Experienced U-boatmen would be able to listen to the smacking sound of a depth charge hitting the water and, based on the assumption that the bomb sank at a rate of four metres per second, use a stopwatch to calculate the depth-setting. Even if they got lucky, it was difficult for the navy to confirm a ‘kill’, although the smell of oil in the air and the sound of bubbling on the headphones implied success. If they could be spared, two escort ships would sit over the site of the suspected hit for forty-eight hours, a sufficient amount of time that, if the U-boat was merely playing dead, its air
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