You only have 75 depth charges for the entire voyage, so you don't want to waste them. ASDIC is notorious for giving false readings due to old wrecks, thermoclines - or layers of water at different temperatures, or even whales or schools of fish. You drop a diamond pattern of four charges. The explosions are impressive but there are no conclusive results. There is no further contact so your ship resumes its regular duties hoping you have at least scared the U-Boat off.