Why do ships zigzag

By an examination of the danger areas it has been determined that a ship should, in order to be sure that the data upon which the submarine is basing its attack shall be changed at least once during the attack, change course at intervals of time which should not greatly exceed the ordinate shown in minutes of the point on the middle curve of Figure XVI, facing page 21, corresponding to the speed of the vessel.

There still remains to be considered the effect on the danger areas, a of the extent of the changes in course, b of the length of time steamed on each course. If the vessel continued on the course A E she could be successfully attacked by a submarine lying, at the time the ship was at A, anywhere in the area BCA'. If upon reaching E the vessel is to change course and head for F, a certain portion of this area is eliminated as a danger area because the submarine to attack at E must have been within a distance eight-twelfths of AE at the time the vessel was at A.

A circle about E with a radius eight-twelfths of AE is the limit of the danger area for an attack at E. The crosshatched area X is the area eliminated as a part of the danger area by the change of course at E. Now, had the course not been changed, a submarine in this area could not have attacked.

These diagrams show that to avoid a submarine whose position is unknown by making large changes of course is impracticable. Large changes of course increase the danger area. It has been suggested that the zigzag should be such as to throw out one-half the danger area each time the course is changed. If, however, the area added to the danger area by each change of course is greater than the area eliminated, and if this ratio is greatest with the largest change of course, then it can be no advantage to zigzag from the point of view of reducing the danger areas; in fact, the danger area is increased by zigzagging.

There is but one other factor to consider as affecting the danger areas. This is the time on each course. It has been shown previously that it is disadvantageous to remain on one course long, but that was the result of consideration from a different point of view. A indicates the danger area if the ship steaming at 12 knots changes course each 15 minutes.

B is the danger area if course is changed each 20 minutes. In each case it is noticeable that the danger area decreases as the time on each leg increases. From these diagrams it is determined that- a Zigzagging increases the danger areas.

From all the foregoing discussion, the zigzag appears to be disadvantageous. Its only advantage of consequence lies in the chance that the course will be changed during the time the submarine is making its submerged attack and thereby the estimate of the submarine's commander will become changed and in order to fire the torpedo the final calculations will be rapidly made and with a high chance of error.

The difficulties of estimating course and speed from a periscopic view are known to be great and frequent changes of course irregularly made, and without following a fixed sequence which is repeated frequently, are undoubtedly the greatest factors in confusing enemy submarine commanders. It has been shown that large changes of course increase the danger areas and the time in the danger zone.

The angle through which the course is to be changed should therefore be reduced to as. There is little hope that changes of course in zigzagging will be any real protection against torpedoes actually fired and unseen. The range which the enemy submarines attempt to reach is between and yards and at such distance a well-aimed shot will not miss no matter what is done by the ship. That zigzagging - a Increases the chance of being sighted; b Increases the time in the war zone; c Increases the danger area; d Has little effect in avoiding torpedoes properly aimed; but e The advantages gained by confusing the commanders of enemy submarines in obtaining the desired position for firing and in making their calculations for firing are such as to overcome the disadvantages.

In zigzagging it is advantageous-- a To change course at intervals approximately equal to the average time required for attacks a from ahead-- b for the normal attack from the limiting position. Fulfilling the conditions required by the previous decision, a knot-vessel should use a zigzag somewhat of the following nature: a Changes of course at intervals varying from 10 to 20 minutes.

Figure XX shows a diagram of a zigzag for one hour which fulfills the above conditions. It is apparent that the duplication of a zigzag of a construction as simple as this would hardly complicate the operations of the enemy submarine in so far as obtaining a position from which he could fire a torpedo is concerned. The more simple the zigzag, the more important it is that it shall not be duplicated.

Even if the zigzag is not duplicated, a simple zigzag constructed upon the same base course for two or more successive hours can not be a serious detriment to the enemy submarine as far as reaching a. In order then to reduce the chance of the enemy predicting future changes of course, or even a prediction as to the base course, it is desirable to make the zigzag extend without duplication over a long period of time, preferably at least one-half of the hours during which it is contemplated zigzagging on any one day, and the zigzag to be such that the apparent base course changes at intervals varying from about 40 minutes to 1 hour 20 minutes.

The effect of this will be to confuse the enemy submarine as to the base course and as to. Figure XXI indicates the track of a vessel for 2 hours 40 minutes of such a zigzag.

The navigational factor will be reduced somewhat by the wide departure from the base course, but the difficulties encountered by the enemy in predicting future changes of course, or the base course upon which the zigzag is constructed, will be so increased as to warrant the reduction in virtual speed imposed.

Additional factors which play an important part in successful submarine attacks are the conditions of sun glare, light, wind, and sea. A few rules which submarines are supposed to follow and which observations have confirmed are given to indicate the importance of the consideration of these factors in zigzagging. Even the track of bubbles will probably not be seen until it is too late to avoid the torpedo. The ship appears as a black silhouette and the bow wave and wake completely vanish in the glittering water.

RULE: Attacks at night in smooth water from awash or surface condition have proven effective. Clear twilight offers very favorable light conditions for shots from the submerged condition. REASON: With a favorable background with reference to clouds and moon a submarine can approach on the surface to within yards without being sighted. Under these conditions a ship would probably be visible 1, to 1, yards.

In twilight attacks a submarine will endeavor to attack on the side away from the bright background. At morning and evening twilight a ship should steer so that the light background covers the danger angle. When in cloudy weather, light conditions during the day approach to normal twilight conditions, the same remarks apply.

A periscope moving to windward will cause a large amount of spray even when moving at the slowest speed. Lookouts to windward should be increased and frequently relieved. RULE: In a light breeze a submarine may attack from either windward or leeward side. If the attack is from windward, a torpedo fired is probably a bow shot.

If from the leeward, it probably is a stern shot. REASON: In a light breeze the submarine will travel with the wind and sea, therefore, from windward it will use its bow tubes; from leeward, its stern tube. REASON: In a calm sea a periscope will generally be sighted before the submarine reaches firing range, and if the torpedo is fired it can be seen sufficiently early to be avoided in most cases.

RULE: Force of wind and a whitecap sea present the most favorable conditions for submarine attack. REASON: a The sea just washes over the periscope if the latter is shown sparingly, and, without masking the view from the periscope, makes it difficult for the ship to recognize the periscope. RULE: If the wind is and the sea moderately rough, it will be difficult for the submarine to fire. If she does, the torpedo will probably come from a direction parallel to the trough of the sea.

REASON: From any other direction it is difficult to reach firing range without exposing the conning tower or at least a large portion of the periscope. The torpedo is apt to broach unless fired in the trough of the sea also its propellers may break the surface and give away its position. To avoid giving the submarines these favorable conditions a ship should: a Avoid steaming on such a course that the bearing of the sun will lie within the danger angle; b when possible, steam with the sea; c at twilight, steam toward the brightest part of the horizon.

Considering all the points covered in the discussion of speed, changes of course, and weather and visibility conditions, the following conclusions are reached:. For speed of ship 12, and submerged speed of submarine, these times are, minimum 10 minutes, maximum 20 minutes. Naval History and Heritage Command. Print Friendly. The Sextant. Social Media. Toggle left navigation Nav.

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The answer seems to be yes, even after the Germans developed acoustically guided torpedoes that could home in on ship engines, regardless of course changes. But the advantages of zig-zagging may have been marginal compared to the enormous effort it took to implement.

While the analysts determined that, on balance, zig-zagging was better than straight line sailing, they also noted that it slowed the movement toward the destination, thus exposing ships to longer times in danger zones. The increased space covered by zig-zagging ships was also hazardous, giving enemy submarines more opportunities for attack. All manner of guided missiles and torpedoes have rendered zig-zagging convoys of ships obsolete. The Zig-Zag Clock, once an indispensable tool of war, is now an obscure relic of a bygone age of sea power.

Image from Royal Museums Greenwich. Recent Posts. Go to Top. You took an observation when the convoy was about 2, yards away and all is good. It looks like when you next stick the scope up they will be right where you want them. However Captain Murphy ordered a 60 degree turn to starboard 10 seconds after you put your scope down.

When you raise the scope again expecting to see the bows of several ships, you see the stern quarter getting further away each second. You can travel knots submerged, the convoy is going Launching torpedoes in those days was like firing artillery, except the target is moving and the shell travels in slow-motion. Zig-zagging, i.

WW1 subs had to do that. An unexpected zig or zag in the enemies path screws up everything. The good zig-zag plans were those that were irregular. Sloppy navigators would make a regular pattern, and with enough observation time, a sub captain can figure out that pattern and get in a hit anyway.

Compare the difference in difficulty in completing a long range pass in American football compared to a short one. The longest ranged USN hits were made at yards, IIRC, but the strategic surveys done after the war calculated that the majority of hits were made from to yards. Towards the end of WW2, the U. The entire crew. Subs of the day were small feet or so in length, with a smaller internal volume devoted to crewed spaces , and divided into only a few water tight sections.

Heat, smoke, cooking odors, etc, quickly made their way to other sections of the boat. Without the snorkel, you had to make do with whatever air you had in the boat when you dived, and this gets circulated throughout the boat.

The air in the air tanks was used for forcing water out of the various diving and trim tanks, not for breathing. Most of the British subs did not, as well as the Japanese types, and their crews suffered in the tropical environments of the Pacific, reducing their effectiveness somewhat.

If one ship makes a 60 degree turn away from you, that means another is making a 60 degree turn towards you. And the one that turned away from you is turning into your buddy.

Assuming of course that you are being attacked by a wolf pack, and the other boat is in the correct position, not too close and not too far away. Neither of these are a dead certainty. I see a cumulative effect here. I wonder if any research has been done on the probability of a hit when zig-zagging vs. Coordinate for them meant that all the subs responded if one detected the ship. Coordinated to me means having spotter subs and hunter packs working together.

Another factor was determining the long range route of the convoy. If the convoy just headed in a straight line than any submarine or plane could observe their current course and speed and easily predict where they would be several hours in the future. A quick radio call could then summon other submarines to that point to await the convoy. By zig-zagging a convoy could conceal its true course from an observer.