I took a look at Uboat.net
Quote:
Possibilities of underwater sound detecting in WW2:
Experienced underwater sound detector operator was able to determine is a detected ship a cargo ship or a warship (type of the warship), approximate speed or manoeuvre of the ship (approaching, digression, increasing or decreasing of speed and depth, if detected object was a submarine).
When detected (submerged) submarine stopped her moving, the operator was able to determine which mechanisms of the submarine still operated. To avoid own noises, a submarine could use underwater sound detector if her speed was up to 6 knots. If a submarine speed was 4 knots, the submarine's underwater sound detector average distance of detecting another object was:
- for a destroyer- 5 to 10 nautical miles, (9-18km)
- for a cargo ship- 3.5 to 7.5 nautical miles, (6-14km)
- for a convoy- up to 50 nautical miles. (90km)
If a submarine speed was 15 knots, the submarine's underwater sound detector possibility of detecting another object was rapidly decreased [although no submarines except XXI and XXIII were able to reach that speed during WWII]. In that case, average distance of detecting another object was up to few hundred metres.
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Ok perhaps in game hydrophone is bit overpowered if it always hears Merchant at 25 km. On the other hand, these are average ranges at 4 kn speed, so at 0 kn or optimal conditions ranges could be up to 25 km.
So it is hard to tell a final conclusion. Seems that we are left with the first hand accounts of the people using the hydrophone. Also I have to believe that if using hydrophone at night would have granted a significant advantage, surely the U-boat crews would have seized it. Hundreds of crews had spent thousands of hours at sea, so I imagine they tried everything to increase tonnage sunk.