Essential Information

National Maritime Museum

09 Dec 2014

This zig-zag clock – a rare object from the Second World War, now on display at Royal Observatory Greenwich – was part of a Top Secret, and incredibly dangerous, defence tactic used by Allied convoys throughout the Battle of the Atlantic, the fiercest naval struggle in modern history.


U.S. Maritime Commission Zig-Zag Control Clock, circa 1942-1950 ZAA0190


An unusual timepiece, it was built for Allied convoys – groups of merchant ships travelling with naval escorts – that ferried a lifeline of supplies including food, trucks, arms and men from North America to Europe. The convoy system was at the heart of Britain’s war effort; but it was not easy to integrate the culture and methods of the merchant marine – professional seamen working privately-owned vessels – within the training, demands and rigid discipline of the Royal Navy.   In 1942, with the building of new U-boat submarines, the Germans had the advantage. The Allies suffered their worst setbacks, and in November alone, over one hundred merchant ships were sent to the bottom of the sea. The loss of life and tonnage of war supplies was staggering. The convoy system was in danger of collapse.  

Study of a surrendered German submarine


With few naval escorts available, merchant vessels had to defend themselves as much as possible. Ships were completely darkened at night; radio communication was prohibited; merchant ships were armed; convoy destinations were kept secret; and in especially dangerous areas, convoys didn’t travel in straight lines.   Zig-zagging – the practice of frequently altering direction to port or starboard – was designed to disguise a convoy’s true course and confuse the enemy. All ships followed the same pattern, one of several top-secret zig-zag diagrams, created by Admiralty anti-submarine experts. Before leaving port, the convoy commodore issued each ship’s master with the zig-zag diagrams and signals to be used en route.   At sea, communication between ships was limited. To coordinate the convoy’s movements, zig-zag clocks were synchronized by a signal from the commodore’s flagship. At predetermined times, the clock sounded an alarm, indicating when to change course.

‘On the Bridge’: merchant marine lookouts in the Second World War. Drawing by John Kingsley Cook, who served in Atlantic convoys. ZBA5205


This practice required steady nerves and excellent seamanship. In World War Two, convoys of over a hundred vessels – of differing capabilities with respect to speed, manoeuvrability and crew experience – were not uncommon. Merchant seamen described the experience of zig-zagging in convoy from a ‘really a screwed up mess most of the time’ to ‘the biggest thrill of my life’.   Thomas Burton, a merchant seaman from Newfoundland, described the perils of zig-zagging aboard the Empress of Britain, a luxury liner then working as a troopship. The convoy was traveling at a blistering 24 knots: ‘We had to zig-zag […] and if anyone did a wrong one, you’d probably be right up against someone before you knew it.’ Twenty-five other troop ships zig-zagged together in Burton’s convoy, carrying as many as 15,000 men each.  A slightly wrong turn, a misunderstood signal or a poor lookout could spell disaster.  

Empress of Britain was the largest liner to be sunk in the war. SLR2783


Such an accident happened in October 1942. Carrying 10,000 troops, the great ocean liner RMS Queen Mary collided with her escort, HMS Curacoa (1917). The Queen Mary was over a thousand feet long and displaced almost 82,000 tons. When she hit the light cruiser, the Curacoa instantly broke in two. Enoch Foster, a lookout on a nearby vessel, recalled how quickly the accident happened: ‘all that destruction in the time it takes to light a cigarette.’ The Curacoa caught fire and sank only six minutes after the collision – taking over three hundred men to a watery grave.  

Considering the challenges of communication, it is remarkable that the Curacoa disaster was not repeated. And, given the high stakes, it is interesting that zig-zagging was ordered by the Admiralty throughout both World Wars, despite a U.S. Naval Intelligence report in 1918 that questioned the effectiveness of zig-zagging in avoiding submarine attack. Toward the end of World War Two, a top secret Admiralty report concluded that, at best, zig-zagging could only decrease the chance of being sunk by fifteen per cent.  

Ziz-Zag 'Dazzle' painting was another tactic used in WW1 to confuse the enemy. See Everett's Dazzle paintings in our War Artists at Sea exhibition.


Although zig-zagging was perhaps a poor ruse, the practice helped forge a stronger defence. To avoid collision, merchant crews – from masters and deck officers, to engine departments and able seamen – had to drastically change their peacetime routines. No longer independent vessels, accustomed to traveling alone, merchant ships in convoy became subject to the culture and discipline of navies. Henry Beston, a notable American journalist who travelled with the British Grand Fleet, described how the practice of zig-zagging aided the convoy system in World War One:   ‘If [a convoy] is to make the most of its chances of getting through the German ambush, it must act as a well coordinated naval unit, obeying orders, answering signals, and performing designated evolutions in the manner of a battleship squadron.’   Zig-zagging continued to bring convoys together in the Second World War. Primarily, it enforced naval hierarchy. Risking collision or separation, merchant masters could not afford to ignore the orders of convoy commodores. Crew priorities and responsibilities changed. Merchant vessels now needed well-trained signalmen aboard in order to follow zig-zag commands. To keep station, vessels needed quick and reliable communication between the helm and the engine room; an apprentice running between decks, still used on older ships, was no longer adequate. And, as in John Cook’s drawing above, merchant seamen began working as keen lookouts, rather than performing other tasks.   In this way, zig-zag clocks shaped how the merchant and naval fleets interacted, helping to make large convoys possible. Now largely a forgotten technology, the mysterious zig-zag clock at the Royal Observatory not only sounded the alarm when to change course – it helped navigate between the culture of the autonomous merchant marine and the rigid discipline of the Royal Navy.   Erika Jones (@erikajones225) explored the technology and practice of zig-zagging as part of a student research internship at the National Maritime Museum in August 2014. She is a PhD student in Science and Technology Studies at UCL and blogs here.     Sources:   The National Archives, Kew.   Beston, Henry. “With the Convoy.” The North American Review, 208: no. 756 (Nov. 1918): 686-701.   Burn, Alan. The Fighting Commodores: The Convoy Commanders in the Second World War. Annapolis: Naval Institute Press, 1999.   Halley, Morgiana. An Ethnography of Marine Convoys During World War II. Department of English Language and Linguistics, University of Sheffield, 1997.   Lane, Tony. The Merchant Seamen’s War. Manchester: Manchester University Press, 1990.   Thomas, David Arthur. Queen Mary and the Cruiser: The Curacoa Disaster. Annapolis, Maryland: Naval Institute Press, 1997.   Morison, Samuel Eliot. The Two-Ocean War: A Short History of the United States Navy in the Second World War. Annapolis: Naval Institute Press, 2007.