[An edited version of this article appeared in GURPS WWII All The King's Men.]
In WWI, by 1917 the German U-boat fleet was sinking 30% of all British merchant shipping, mainly due to Admiralty incomprehension of the U-boat menace. At the brink of Britain's defeat by starvation, institutional paralysis was overcome and convoy operations (a technique dating back to before the Napoleonic wars) were introduced. Losses immediately fell to less than 1%. Shaken by this brush with catastrophe, the Admiralty was ready with convoy plans the moment WW2 began.
There was a major problem with the Admiralty's plans: there weren't enough escorts. This was partly because of Britain's failure to build enough warships before the war, and partly due to a reluctance to divert destroyers away from defence of the home waters after the Fall of France. RN losses associated with the Fall of Norway only exacerbated the problem.
Further problems were caused by the insistence of some in the Admiralty (and Churchill himself) that convoy ops were too defensive, and that the way to carry the battle to the enemy was for convoy lanes to be patrolled by "hunter-killer" groups. In the event, the patrols were totally ineffective, their proponents having severely overestimated the capabilities of ASDIC and underestimated the U-boats' ability to avoid the patrols.
At first the lack of escorts was dealt with by only providing them east of a demarkation line. Merchants heading west would travel in convoy formation to 12.5° West, then disperse, the escorts then meeting up with convoys (usually defended across the Atlantic by a single sloop or armed merchant) heading east. As the U-boats caught on to this, and as more escorts were built, the line was drawn further west, eventually up to 17° West.
But this strategy was only effective as long as the German Navy had no Atlantic bases and the Luftwaffe had no long-range maritime aircraft. By late summer of 1940, U-boats operating from the west coast of France, supported by the long-ranged FW-200 Kondor, were able to range past 17° West deep into the Atlantic, causing chaos among the nearly undefended merchantmen.
In September 1940 Churchill, desperate for warships that President Roosevelt was politically unable to sell at that time, concluded a deal exchanging several military bases in Newfoundland and the Caribbean in exchange for 50 WWI-era destroyers. Although obselete, these proved invaluable, and briefly evened the odds against the steadily increasing number of U-boats. But Dönitz's U-boats had been practicing wolf-pack tactics since the '30s, and when they ran out of unescorted ships were quickly able to adapt to attacking escorted convoys. And in the first half of 1942, after America joined the war, they were able to take full advantage of American over-confidence in their home waters.
This 'Happy Time' for the U-boats coincided with Dönitz's decision to upgrade his naval Enigma machines to four rotors. Up to this point, Bletchley Park had been making sporadic progress on the three-rotor Enigma, usually decrypting daily keys within a couple of days, sometimes quickly enough to steer convoys around lurking U-boats, and always adding to the store of Allied intelligence about U-boat operations. With the four-rotor Enigma in operation it could take over two weeks to break a single day's key, if at all, a condition that lasted almost a year.
The peak of the Battle of the Atlantic came in March and April 1943. The number of U-boats in the North Atlantic reached its peak, as did the number of losses among the convoys. The Admiralty was on the brink of giving up the convoy system in desperation when losses abruptly fell. This was due to the combination of several factors coming into play at nearly the same time: most escorts and maritime patrol aircraft were now fitted with centimetric radar, making it possible to spot surfaced U-boats at beyond-visual range; escort carriers had made their first appearance; escort crews were gaining in experience, while the U-boat fleet was losing its veterans to attrition; convoys were increased in size, reducing the chance of them being spotted and allowing more escorts per convoy; convoys, escorts, and patrol aircraft were at last working effectively together; and Bletchley Park had emerged from its naval Enigma blackout, thanks to a timely pinch of Enigma keys and American-built high-speed decoding machines.
During the first 22 days of May 1943, 31 U-boats were sunk; an unsustainable loss. On 24th May, Dönitz ordered the U-boats withdrawn from the North Atlantic convoy routes, so conceding victory in the Battle of the Atlantic to the Allies.
The ideal convoy was composed of around 35 (later fifty or more) merchantmen escorted by five or more warships. The formation would cover 5 square miles, and would be arranged in a block formation, say seven columns of five ships each. This reduced the size of the target that the side of the convoy presented to U-boats.
Fast convoys, designated HX, were comprised of ships with speeds between 10 and 17 mph [9 and 15 knots]; slow convoys, designated SC, had a minimum speed of 8 mph [7.5 knots]. Other convoy codes included PQ for Iceland to Northern Russia, and OS for convoys southbound from Britain.
Some ship-owners, especially those of ships faster than the HX convoys, had no patience for the time lost to convoy formation and the need to keep pace with slower ships, and attempted to proceed across the Atlantic unescorted, relying on speed and luck to keep their ships safe. These suffered the losses that WWI experience should have taught them to expect. Few ships could outrun a torpedo, and none could outrun enemy aircraft.
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