If you are tracking a convoy, you can pick up the lead contact on each measurement. In convoys, every ship travels at roughly the same speed. Even if there are escorts in front zigging and speeding up/down, it's overal speed is still roughly the same as it tries to maintain a fixed distance to the rest of the pack. It gets a little bit more complicated if they know you are around and escorts start weaving out to the sides though. Also, your relative position may cause problems using this method (ie. if you overtake them, another ship may become lead contact).
You can try to take over the hydrophone yourself as well, and listen for the first merchant it's screws, which rotate significantly slower. Again, pick the first contact and mark him. Alternatively, I've heard people count RPM to have a rough estimate of target speed (using a RPM/speed table), but I don't use this myself.
After you got a speed estimate, you can be fairly sure the other ships in the convoy travel at the same speed. So, in short, track the first contact.
I'm not a hydrophone guru, but I use this method alot.
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