Western Approaches Tactical Unit

The entrance door to WATU. The crest was a relic from the destroyer HMS Tactician, decommissioned in 1931.

The Western Approaches Tactical Unit (WATU) was a unit of the British Royal Navy created in January 1942 to develop and disseminate new tactics to counter German submarine attacks on trans-Atlantic shipping convoys. ^[1] It was led by Captain Gilbert Roberts and was principally staffed by officers and ratings from the Women's Royal Naval Service (Wrens). ^[2] Their primary tool for studying U-boat attacks and developing countermeasures was wargames. After the U-boat threat to merchant shipping was defeated, WATU continued to develop anti-submarine tactics for later stages of the war, including Operation Overlord and the Pacific War. WATU trained naval officers in its tactics by hosting week-long training courses in which the students played wargames. WATU formally ceased operations at the end of July 1945.

Mission

During World War I, German submarines (U-boats) sank merchant ships in the Atlantic Ocean so as to deny supplies to Germany's enemies in Europe. Britain reacted by organizing the merchant ships into convoys which were escorted by warships armed with depth charges. This strategy proved effective at repelling U-boats. During the inter-war years, Germany secretly developed new submarine tactics to counter the convoy system. The products of this research were the "wolfpack" tactics, wherein submarines would attack convoys in groups, exploiting the weaknesses of the convoy system, and new advances in submarine technology. The British, by contrast, had neglected to study submarine tactics during the inter-war years. They entered World War II assuming that the U-boats would operate much as they had during the previous war, unaware that the Germans would come at them with new tricks. ^[3]

As soon as Britain declared war on Germany (3 Sept 1939), Germany sent its U-boats to attack transatlantic shipping. The U-boats had a devastating effect. In 1938, Britain had received 68 million tons of imports, but in 1941 the U-boats reduced this to 26 million. ^[4] Britain was not a self-sufficient nation, and eventually its reserves of food would run out and it would be forced to capitulate to prevent a famine. ^[5] In March 1941, Prime Minister Winston Churchill declared that Britain was fighting "the Battle of the Atlantic", and made anti-submarine warfare a top priority.

The Royal Navy understood from intercepted radio transmissions that the U-boats were operating in coordinated groups but did not know the specifics of their tactics. ^[6] On 1 January 1942, Admiral Cecil Usborne assigned Commander Gilbert Roberts to establish a wargaming unit at the Western Approaches Command in Liverpool, to analyze the submarine attacks and develop defensive tactics. ^[7] Roberts had designed naval wargames during a two-year stint at the Portsmouth Tactical School, using them to develop new strategies and tactics. Additionally, Roberts was a gifted communicator who would be able to train commanders in the tactics he was to develop. ^[8]

Roberts moved to Liverpool to set up his tactical unit on the top floor of the Western Approaches headquarters. This assignment officially began on 23 February 1942. ^[9] Most of the staff at Western Approaches were women from the Women's Royal Naval Service (colloquially referred to as "Wrens"), and likewise Roberts recruited most of his staff from the Wrens. A total of sixty-six Wrens served at WATU from 1942 to 1945. ^[10]

Roberts and his team reviewed battle reports from convoy escort commanders, recreated the battles in wargames in order to deduce how the U-boats were operating, and then devised tactics by which the escorts could defeat the U-boats. Their first product was a tactic codenamed Raspberry (see below). As well as devising tactics, WATU also trained naval officers in their use by having them participate in wargames. The training course lasted six days, from Monday to Saturday, and was held every week from February 1942 to the last week of July 1945. Up to fifty officers at once took the course. ^[11] WATU not only trained British officers, but also officers from other countries such as Canada, the United States, New Zealand, Norway, South Africa, Poland, and Free France. ^[12]

In May 1943, Admiral Karl Doenitz ordered the U-boats to withdraw from the Atlantic, allowing merchant convoys to pass unmolested.

By 1944, WATU's existence was public knowledge. A journalist visited WATU in January 1944 to observe a wargame and published a short article in The Daily Herald. ^[13] An exposé appeared in Illustrated magazine the following month. ^[14]

WATU continued to develop anti-submarine tactics and train officers until the end of the war. It officially ceased operations at the end of July 1945. It had trained close to 5,000 officers over its lifetime. ^[15]

Legacy

After WATU was closed, Admiral Horton sent the following signal to its former members: "On the closing down of WATU I wish to express my gratitude and high appreciation of the magnificent work of Captain Roberts and his staff, which contributed in no small measure to the final defeat of Germany." ^[16] Admiral Noble sent Roberts a letter in which he wrote: "...you had a great deal to do in winning the war because if we hadn’t won the Battle of the Atlantic we should undoubtedly have lost the war!" ^[17]

What makes WATU a noteworthy episode in the history of military wargaming is that it was an early instance where wargames were used to develop solutions to problems that were occurring in an ongoing war. Up to that point, most wargames were played during peacetime to prepare officers for potential wars, and the scenarios they explored either were hypothetical or happened many years ago. This was made possible by communications technologies that were not available to wargamers in earlier eras (radio and telephone).

The training course provided by WATU appears in the 1951 novel The Cruel Sea, referred to as the "Commanding Officers' Tactical Course". The author, Nicholas Monsarrat, had attended WATU during his service in the war. ^[18] The scene did not appear in the 1953 movie adaptation.

The role of WATU in the Battle of the North Atlantic is highlighted in the 2022 television series 'U-Boat Wargamers', ^[19] and which also emphasises how the unit's Royal Navy Wrens significantly contributed to WATU's success.

Headquarters

Western Approaches Command was an operational command of Britain's Royal Navy, tasked with safeguarding British shipping in the Western Approaches (the seas to the west of Ireland and Britain). Initially headquartered in Plymouth, on the southern coast of Britain, it was moved north to Liverpool in February 1941. After France had fallen to the Germans, North Atlantic shipping convoys had been diverted around the north of Ireland to evade the German navy. Relocating Western Approaches Command to Liverpool sped up communications. Its headquarters was Derby House, a building located behind Liverpool's town hall; today the headquarters is a museum. ^{[lower-alpha 1]} The top floor, comprising eight rooms, was allocated to WATU. ^[20] Most of the staff at Western Approaches HQ were women from the Women's Royal Naval Service. Colloquially, they were referred to as "Wrens". When Roberts arrived at Western Approaches in January 1942, its commander-in-chief was Admiral Percy Noble, who was replaced by Admiral Max Horton in November 1942.

Staff

A total of sixty-six women from the Women's Royal Naval Service (Wrens) served at WATU from 1942 to 1945. ^[10]

• Captain Gilbert Roberts - Director of WATU. Once commanded a destroyer but contracted tuberculosis in late 1938 and was barred from serving at sea.
• Mary Poole - WRNS officer. The first female officer to take the WATU course.
• Laura Janet Howes - WRNS officer. Born in Antigua. Mathematical prodigy. Nicknamed "Bobby". ^[21]
• Elizabeth Drake - WRNS officer. When WATU was established, she was already working at Derby House as a plotter.
• Nancy "Nan" ^[22] Sofia Wailes - WRNS officer. Born in Kingston upon Hull, joined the Wrens in 1941. She was a passionate hockey player, and was selected for her understanding of team tactics.
• Jean Laidlaw - WRNS officer. Scottish. Former Sea Ranger and chartered accountant. ^{[23] [24] [25] [26] [27] [28]}
• Janet Okell - WRNS rating. She was only 19 years old when she joined WATU in January 1942. She served at WATU for the whole duration. Okell was initially trained as a plotter, but she was soon participating in wargames as a player, commanding U-boats and escort ships alike.
• June Duncan (1924-2014) - WRNS rating. One of WATU's longest serving members, in the 1950s, became one of Britain's top fashion models. ^{[22] [29] [30] [31]}
• Dorothy N. G. "Judy" Du Vivier - WRNS ^{[32] [33]}
• Mary (née Carlisle) Hall - WRNS ^[34]
• Elizabeth Hackney - WRNS ^[35]
• Doris Lawford - WRNS ^[35]
• Pauline Preston - WRNS ^[35]

• 
  Captain Gilbert Roberts, director of WATU.
• 
  Mary Poole
• 
  Laura Janet Howes
• 
  Nancy "Nan" Wailes ^[22]
• 
  Janet Okell
• 
  June Duncan ^[22]

Overview of the wargames

Gilbert Roberts was first introduced to wargaming during a stint at the Portsmouth Tactical School from 1935 to 1937. Roberts took to wargaming with great enthusiasm, and developed his own rulesets. Roberts' wargames were based on the wargames developed by Fred T. Jane in 1898 (Jane Naval Wargame and Fighting Ships). ^[36] Despite the strong effect that U-boats had during World War I, Roberts' wargames at Portsmouth did not simulate submarine warfare, nor attacks on merchant convoys. In fact, nobody in the Royal Navy studied submarine warfare through wargames until the establishment of WATU in 1942. ^[3]

At WATU, the wargames were conducted in the largest room of the top floor of Western Approaches HQ. The floor was covered with brown linoleum and in the center was a painted grid. This grid was the game board, or "the tactical table" as the Royal Navy referred to it. The gridlines were spaced ten inches apart, representing one nautical mile. Around the grid were vertical screens of canvas that had peepholes cut into them. The players who controlled the escort ships had to stand behind the screens and could only view the game board through the peepholes. The players who controlled the U-boats did not stand behind the screens and had an unrestricted view of the game board. The ships and surfaced U-boats were represented on the game board by tiny wooden models. ^[37] The U-boats' movement lines were drawn in green chalk, a color which contrasted poorly with the brown tint of the floor, such that when viewed from an angle, these lines were practically invisible, so the players behind the screens couldn't make them out. The escort ships' movement lines were drawn with white chalk, which could be clearly perceived by the players behind the screens. ^{[38] [39]}

The players were given two minutes per turn to make decisions and give orders. ^[40] The players issued their orders for their imaginary ships on pieces of paper that they passed to the Wrens—this prevented their opponents on the other side of the room from overhearing. The Wrens would then get down on the floor and compute the outcomes of the players' orders, drawing the trajectories of the ships in chalk. Roberts provided the Wrens with the performance characteristics of all ships concerned: the range of the U-boat's torpedoes, the speed of the ships, their turn speed, the precise capabilities of the escorts' sonar (then known as ASDIC), how engine noise might distort listening attempts, visibility at night, etc.

• 
  A wargame in progress. The men on the far-right command the escort ships and may only view the game board through holes in the screen. The woman in the foreground is Jean Laidlaw.
• 
  Wrens computing the outcomes of player decisions.
• 
  A Wren organizing game materials. The wooden pieces represent escort ships, the black board with white shapes represents a convoy, the wire wool represents smoke.

Tactics developed

Notes on WW2 submarine warfare

Submarines of this era were powered by diesel engines and batteries. They could only use their diesel engines when surfaced, as these needed to breathe air to work. When submerged, the submarine used lead-acid batteries. The batteries were less powerful than the engine, so the submarine was reduced to about half-speed: e.g. a Type VIIC U-boat could travel at 17.7 knots (32.8 km/h) on the surface but only 7.6 knots (14.1 km/h) underwater. The batteries could be exhausted after an hour or so of maximum speed underwater. When the batteries were exhausted, the U-boat would be forced to surface for air and recharge the batteries with the diesel engine.

The British were the first to equip their warships with sonar to hunt enemy submarines. They called this technology "ASDIC"; the term "sonar" was later coined by the Americans (this article will prefer the more general term "sonar" as it is more widely known). This technology sent loud pings into the water and located a submerged submarine by the echoes. The U-boats could hear these pings, of course, so they would know they were being hunted. In practice, ASDIC had an average detection range of 1,300 yards (1,200 m). ^[41] ASDIC could be ineffective if there was too much ambient noise in the water. The maximum speed at which a ship could travel while using its ASDIC was about 15 knots (28 km/h), beyond which the noise of its own propeller and engine would drown out the echoes. ^[42] Both the U-boats and the warships also had hydrophones with which they could passively listen for sound in the water.

Merchant ships travelled in convoys, surrounded by a ring of escort warships. They were either slow or fast convoys. The slow ones travelled at about 7 knots (13 km/h) –a speed at which tactical re-routing was not practical. ^{[43] : 80}

U-boats usually attacked at night. The cover of darkness allowed them to travel at surface depth with less risk of being spotted by look-outs. Escort ships were equipped with star shells, which when fired in the air would release a burning flare held aloft by a small parachute. This would illuminate the surface of the water, making it easy to spot a surfaced U-boat.

Warships initially used solely depth charges to sink submarines. These are explosive charges on a depth sensitive fuse that were dropped into the water around the U-boat. The hydraulic shockwave produced by the explosion would seriously damage if not sink any submarine within 10 metres.^{[ citation needed ]} Later, ahead-throwing anti-submarine weapons were also used, which had contact fuses.

Raspberry

This variation of the Raspberry maneuver was designed for a six-escort formation with no forward escort.

This variation of the Raspberry maneuver was designed for an eight-escort formation with one forward escort.

During World War I, the U-boats typically attacked convoys from outside the formation, striking ships at the perimeter. But reports from convoys in 1942 showed that U-boats were sinking ships at the center of the formation. Roberts surmised that the U-boats were somehow sneaking into the formation undetected before firing their torpedoes. ^[44] Roberts and his team tested various ways by which a U-boat might sneak into a convoy, sink a ship, and escape undetected. Only one tactic worked on the game board: The U-boat snuck into the convoy from the rear, on the surface so that it could use its diesel engine to outpace the convoy. Since these attacks happened at night and the look-outs tended to focus on the front, the U-boat was not easily spotted, and once inside the convoy it was indistinguishable from the other ships on radar. The U-boat would then sink a merchant ship with a torpedo at close range, then submerge to make its escape. ^[45]

Roberts and his team developed the Raspberry maneuver. Upon seeing a convoy vessel being torpedoed, any escort was to fire two white rockets or Roman candles, then say the word "raspberry" over the radio to commence the maneuver. Any forward escorts maintain course, firing star shells. The escorts to the rear and flanks of the convoy converge towards the convoy at a speed of 15 knots. One rear escort sweeps the stern of the convoy. The other escorts, upon nearing the edge of the convoy, turn around and sweep away from the convoy firing star shells for 10 to 12 minutes. Then they turn and sweep back to their starting position. While doing this, all the ships fire star shells outwards to light up the surface of the water. ^[46]

Raspberry is likely a development of an earlier maneuver known as Buttercup, which was developed by Frederic J. Walker, an escort commander. Buttercup was criticized by the Admiralty for not having all the escorts sweep for the U-boat in all directions. In Buttercup, the escorts would only sweep to the side of the convoy where the attack was thought to have come from, which meant the U-boat could escape if the escort commander made the wrong call. Additionally, only the escort commander could order the maneuver, which could have caused a fatal delay. By contrast, Raspberry has all the escorts sweep the entire perimeter, and any of the escorts could order the maneuver. ^[47]

It was Jean Laidlaw who proposed the name Raspberry, as in "blowing a raspberry at Hitler".

Raspberry was cancelled in May 1943. ^[48]

Pineapple

The Pineapple maneuver, adapted for a six-escort formation when a second attack is expected to come from ahead.

The Pineapple maneuver, adapted for a seven-escort formation when a second attack is expected to come from astern.

Pineapple was designed to be an alternative to Raspberry and Banana. It was to be used when more than one U-boat was attacking the convoy, and was designed to do four things: (1) Force a second U-boat to dive before being able to attack; (2) Give a chance of detecting a U-boat which has already fired torpedoes; (3) Prevent another U-boat from sighting the convoy in any illumination used to detect the U-boat which has already attacked; and (4) Increase chance of detection of any submerged U-boat which is about to attack and to form a greater physical obstruction in the submarine attacking area.

Upon noticing a ship getting torpedoed, any escort fires two white rockets and then says the word "pineapple" over the radio to commence the maneuver. Depth charges are to be set to ten-charge shallow pattern. The escorts keep moving forward in zigzag patterns that are 2 miles wide, searching for the U-boats using sonar, radar, and star shells. The escorts facing the expected direction of the second U-boat fire star shells away from the convoy to search for the second U-boat. They were not to fire star shells into the convoy, and the escorts on the opposite side of the convoy were not to fire star shells at all — this was probably to avoid lighting up the convoy, which would have made it easy for the second U-boat to spot. ^[49]

Banana

Banana variant for a six-escort formation.

Banana variant for a nine-escort formation.

Banana was intended for when a single U-boat attacked the convoy (unlike Pineapple which was intended for pack attacks). Along with Pineapple, it supplanted Raspberry.

Upon seeing a merchant ship being torpedoed, the escort was to fire two white rockets or Roman candles and signal "banana" over the radio, then begin sweeping with sonar and radar at maximum viable speed. The rear escorts fire star shells outwards, then move into the convoy to sweep with sonar in a zigzag pattern. The first of these escorts to reach the wreck of the torpedoed vessel carries out Operation Observant in the area. The escorts to the flanks of the convoy sweep towards the rear of the convoy until they were abreast of the rear escorts, after which they turn around and sweep forward in a zigzag pattern. The forward escorts were to sweep forward in a zigzag pattern. ^[50]

Beta Search

Alpha Search, developed by Frederic Walker.

Beta Search, developed by Roberts' team at WATU.

Beta Search was a maneuver by which an escort might be able to locate a U-Boat that it had spotted shadowing a convoy. The manual for escort commanders (Atlantic Convoy Instructions) noted that a single escort had a poor chance of finding and destroying a U-boat, but by assuming the most likely course the U-Boat would take and searching within that vicinity, the odds of success could be raised somewhat. Beta Search was a development of an earlier maneuver developed by Commander Frederic Walker called Alpha Search. The advantage of Beta Search over Alpha Search is that Beta Search persuades the U-boat to move in a specific direction. The disadvantage of Beta Search is that the escort must be fitted with "special plotting equipment" to use it. ^[51]

In Alpha Search, the escort turns to head straight for the U-boat, which will prompt the U-boat to dive. The escort then turns 20 degrees. When the escort reaches the U-boat's "furthest towards" circle, it alters course towards the position the U-boat dived, moving in a zigzag with short legs ^{[lower-alpha 2]} pattern. Upon passing the location where the U-boat dived, the escort drops a marker in the water, then proceeds for the same distanced zigzagging. Then it turns 90 degrees towards the convoy and begins a search pattern known as Operation Observant.

In Beta Search, the escort turns towards the U-boat, but not directly towards it. The U-boat would likely react by submerging and following a course parallel to the convoy. The escort then moves towards the U-boat's predicted position along that course (this would be on a roughly 15 degree angle from the bearing on which the U-boat was spotted). When the escort reaches the U-boat's "furthest towards" circle, it starts "zigzagging with short legs". When the escort passes over the U-boat's predicted line of escape, it was to drop a sea marker in the water. When the escort reached the U-boat's "furthest away" circle, it was to turn 90 degrees towards the position where the U-boat dived and begin Operation Observant.

Step Aside

Step Aside was a maneuver by which a warship could attack a U-boat armed with acoustic torpedoes, specifically the T5 Zaunkönig torpedo, which the Royal Navy referred to as the GNAT (German Navy Acoustic Torpedo). This torpedo used built-in hydrophones to guide itself to its target by sound. The first use of this torpedo was on 20 September 1943 against convoy ON 202. The Royal Navy was already aware that the Germans had developed an acoustic torpedo through interrogations of captured Germans and decrypted communications. ^[52] In the attack on ON 202, a frigate was hit in the rear while it was bearing down on the U-boat, something which could only have been done by a guided torpedo. The torpedo was drawn to the sound of the frigate's propeller. Roberts and his team at WATU developed a maneuver known as Step Aside, in which the warship made sharp turns to dodge the acoustic torpedoes the U-boat might fire as the warship maneuvered into attack range. Step Aside was communicated by radio to escort commanders at sea on 23 September 1943. ^[53]

Upon sighting a U-boat within 6,000 yards, the ship heads straight for the U-boat at best speed for 2 minutes. It was expected that the U-boat would fire its acoustic torpedo when it saw the ship heading straight for it. ^[54] After 2 minutes, the ship turns 60 degrees left or right and holds this divergent course for 3 minutes, thereby putting itself outside the acoustic torpedo's detection range. ^{[lower-alpha 3]} Then the ship turns to head straight for the U-boat again. If the U-boat has not yet dived, the ship repeats the maneuver: sail towards the U-boat for 2 minutes, then turn 60 degrees, etc. The warship repeats this process until the U-boat dives. When the U-boat dives, the ship rushes to the U-boat's diving position, then slows down to do a sonar search. ^[55]

Step Aside was intended for warships that could not go faster than 24 knots (44 km/h) and which were not equipped with Foxer decoys. ^[55] Most convoy escorts were frigates and corvettes, with a maximum speed of 16 to 20 knots. 24 knots was the speed at which the T5 torpedo moved, so ships which could go faster than that (destroyers) could just outrun the torpedo.

See also

• Rodger Winn

Footnotes

1. https://liverpoolwarmuseum.co.uk/
2. Make turns at short intervals (3 to 7 minutes) as opposed to long-leg zigzags of 10 to 30 minutes.
3. The T5 acoustic torpedo had a speed of 24 knots and a range of 5,000 yards, though these exact details were likely unknown to WATU. The fastest torpedo in the German arsenal was the (non-acoustic) G7a, which could travel at 40 knots.

References

1. Strong, Paul Edward. "Wargaming the Atlantic War: Captain Gilbert Roberts and the Wrens of the Western Approaches Tactical Unit" (PDF). paxsims.wordpress.com/. PAXsims. Retrieved 11 November 2019.
2. "Women winning battles: Recreating the Wrens unit which helped win the War". GOV.UK. UK Government. Retrieved 11 November 2019.
3. Parkin (2019). A Game of Birds and Wolves, chpt. 7: "Despite the fact that during the First World War the Germans had used submarines to great effect to disrupt the convoys bringing food and supplies to Great Britain, neither U-boats nor convoys featured in the wargames of 1935. [...] ‘Submarines were not mentioned,’ Roberts wrote of the games he was tasked with designing. ‘Nor were convoys and attacks on them. Nobody connected Hitler’s rise … to the possibility of another Battle of the Atlantic. Nor did I, to be absolutely fair.’"
4. Overy (1995), Why the Allies Won. p. 31
5. Parkin (2019). A Game of Birds and Wolves, chpt. 3: "‘Unless something was done in the Battle of the Atlantic,’ Roberts was told, ‘we were going to lose [the war], merely because vital food and war supplies would not arrive.’"
6. Parkin (2019). A Game of Birds and Wolves, chpt. 8: "While the British could see from intercepted radio transmissions that U-boats were increasingly working together, providing the German vessels with safety in numbers, the specifics of their highly effective tactics could only be guessed at."
7. Parkin (2019). A Game of Birds and Wolves, chpt. 8: "On the first day of 1942, Roberts was told to report to the Admiralty offices with an overnight bag. On arrival he met two of the navy’s most senior officers, the Second Sea Lord, Sir Charles Little, and Admiral Cecil Usborne, the former director of naval intelligence, now an aide to Winston Churchill. Usborne was responsible for overseeing the development of anti-U-boat weapons."
8. Parkin (2019). A Game of Birds and Wolves, chpt. 8: "Churchill’s aide believed that Roberts, who had shown himself to be a talented strategist in Portsmouth and an enthusiastic proponent of games as a way to prepare for war, was the ideal person to evolve anti-U-boat tactics. Moreover, as a gifted communicator he was qualified to train escort commanders in those tactics."
9. Service record of Gilbert Howland Roberts, (UK National Archives, ADM 196/124/8) (archived at Dropbox)
10. Parkin (2019). A Game of Birds and Wolves, Postscript: "By 1945, a total of sixty-six Wrens had completed the course in order to become staff at WATU or its sister units."
11. Parkin (2019). A Game of Birds and Wolves, chpt. 12: "Each course, which lasted from Monday to Saturday, and which ran weekly without interruption from the first week of February 1942 to the last week of July 1945, involved up to fifty officers at once."
12. Parkin (2019). A Game of Birds and Wolves, chpt. 12, fn 1: "Roberts recorded that on Sundays the team would often be called upon to give demonstrations to visiting Americans."
13. The Daily Herald. 17 January 1944
14. A. J. McWhinnie (26 February 1944). "Behind the Atlantic Battle" (PDF). Illustrated. Odhams Press.
15. Parkin (2019). A Game of Birds and Wolves, postcript
16. Parkin (2019). A Game of Birds and Wolves, Postscript
17. Williams (1979). Captain Gilbert Roberts, p. 179
18. Parkin (2019). A Game of Birds and Wolves
19. Considine, Pippa (24 November 2022). "WMR focus on WRNS for WW2 History series". televisual.com. Televisual Media UK Limited. Retrieved 24 November 2022.
20. Parkin (2019). A Game of Birds and Wolves, chpt. 10: "Roberts was to be given the entire top floor of Derby House, [...] comprising eight rooms."
21. Parkin, Simon (28 January 2020). A Game of Birds and Wolves: The Ingenious Young Women Whose Secret Board Game Helped Win World War II. Little, Brown. ISBN   978-0-316-49208-9 . Retrieved 6 July 2024.
22. "Wrens, Wargames and the Battle of the Atlantic". Historic UK. Retrieved 6 July 2024.
23. Killelea, Amanda; Dresch, Matthew (18 February 2023). "Inside life of women who outwitted Hitler and won brutal Battle of the Atlantic". The Mirror . Retrieved 6 July 2024.
24. "Wargaming Case Study: RN Western Approaches Tactical Unit" (PDF). International Symposium on Military Operational Research. Retrieved 6 July 2024.
25. Killelea, Amanda; Dresch, Matthew (18 February 2023). "Inside life of women who outwitted Hitler and won brutal Battle of the Atlantic". The Mirror . Archived from the original on 6 July 2024. Retrieved 6 July 2024.
26. "A Game of Birds & Wolves". General Staff. 21 September 2020. Retrieved 6 July 2024.
27. Parkin, Simon (28 January 2020). "A Nazi U-Boat Tactic Stumped Allied Forces — Until a Retired British Naval Officer Designed a Game to Reveal How It Worked". TIME. Retrieved 6 July 2024.
28. "U-Boat Wargamers: A historic timeline of events". Sky HISTORY TV channel. Retrieved 6 July 2024.
29. "'Never at Sea'... well, never say never". National Museums Liverpool. Retrieved 6 July 2024.
30. "Glamour of the 1950s captured in stunning exhibition at the Lady Lever Art Gallery". Creative Boom. 2 October 2017. Retrieved 6 July 2024.
31. Wright, George (13 November 2017). "Name to a face". Big Issue North. Retrieved 6 July 2024.
32. "P/O R A L Du Vivier". Battle of Britain London Monument. Retrieved 6 July 2024.
33. "Collection: Reminiscences of Vice-Admiral Sir Peter Gretton". archivesearch.lib.cam.ac.uk. Churchill Archives Centre. Retrieved 6 July 2024.
34. "Mary (née Carlisle) Hall – WRNS". Second World War Experience Centre. Retrieved 6 July 2024.
35. "A Game of Birds and Wolves review: The Young Women Whose Secret Board Game Helped Win World War II". thegazette.com . Retrieved 6 July 2024.
36. Parkin (2019). A Game of Birds and Wolves, chpt. 7: "The progenitor of the wargame on which Roberts based his games in Portsmouth was Fred Jane [...] It was a version of the Jane Naval Wargame that Roberts adapted at his posting in Portsmouth"
37. Parkin (2019). A Game of Birds and Wolves, chpt. 11: "The floor in the centre of the room was covered in brown linoleum, painted with white gridlines and punctuated with tiny wooden models [...] each white line was spaced ten inches apart, representing one nautical mile, while the counters represented ships and surfaced German U-boats.""
38. A. J. MchWhinnie (26 Feb 1944). "Behind the Atlantic Battle". Illustrated: "Then he [Roberts] bends down and makes a series of small green chalk marks on the lino to indicate submerged U-boats on the flank of a distant convoy.
39. Parkin (2019). A Game of Birds and Wolves, chpt. 11: "The movements of the U-boats were drawn in green chalk on the floor, a colour chosen as it was impossible to make out against the floor’s tint when viewed from an angle. This ensured the U-boat positions were undetectable to the players peering through the canvas screens. The escort ships’ movements would then be added to the floor in white chalk, which was, in contrast to the green markings, legible to those peeking from the canvas holes."
40. Parkin (2019). A Game of Birds and Wolves, chpt. 11: "Players were given two minutes in which to submit their orders for the next ‘turn’, to replicate the urgency of a real battlefield."
41. Akermann (2002). Encyclopedia of British Submarines 1901-1955, p. 47
42. Brown (2000). Nelson to Vanguard
43. Milner, Marc (2011). Battle of the Atlantic. Stroud: The History Press. ISBN   978 0 7524 6646 0.
44. Parkin (2019). A Game of Birds and Wolves, chpt. 11: "If the U-boats were firing from outside the perimeter of the convoy, how had Annavore, which was in the centre of the convoy, been sunk? Might it be possible, he wondered, that the U-boat had attacked the ship from inside the columns of the convoy?"
45. Parkin (2019). A Game of Birds and Wolves, chpt. 11: "Between them, Roberts and the two Wrens began to plot different scenarios that might have enabled the U-boat to sneak into the convoy without being detected. Only one checked out: the U-boat had entered the columns of the convoy from behind. And it must have done so on the surface, where it was able to travel at a faster speed than the ships. By approaching from astern, where the lookouts rarely checked, the U-boat would be able to slip inside the convoy undetected, fire at close range, then submerge in order to get away."
46. Article 120, Atlantic Convoy Instructions
47. "Buttercup vs Raspberry Connections Online 2022" (PDF). www.admiraltytrilogy.com.
48. Admiralty order P. 197/43
49. Article 134, Atlantic Convoy Instructions
50. Article 136, Atlantic Convoy Instructions
51. Article 106, Atlantic Convoy Instructions
52. Polmar & Allen (2012). World War II: the Encyclopedia of the War Years, 1941-1945, p. 343
53. Williams (1979) writes that the captain of HMS Orchis received Step-Aside over radio "a few hours" after his convoy had been attacked by U-boats, during which HMS Itchen was sunk. Itchen was lost in the early morning of 23 September 1943.
54. Williams (1979). Captain Gilbert Roberts, p. 129: "It was obvious that the U-boat did not fire its acoustic torpedo until it was sure the escort was attacking."
55. "ADM 239-298 C4P381 Step-Aside.pdf" (PDF). Dropbox. Retrieved 2024-02-08.

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