Raspberry hunts a U-boat inside the convoy

WATU examined how a U-boat could approach a convoy from astern on the surface, use diesel speed to enter the formation at night, and become hard to distinguish from merchant ships at close range. The response depended on the number of ships in the close escort, because those vessels rather than distant support had to react inside the formation. Raspberry used two white rockets and a radio code word to trigger a rapid inward sweep by rear and flank escorts, with star shells fired outward. Jean Laidlaw later described the code word as blowing a raspberry at Hitler. The U-boat might still fire before the escorts trapped it, so the aim was to prevent an easy escape and make repeated attacks costly. Unlike Walker's Buttercup, Raspberry avoided a single uncertain side choice that could delay the search. It was withdrawn in May 1943 as Pineapple and Banana separated the response to pack attacks from the response to a single boat.