When did the KDB hydrophone enter service?

[Cybermat47]

So yeah, when did the KDB enter service? And which U-boats actually used it?

[Aktungbby]

http://www.cdvandt.org/GHG1996.pdf (http://www.cdvandt.org/GHG1996.pdf) Cannot find KDB info but several top secret Brit reports indicate that several u-boats discarded KDB and returned to GHG...:hmmm: A rotatable base system was first introduced in 1918 until 1925 (pg 4 of link) The krauts were way ahead of Britain and the US in passive crystal in-hull receptor arrays.