Foxer

Image:12knot ship single noisemaker.jpg

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 decibels 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 work effectively only 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.

Image:Development of towed noisemakers fig8.jpg

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.

The Foxer consisted of one or more {{convert|3000|lb|kg}} arrangements of hollow metal pipes with holes cut in them. These were towed through the water about {{convert|200|m|yd|abbr=on}} 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 on the loudest cavitation sound.

The German U-boat crews called it the Kreissäge (circular saw) or Rattelboje (rattle buoy) and 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 {{convert|14|kn}} and because of the noise it effectively rendered the towing ship's sonar useless.{{sfn|Williamson|2012|p=45}} 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 Zaunkönig II torpedo, designed to ignore towed decoys and noisemakers. However, it was never actually employed in wartime as Germany had surrendered by the time testing was fully completed.

{{Reflist}}

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• {{cite book|title=Antisubmarine warfare in World War II|publisher=Operations Evaluation Group (U.S. Navy)|author=Charles M. Sternhell, Alan M. Thorndike}}
• {{cite book|title=Secret Naval Investigator|publisher=William Kimber and Co. Ltd, London (1961)|author=Commander Ashe Lincoln, Q.C.}}
• {{cite book|last=Showell|first=Jak|year=2009|title=Hitler's Navy: A Reference Guide to the Kriegsmarine 1935-1945 |location=Barnsley, UK|publisher=Seaforth Publishing, 2009|isbn=978-1848320208}}
• {{cite book|last=Williamson|first=Gordon|title=U-boat Tactics in World War II|location=London, UK|publisher=Bloomsbury Publishing|year=2012|isbn=978-1849081740}}
• {{cite book|last=Morison|first=Samuel|title=History of United States Naval Operations in World War II Vol 10, The Atlantic Battle Won, May 1943 – May 1945|location=Champaign, IL|publisher=University of Illinois Press |year= 2002|isbn=978-0252070617}}

{{refend}}

{{Commons category|Foxer (acoustic decoy)}}

• {{webarchive |url=https://web.archive.org/web/20080509184538/http://www.de220.com/Armament/Decoys/Decoys.htm |date=May 9, 2008 |title=Decoys and Obscurants }}
• {{cite web | url=http://stonebooks.com/archives/030202.shtml | title=Second World War Books Review | access-date=December 4, 2014 | author=Campbell, John}}

Category:Sonar decoys