Remember that the hydrophones only have so much range; I think it is something like 20 km. Also, depending on what model hydrophone you have, your coverage could vary. Just sitting in the open ocean listening isn't gonna find anything.
The way I use my hydrophone to hunt down escorts is like this:
If I get a contact report, I plot its estimated course. 'Slow' means its traveling about 6-7 kts, medium, 8-10 kts, fast, 11-30 kts. I estimate the fastest speed. So a contact traveling slow, I will trace its bearing and estimate it going 7 kts, that means 3 hours later it will be 21 nautical miles away. Using this, I can get a 'lane' that I can retrace backwards.
In good weather, and if my plotting is accurate, my watch officer will spot the ship. However, if I'm off, or weather's bad/dark, by retracing this 'lane' I'm close enough to the ship's actual position to pick it up on hydrophone. The better your crew is, the easier it is to pick up ships on the hydrophone.
Once you get sonar, things get even easier since you can 'ping' the ship to find its exact distance from you. This makes it much easier to intercept, and a single ping isn't as noticible to merchant traffic as having radar on (I think).
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