anyone who has watched tv like The Blue Planet will know sonar can carry for miles underwater. Dolphins, whales etc can hear 'conversations' hundreds of miles away. Water is the best conductor of sound - ever! Radar is limited by the horizon, at what? 12000 - 14000 yards away. If radar is mounted on the deck, thats the limit but if higher, on the tower mast, you may get another 2000 yards. A good trained asdic operator will pick out the turning of screws at a reasonably great distance. Depth, salinity and temperature were of vital importance in the effectiveness of the British ASDIC, first researched in WW1 by French, British and US scientists. It later became known by the USA word, Sonar. The worlds first vessel to receive ASDIC was HMS P59 in 1919.
http://jproc.ca/sari/asd_gen.html
Sonar v U Boats
https://uboat.net/allies/technical/asdic.htm
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Captain!, there's a destroyer on the por........ periscope is flooded Sir!
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