My experience is they completely ignore the decoy 99 times out of 100, and if they do key on the decoy they only do it once within the first five minutes. Timed it once with the stopwatch, decoy stops bubbling at 10 minutes on the mark, but going 8 knots you only cover about 2700 yards in 10 minutes.
I read someplace that the decoy ingame doesn't make any noise so wouldn't be heard on passive sonar (hydrophone) but the cloud of bubbles would reflect an echo for active sonar (pong for the ping). Dunno if that's true or not, or how they actually tested it, assuming it's a fact rather than a theory. (Lot of opinions and theories on the internet stated as if they were proven facts, so I take everything I read with a grain of salt.)
Actual experience over the last seven years ingame, it strikes me that they SHOULD be at least twice as effective as they are, or they don't deserve the name "decoy".
