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Zig-zag, the clock that saved one million lives

In 1916 the German Naval surface fleet had been contained by the British Navy after the battle of Jutland. They were no longer a threat in the Atlantic to our Supply and troop convoys, however they then put all their Naval energy into their Uboats.
As Admiral Karl Donitz Had been a Uboat Commander in the first world war and he was the flotilla commander of the only uboat flotilla at the start of the second World War, Adolf Hitler quickly made him commander of the fleet and he then turned the uboats into a serious threat to Britains survival. Making the Atlantic a dangerous place for our convoys.
One of the defences of our convoys being tracked and having our courses plotted so we could be ambushed was “Zig Zagging” Zig Zagging with a single ship is fairly straightforward, however with a convoy of up to and beyond 100 vessels it required steady nerves and first rate seamanship. Now the ships in the convoy were manned by a mixture of crews, Merchant, Royal Navy, and many foreign nationals, as well as ships of varying sizes and abilities, not only that some of these ships were carrying up to 15,000 troops, it was a tricky undertaking and naturally there were accidents.
The way it worked was this:-A Zig Zag plan or plans would be plotted taking in to account tides, winds, currents, and weather, to the disadvantage of the uboats. This was especially effective at night as the ships were blacked out and 24 hour radio silence would usually be observed.
When approaching the danger zone, the Commodore on the flagship would hoist the “Zig Zag A Flag and the whole convoy would turn to the first heading. And start the zig zag clock. The Octo and later called the Mercer clock would have been calibrated and set prior to departure and all the ships would have one. They had a ring with moveable indicators on it and they were wired to a buzzer on the bridge. Usually they would sound a buzzer 10 minutes after the zig zag had commenced and every hour thereafter to indicate the time to change the Course of the ship to the next heading.
As you can imagine this did not always go as planned:- One notable accident occurred in October of 1942 when the RMS Queen Mary carrying 10,000 troops collided with her escort the Curacoa. The Queen Mary is over 1000 feet long and displaced 82,000 tons. The Curacoa was a light cruiser, the impact caused the Curacoa to break in two. A sailor named Enoch Foster was on look out on a nearby ship, he recalled how quickly the disaster unfolded.
He said “All that destruction in the time it takes to light a cigarette” The Curacoa caught fire and sank in 6 minutes taking over 300 men down with her. At the end of the war a top secret admiralty report concluded that zig zagging at its best could only decrease the chances of being sunk by 15% In WW2 Uboats sank 2779 ships for a total of 14.1 million tons. 15% of that is 416 ships and crew. Due to technological advances towards the end of the 2nd world war, uboats instead of the hunters became the hunted whilst the convoys lost less and less ships.

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