Foxer Foxer , was the codename for a British built acoustic decoy , used to confuse German acoustic homing torpedo es like the G7es torpedo during the Second World War . A US version codenamed FXR was deployed in 1943 . A Canadian version was also built called the CAT ( Counter - Acoustic Torpedo ) . It was replaced in US service by the Fanfare noisemaker . The device consisted of one or two noise making devices towed several hundred metres behind the ship . The noise makers mechanically generated huge amounts of noise , far more than produced by the ship itself . This noise seduced the simple guidance mechanisms of acoustic torpedoes away from the rear of the ship into a circling pattern around the noise maker until the torpedo ran out of fuel . Development . By 1943 Allied intelligence sources indicated that Germany had produced a passive acoustic homing torpedo , which homed on the sound produced by a ship ' s propeller as it moved through the water . To counter this threat various countermeasures were considered . There was already a towed noise maker developed to mine - sweep for acoustic mines , consisting of two steel bars fixed parallel , with a gap between them to allow water to flow between , and towed perpendicular to the flow of water . The water flowing between the two bars caused them to flex and bang together generating far more noise than the towing ship above 1 kHz . This noisemaker produced around 20 decibel s more noise than the towing ship , and would last for around 25 hours when towed at 12 knots . The US developed this existing noise - maker into the FXR , which was to be towed singly around 500 feet behind the ship . This entered production by July 1943 . The single noise - maker however would only work effectively if the torpedo was equally sensitive to sounds from both the front and rear . If a torpedo passed directly over the noise - maker from the rear the ship itself would be directly ahead , if the torpedo had a reduced sensitivity to noise from behind , it might ignore the greater output from the noise - maker , and proceed to home on the ship . After successful attacks on ships in Convoy ONS 18 and Convoy ON 22 in which nine ships were sunk , more detailed studies were begun into the torpedo behaviour . These studies revealed that against a torpedo with reduced sensitivity to the rear ( described by its sensitivity pattern as a Cosθ torpedo ) , a single towed noise - maker would not be sufficiently effective . Description . The Foxer consisted of one or more 3 , 000 lb ( 1 , 360 kg ) arrangements of hollow metal pipes with holes cut in them . These were towed through the water about behind the ship . The water rushing through the holes and the pipes banging together created cavitation noise , much greater than that coming from the ship ' s propeller . This worked because the German homing torpedoes were tuned to home in on the sound frequencies generated by cavitation , and to home in on the loudest cavitation sound . The Germans U - boat crews called it the " Kreissäge " ( circular saw ) or " Rattatelboje " ( " rattle buoy " ) , estimated the volume of noise generated by Foxer at 10 to 100 times greater than that generated by a ship . The limitations of the system were that it could not be towed faster than 14 knots ( 26 km / h ) and because of the noise it effectively rendered the towing ship ' s sonar useless . The drums also wore out quickly and the sound could be heard underwater over a long distance giving away the position of the towing ship to U - boats searching for convoys . In the closing stages of the war the Germans developed the T11 " Zaunkoenig II " torpedo , designed to ignore towed decoys and noisemakers . However , it was never fired .