Western Approaches Tactical Unit

[1] It was led by Captain Gilbert Roberts and was principally staffed by officers and ratings from the Women's Royal Naval Service (Wrens).

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.

During the inter-war years, Germany secretly developed new submarine tactics to counter the convoy system.

[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.

[8] Roberts moved to Liverpool to set up his tactical unit on the top floor of the Western Approaches headquarters.

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

A journalist visited WATU in January 1944 to observe a wargame and published a short article in The Daily Herald.

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.

Initially headquartered in Plymouth, on the southern coast of Britain, it was moved north to Liverpool in February 1941.

[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.

"[3] However, Roberts told the Royal United Services Institute in 1947 that before the war "In the Tactical School, a short game was played in which a convoy was escorted by a fleet and the enemy force included some submarines.

"[37] At WATU, the wargames were conducted in the largest room of the top floor of Western Approaches HQ.

The escort ships' movement lines were drawn with white chalk, which could be clearly perceived by the players behind the screens.

[41] 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.

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.

Escort ships were equipped with star shells, which when fired in the air would release a burning flare held aloft by a small parachute.

During World War I, the U-boats typically attacked convoys from outside the formation, striking ships at the perimeter.

[45] Roberts and his team tested various ways by which a U-boat might sneak into a convoy, sink a ship, and escape undetected.

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.

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.

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.

The escorts keep moving forward in zigzag patterns that are 2 miles wide, searching for the U-boats using sonar, radar, and star shells.

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.

Upon passing the location where the U-boat dived, the escort drops a marker in the water, then proceeds for the same distanced zigzagging.