The principle of hydrophones was simple enough. It consisted of two pairs of underwater microphones which listened to the sound of ships’ propeller noises. By measuring the amount of time it took for sound to arrive at each of the microphones, the device could triangulate the bearing of the vessel from the U-boat. The radioman could also tell if it was a merchantman or warship, but not the range, direction or speed it was moving. Because sound travels much further underwater, hydrophones could pick up distant convoys traveling up to 100 kilometers away. For maximum effectiveness however, the U-boat had to submerge and stop all engines while the hydrophones listened in for a few minutes. It also had an added bonus of being passive.
The standard U-boat hydrophone, the GHG (Group Listening Apparatus) was installed in U-boats from 1935 onwards. It consisted of two sets of hydrophones mounted on each side of the bows, covering two arcs of 140 degrees on the sides of the U-boat. Because the hydrophones could not be rotated, the triangulation was most effective with sound sources coming from the sides, with deteriorating accuracy as the source moved to the front or rear of the boat. Consisting of 24 hydrophones, the GHG could pick up lone vessels up to 20 kilometers and convoys up to 100 kilometers away. The detection range however was also dependant on sea conditions.
The KDB (Crystal Rotating Base Apparatus) was an improvement of the GHG in that it was rotatable and hence able to provide more accurate readings from any direction. The disadvantage however was its extreme vulnerability to depth charges.
The Balkon Great (Balcony Apparatus) was an improved version of GHG. Where the previous had 24 hydrophones, the Balkon had 48 hydrophones and improved electronics, which enabled more accurate readings to be taken. The Balkon was standard on the Type XXI and was also fitted to several Type VIIs.
Designed as an early warning system for incoming torpedoes, the TAG was installed on the Type XXI and intended for future generation U-boats. It was connected to a loudspeaker inside the pressure hull which would give audible warnings on an approaching torpedo. It functioned by listening in to certain pre-programmed sounds, which would trigger the alert status.
Sondergerat Fur Aktive Schallortung (S-Gerat)
The S-Gerat (Special Apparatus for Active Sound Location) was essentially sonar. It transmitted sound pulses and timed the returning echo to detect underwater objects. Because it emitted sound pulses which also broadcasted the U-boat’s location, this device was not very popular. It could detect objects up to 4,000 meters away.
Sonderapparat (SU-Apparat) – Nibelung
This device was one of the latest sensors being installed on the Type XXI, which coupled with the acoustic torpedo, would enable the boat to launch its torpedoes blind from up to a depth of 50 meters.The SU-Apparat (Special Apparatus for U-boats) or also known as Nibelung functioned as an active/passive sensor. It consisted of two basic elements – a hydrophone and a sonar device. Once the hydrophone picked up the bearing of a surface vessel, the sonar would ping in that direction to determine range. Up to three short pulses may be needed, which when plotted together, enabled the operator to determine the target’s speed, range and direction. An acoustic torpedo could then be programmed and fired at the target. The maximum range was 4,000 meters, and also depended on sea conditions.
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