Hydrophone issues?
 

Was playing last night, and my sound man detected a merchant travelling slowly, but when I went to look at it myself and begin analysis, I could hear nothing and couldn't get a ping on him. I've read that when a merchant is going under half his maximum speed, he will not show up on sonar. Is there some manner of fix for this? I tried using Vickers Sonar Target Fix, but that didn't seem to solve my problem.

what range was the merchant supposedly at? What was the weather - stormy? Were you on the surface? Was it a voice message eg: 'Sir merchant at 315, closing?' or similar. Because I have found when I surface I get an old message about something that the sonar found but not recent. And no ship is there, its a bug.

on the same idea, in real life, a merchant/warship travelling under 7 knots could not be detected by an acoustic torpedo.

Stock bug! When an enemy ship is not moving at least half its top speed the crew can detect them but you can't hear them on the hydrophone. We're working on it with the FOTRSU mod. No other modding group has even known the bug exists, or at least they never talked about it.