'Buttercup' (i) was a British tactic devised by Commander F. J. Walker of the 36th Escort Group as the standardised and primary means of defeating a surfaced U-boat’s nocturnal attack on a convoy (December 1941/May 1945).
As soon as the convoy was attacked, all of the escorts turned outward without awaiting an order and fired star shells ('snowflakes') away from the convoy for a period of 20 minutes. This was designed to achieve the double purpose of illuminating any U-boat on the surface and back-illuminating any U-boat between the star shell and the escorts. Any U-boat thus illuminated was to be attacked with gun fire or, if the boat submerged, to be detected and tracked by Asdic (sonar) with the escort vessels communicating by radio to co-ordinate the tracking and subsequent depth-charging of the boat. If not destroyed, the tracked boat had eventually to surface to replenish its air and charge its batteries, whereupon it could be located and attacked once more.