hour due to the retarding effect at the turns. Her speed along the base course then may be considered as 16 1/2 knots. (It may be said incidentally that the width of the dangerous zone is increased by this reduction in effective speed by 1.4 miles, to 19.2. The angle bounding the area of vulnerability is increased from 34 to 37 degrees, on either side of the base courses).
10. Let us take the condition most difficult for the submarine. Consider that the submarine is just at the limiting angle when it begins the attack. Consider that the target ship is just starting to head away 30 degrees from the base course in the direction away from the submarine when the attack begins. For 10 minutes the submarine will lose bearing, for the next 10 minutes when the target ship heads in 20 degrees toward the base course, the submarine will still lose bearing but less rapidly. When the target ship changes to the next course, 10 degrees from the base course in the direction toward the submarine, the submarine will gain bearing slowly. When the target ship heads 20 degrees further toward the submarine, the submarine will gain bearing more rapidly. And so on until the cycle is finished, and it will be found the bearing has been maintained just the same as if the target ship had been steaming on the base course at 16 1/2 knots. The submarine has been able to attain position and get in his shot just as would have been the case under the above mentioned conditions.
11. Let me repeat. The zigzag, instead of rendering it more difficult for the submarine to attain position for firing, has actually facilitated its doing so. That is to say, it has increased the area from which it is possible for the standard submarine to arrive at position for firing successfully her torpedo, or he has made it possible for a slower submarine to attain that position from areas possible only for the standard one if the target ship steam steady on its course.
12. What other advantages are there that attach to zigzagging? It has been claimed that it is more difficult for a submarine to attain position for firing her torpedo due to the difficulty of estimating the target’s course and speed when the latter is zigzagging. In the first place, this is not borne out by my experience. I find it about as easy to estimate the course and speed of other ships when they are zigzagging as when they are on steady course. Some forms of camouflage appear to increase the difficulty, notably one side of the MOUNT VERNON, but as I have already explained, it is not necessary to make these estimations to gain the desired position. All that is necessary is to bring the target ship abeam, then go ahead and take successive bearings of it while maintaining course and speed. It has also been claimed that it makes it more difficult to estimate the data necessary to put on the torpedo director with the target ship zigzagging, or that if the torpedo is fired set for one course of target ship and the target ship changes to another, the result will be a miss. This I believe to be a fallacy. A ship’s course and speed looks about the same whether or not she has shortly previously changed course; and granting that she changes course to the maximum (30 degrees) the instant a torpedo is fired, if the torpedo runs at 35 knots per hour and is fired at a distance of 1500 yards, the ship would not be able during the time of run (less than a minute and a half) to more than turn on the new course. The chance of having caused a miss thereby is negligible. In short, I cannot by calculation arrive at any advantage or increased safety from attack from submarines, resulting from the practice of zigzagging.
13. On the other hand, there are some disadvantages attaching to the practice. I believe that the lookout is less effective when the ship is zigzagging. Certain it is that the effective speed is reduced. Granting that plan 3 reduces the speed of an 18 knot ship to 16 1/2 knots, the reduction is somewhat over 8%. In other words, the ship is destined to remain exposed to submarine attack 8% longer. Her efficiency as a carrier due to her slower effective speed is reduced somewhat. It may cause her to lose a tide or even lose the chance of getting away with a certain convoy, with resulting delay till the next one sails. The policy of staying as little time exposed to submarine attack as possible is the one that commends itself to me.
14. To sum up, the successful operation of a submarine in the open sea depends upon 3 things. First, the chance of sighting of the ship to be attacked. This is simply a matter of chance. Considering the assumption set forth herein, the standard submarine guards a zone 17.6 miles wide. If the width of this zone varies as heretofore explained, the chance of a ship being picked up by one therefore varies with it. The conditions as regards visibility and the number of submarines disposed across the steamer tracks, provided they are not too closely spaced, all have their effect on this chance. But after all it is merely a question of chance. Second, successful attack depends primarily upon the submarine attaining a position from which it can fire her torpedo with a possibility of success. This is the most important part of this problem and the one to which the principal part of this discussion has been devoted to. The conclusions have already been set forth. Third, it depends upon the successful sighting and running of the torpedo, which I have announced as believing to be unaffected by the practice under discussion,
15. I am not ready to advocate the abandonment of the practice of zigzagging in the face of the confidence in which it is held by so many; but I confess my confidence in it is somewhat shaken by these deductions. I would be glad to have the opinions of some other officers engaged in the transport service on the question.