The device consisted of one or two noise-making devices towed several hundred metres astern of the ship. The noise makers mechanically generated a far louder cavitation noise than the ship's propellers. This noise distracted the acoustic torpedoes away from the rear of the ship into a circling pattern around the noise maker until the torpedo ran out of fuel. The downside of the "Foxer" was that it also rendered the own ship's ASDIC ineffective and concealed any other U-boat nearby that could approach the convoy.