The use of convoys to protect merchant ships from pirates and privateers probably dates back to the earliest days of sending goods from one place to another by ship. The use of convoys in modern times dates back to 1917 - when the British Royal Navy realized it needed to do something to stop the German navy from starving England into submission during WWI.
A convoy has been
defined as "one or more merchant ships sailing under the protection of one or more warships." A convoy can consist of a number of unescorted ships, too.
In its history perhaps no country has been as dependent on the free flow of materials to its shores as
England and no country in more danger of having that lifeline cut:
Early in the Seventeenth Century, while confined to the Tower of London, Sir Walter Raleigh wrote: ‘There are two ways in which England may be afflicted. The one by invasion ... the other by impeachment of our Trades’. On this Sir Julian Corbett has commented: ‘Herein lies the raison d’être for British sea power throughout the ages. Trade protection and security from invasion both depended on sea power’. As will become apparent in the ensuing account, in the last two World Wars, through insufficient attention to trade protection, Britain was nearly defeated even though she was the dominant naval power and under no serious risk of invasion.
The English use of convoys to protect its trade was already 400 or more years old as World War I broke, but at the start of World War I, however, there was reluctance on the part of the British Royal Navy to institute a convoy system for its merchant fleet. See the discussion in this article titled
"Losing the Initiative in Mercantile Warfare: Great Britain's Surprising Failure to Anticipate Maritime Challenges to Her Global Trading Network in the First World War".
One school of thought is that reluctance was because of new fast, powerful battleships, as set out
here:
In the early 20th century, the dreadnought changed the balance of power in convoy battles. Steaming faster than merchant ships and firing at long ranges, a single battleship could destroy many ships in a convoy before the others could scatter over the horizon. To protect a convoy against a capital ship required providing it with an escort of another capital ship; a very high cost.
Battleships were the main reason that the British Admiralty did not adopt convoy tactics at the start of the first Battle of the Atlantic in World War I. But by the end of 1914, German capital ships had largely been cleared from the oceans and the main threat to shipping came from U-boats. From a tactical point of view, World War I-era submarines were similar to privateers in the age of sail: only a little faster than the merchant ships they were attacking, and capable of sinking only a small number of vessels in a convoy because of their limited supply of torpedoes and shells. The Admiralty took a long time to respond to this change in the tactical position, and only in 1917 . . . did they institute a convoy system. Losses to U-boats dropped to a small fraction of their former level.
Another
analysis points to three other reasons:
At the outbreak of World War I the Admiralty was prompt to introduce the convoy system for troop transports. But while convoys were instituted for troop transports, storeships and other special ships, and while the Grand Fleet never left harbour without an extensive anti-submarine, anti-torpedo-boat screen of escorting destroyers, not until May 1917 was any attempt made to protect the bulk of Britain’s extensive ocean-going merchant shipping through the traditional means of convoy and escort. How does one explain this curious paradox in the light of past history?
It stemmed from three misconceptions at the Admiralty which were shared to a greater or lesser extent by the Government of the day at the outbreak of the war.
Firstly, there was an obsession with the ‘decisive battle’ concept, the conviction that the main function of the Navy was not home defence or trade protection (which were essentially ‘defensive’ measures), but to seek out and destroy the enemy’s fleet (which was ‘offensive’). This thinking persisted right through into 1918 even though as early as 23 September 1916 the Admiralty was writing to the Commander-in-Chief of the Grand Fleet: ‘The British Fleet is vital to the success of the Allied cause. The German Fleet is of secondary importance; its loss would not vitally affect the cause of the Central Powers ...’
Secondly, there was the notion that as the dominant naval power Britain had nothing to fear from a weaker naval power which resorted to the guerre de course, or commerce raiding. But there was a fallacy in this argument. The guerre de course had never been attempted against a power so vulnerable to it as Britain now was. Britain imported nearly two-thirds of her food supplies, all her oil, most of her iron ore and other minerals and metals except coal.
Thirdly, while it was generally recognised in 1914 that the submarine posed a distinct threat to warships, with the notable exception of Admiral of the Fleet Lord Fisher, a former First Sea Lord, there was a complete failure to recognize its possibilities as a commerce raider. Here Admiralty thinking was in accord with that of the German Secretary of State for the Navy, Admiral von Tirpitz. He saw the submarine only as a defensive weapon for German harbours and their approaches. The early war successes of the U-boats surprised him.
The possibility of German submarines sinking merchantmen without warning was discarded in the pre-war Royal Navy as ‘impossible and unthinkable’. Churchill, on 1 January 1914, stated that he did not believe ‘this would ever be done by a civilised Power’. He was not alone in this error.
On the other hand, Lord Fisher, in a memorandum to Churchill in January 1914, was remarkably prescient. He pointed out that the submarine ‘cannot capture the merchant ship; she has no spare hands to put a prize crew on board; little or nothing would be gained by disabling her engines or propeller; she cannot convoy her into harbour; and, in fact, it is impossible for the submarine to deal with commerce in the light and provisions of international law ... There is nothing else the submarine can do except sink her capture’.
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The truth of the matter was that naval thought was focused too exclusively on battle and too little on the protection of shipping. In spite of Jellicoe’s memorandum to Balfour of 29 October 1916, it was still not appreciated that lack of shipping could lose the war without a single major engagement at sea.
Finally, however, with the resumption of unrestricted submarine warfare by the Germans in 1917 the need for merchant convoys became very apparent and their establishment directed by the Admiralty.
In addition to setting up a convoy system, these convoys were directed, under certain conditions to weave their way from their point of origin to destination instead of traveling in a predictable straight course. The idea of the weave was to make it harder for the German submarines to predict exact convoy routes even if the time for the transit of the convoy was increased by the time spent moving back and forth across a base course.
This weaving pattern became known as
"zigzagging":
Zig-Zagging amounts to the main body of the convoy simultaneously steering 
predetermined courses for various lengths of time, which would prevent a submarine captain from easily determining the true course of the convoy. Changes in course were made at specific intervals on the clock thus eliminating any visual or electronic signal, which might alert the submarine captain to an impending change. Each convoy has one ship as a Guide-on. That ship is responsible for precise execution of the zig-zag plan and all other ships in the main body must also execute precisely the same zig-zag maneuver to insure maintaining the formation. The Guide-on was generally one of the foremost ships in the main body. Several standard Zig-Zag plans were available for use and they usually were changed daily or as needed.
Make no mistake, the zigzag of WWI was meant to evade or avoid
attack:
The primary objective of anti-submarine tactics is to destroy the enemy submarines. But, since it is easier and wiser for the larger vessels, transports, and merchantmen to evade the attack, every effort should be made by them to practise tactics of evasion to supplement tactics to destroy. In convoy the latter can much better be conducted by the escorting destroyers.
Zig-zag tactics make attack difficult. Also, a quick manoeuvre the instant a periscope or torpedo is sighted will often save the ship. Alert seamanship is, therefore, a main reliance of capital ships in avoiding submarine attack.
As the Americans entered the war, they joined in the newly established convoy system (see
here for the U.S. Navy publication "Remarks on Submarine Tactics Against Convoys" - a 1917 reprint of a British publication):
To a submarine approaching a convoy, zigzagging has a very confusing effect.
If she decides for the flank attack, she has almost the same difficulties to contend with as when attacking a single ship zigzagging. If attacking the centre, it is much more difficult to decide how the columns are disposed and the intervals between them. When close to the leading ships, she can only afford to show her periscope at fairly long intervals, and must always have the uncomfortable feeling that, owing to an alteration of course, she may find herself right ahead of a ship when she looks again (vide "Hearing Power," Section IV).
As long as there is sufficient light for the submarine to make out the position of the several ships nearest him, whether by night or in thick weather, zigzagging adds greatly to the difficulties of attack and consequently forms a valuable defence.
Another description of the
process sets out the odd beauty of the formation zigzag:
This mighty collection of vessels, occupying ten or twelve square miles on the ocean, skillfully maintaining it formation, was really a beautiful and inspiring sight. When the destroyers had gained their designated positions...the splendid cavalcade sailed boldly into the area which formed the favourite hunting grounds for the submarine.
As soon as this danger zone was reached the whole aggregation, destroyers and merchant ships, began to zigzag. The commodore on the flagship hoisted the signal "Zig-zag A," and instantaneously the whole thirty-two ships began to turn twenty-five degrees to starboard. The great ships, usually cumbersome, made this simultaneous turn with all the deftness, and even with all the grace of schoool of fish into which one has suddenly cast a stone. All the way across the Atlantic they had been practicing such an evolution; most of them has already sailed through the danger zone more than once, so that the manoeuvre was by this time an old story. For ten or fifteen minutes they proceeded along this course, when immediately, like one vessel, the convoy turned twenty degrees to port, and started in a new direction. And so on for hours, now a few minutes to the right, now again straight ahead . . . This zigzagging was carried out according to comprehensive plans which enabled the convoy to zigzag for hours at a time without signals, the courses and the time on each course being designated in the particular plan ordered, all ships' clocks being set exactly alike by time signal.
In a zigzagging convoy, every ship ship must be on the same page as all the other ships in the convoy and the execution must be simultaneous to prevent collisions among the convoy ships. As set out
here:
"The ship escaped the torpedo by zig-zagging." How many times that sentence has appeared in the newspapers! Yet how few persons realize all that that meant! A ship steaming along by herself can change her course at irregular intervals without bothering anyone or without interfering with the plans of anyone unless it be the plan of a submarine commander. However, make up a convoy of a number of ships of different nationalities. On one trip when the George Washington was flagship of the convoy, there were thirteen vessels with over thirty-five thousand men on board to be landed in France. There were vessels representing six different nationalities—there were "native-born" American vessels, "naturalized" ex-Germans, both merchant ships and a former raider, a Hollander, all under the Stars and Stripes, and there was a British vessel, a Russian manned by the British, and an Italian,—the last three chartered to hel
p carry our men over. The British and Italian were under their own flags and officers, and with two American naval officers and a signal force and perhaps guns' crews furnished by Uncle Sam, so that there was always a naval representative to see the convoy orders carried out.
When passing through dangerous waters, or even when approaching possibly dangerous waters the zig-zag was continuous so long as there was light to see a certain distance, for even a darkened ship looms up a long distance, on a night that is not cloudy and overcast.
Imagine five ships in line and 800 yards apart, also a ship 800 yards astern of each of the first line, and then three more ships astern of the middle of the second line similarly spaced. Imagine these three lines of ships with about forty-two thousand souls in all on board going through submarine waters, and each ship zig-zagging. The changes of course must be made simultaneously. A special clock known as the "zig-zag clock" on each ship had to agree to the second with the zig-zag clock on every other ship. Each ship must put the rudder over on time to the second;—each ship must make her turn of twenty or thirty or forty degrees at and in the same time interval or a collision may result. The zig-zag must be begun as dawn breaks or the moon rises, and must continue until darkness is established. It is not dark on a cloudless night. The "Northern Lights" may be, and on some occasions were, as bright as moonlight. The glow of a cigarette may be seen half a mile at night at sea.
How was all this allowed for? What care had to be taken in the preliminary plans to ensure mutual understanding of the plans of the convoy commander? How were the other vessels to know when to start or to stop zig-zag? Suppose one ship broke down? What should be done in case of attack by a raider or by a submarine? No lights could be shown at night, even for signaling. The use of the radio was restricted, even of the "toy" sets that could not carry over five miles. What do in case of an alarm in any direction?
The Commander Cruiser and Transport Force, Rear (now Vice-) Admiral Albert Gleaves, provided for many contingencies in his "Orders in Convoy," but realizing that the man-on-the-spot should not be hampered by too many cast iron orders left much to the initiative of the convoy commander. The results, he has been kind enough to say, have justified his confidence in his commanding officers.
Even now, it would probably not be wise to go too deeply into all the details of the conferences of commanding officers that were held before the convoys sailed. This was discussed, and that was discussed, and the final results were embodied in the orders issued to each ship. The convoy group sailed from New York; at a certain time a ship from Philadelphia dropped into place; at a later time a number of ships from Newport News followed suit; their places were known before they sailed. Dangerous waters were near; "Jig No. 3" was shown by flag signals. Al l ships repeated it; down came the first signal, and at the order from the Officer of the Deck on each ship, the group of ships simultaneously began the first (?) leg of a certain "zig", which each ship followed almost automatically" and to the second until further orders, or until an alarm when each ship—did something else, —and each ship knew what to do! It 
became foggy, so thick it was difficult to see the ship next to you in formation,—a pre-arranged signal by whistle or by "buzzer" and each ship steered a straight course until the weather cleared. A heavy rain,—the same thing happened. One night while in particularly dangerous waters—ships had been reported as torpedoed there the day before—the group was zig-zagging when the weather became thick. If the zig-zag was stopped the group would make land (or rocks) before daylight; speed was reduced there was more danger from the submarines; in any case the destroyers on escort duty were in danger. It can readily be imagined how each captain kept peering first at the place where one ship should be to see if it was still there, and then on the other side to see if the other one was also in position. If his vessel was not the "guide" a change of a revolution or two on the engines kept that vessel in place. And all this time at certain intervals the vessels of the group would turn simultaneously onto a new leg of the zig-zag. Very frequently a heavier bit of rain or a thicker mist and hardly the bow of your own ship could be seen, yet the zig-zag continued whether the other ships were in sight or not. Yet when it lighted a bit, there was that dark mass with a white wave at her bow, and a white wake astern, just where it should be. The officers on deck did their part, whfle those in the engine room saw that the revolutions of the engines did not vary one-tenth of one per cent from the speed ordered. Team Work! And all the time there was that constant vigil by nearly a hundred lookouts for that little white "feather" that a periscope makes. Eternal Vigilance is The Price of Safety.
And so through the long night. There may have been a few more gray hairs when dawn broke on the heads of the captains, but there was a feeling of relief when the ships anchored in harbor in France in the forenoon. One more trip "with the goods", i. e., Yankee soldiers, was behind.
At the Flag Office where the captains reported later for orders, one was asked the question,—"What sort of a trip?" "Oh, pretty fair." "Zig-zag last night?" "Sure. When do we start back?" and so the game went on.You might have noticed in the discussions above mention of the "zig-zag clock" - the purpose of which was to ensure that precision in the time of changing course demanded in a close formation. An example of such a clock from WWII is shown nearby. These clocks functioned as
follows:
To confuse U-boats, convoy clocks were 
used in both World Wars to coordinate the simultaneous zig-zag maneuvering of large groups of vessels, often out of sight, signal or radio contact with each other.
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The clock was wound by hand every six to eight days; the batteries powered a signal bell or buzzer that was mounted elsewhere. The single hand swept by the moveable indicators mounted around the rim, arranged to the convoy's pre-determined turn intervals, thereby connecting the circuit to the signal to let the bridge know when to alter course onto the new leg of the zigzag.
One question that needs to be asked is whether zigzagging actually worked to reduce attacks or to interfere with U-boat targeting. While a later post will discuss the mathematics of convoys, the short answer seems to be that given WWI submarine technology, there may have been some advantage gained by the zig-zag in that it took away the ability of a sub to get out front of a convoy as wait for the convoy to bear down on the sub. In addition, given the relatively short range of the torpedoes of the day, it may have interfered with targeting solutions. On the other hand, if, on occasion you "zig" away from trouble, there will be occasions when you "zag" into trouble.
A U.S. Navy "Analysis of the Advantage of Speed and Changes of Course in Avoiding Attack by Submarine" can be found
here. It makes some interesting points:
81. Considering all the points covered in the discussion of speed, changes of course, and weather and visibility conditions, the following conclusions are reached: (a) Every endeavor should be made to increase the speed of all vessels when passing through the danger zone. (b) The most important increase in speed is that at about the submerged speed of the submarine; i.e., 7 to 9 knots. (c) Zigzagging is in some respects disadvantageous, in that the chances of being sighted are increased and the danger areas increased. (d) The 
advantages of zigzagging, due to the interference with the submarine's calculations for reaching position desired for the attack and for firing, outweigh the disadvantages. (e) In zigzagging large changes of course are disadvantageous as by making such the danger area is increased and the time within the danger zone increased. (f) That changes of course should be made at irregular time intervals, which should not be much less than the time required for a direct attack from ahead or much more than the time required for the normal attack from the limiting position. For speed of ship 12, and submerged speed of submarine, these times are, minimum 10 minutes, maximum 20 minutes. (g) That the changes of course must be sufficiently large to cause a material error in the calculations of the submarine's commander. The angle required should not be less than 20° or over 40°, except that if changes are made, at intervals of time not greater than one-half the maximum time interval for the speed in use, 10° changes may be made. (h) That in view of the possibility that a
submarine may be able to predict changes in courses and to determine the base course from a series of several observations when the zigzag is duplicated each hour, this practice should be abolished and the zigzag so constructed that no two successive hours are similar. (i) In order to make the determination of the base course impossible, the zigzag should extend over a period of at least half a day, the apparent base course being changed at intervals varying from 40 minutes to 1 hour 20 minutes. (j) In constructing zigzag diagrams, possible conditions of sun glare, prevailing wind, and position of light areas during twilight should be considered, and the zigzag selected so as to afford the greatest advantage with regard to light, wind, and sea conditions.
Painting: A Convoy in the First World War - A painting by Herbert Barnard John Everett illustrated in Bernard Ireland's "The War At Sea 1914-1945"
Update: Added some more art.
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