Just to echo a few things that have already been said. Assuming you have a decent Hydrophone operator you'll pick up contacts well outside visual range. With experienced operators you'll get contacts reliably up to 20km out. Up to 25 if the target is extra noisy and in good conditions.
Visual range even with mods is going is usually 5-10km in average conditions. So the hydrophone is usually more efficient in finding contacts. Just remember that the target has to be actively underway to find it. I have popped up dangerously close to a parked destroyer because the hydrophone scope was clear and I got sloppy in forgetting to do a periscope check. Also your target has to be in the water. Land gun emplacements and airplanes are other things that need to be checked for visually.
Also, you can actually hear ships out to about 35 km even though they won't be confirmed as contacts until they get closer (and their engine/screw noise is louder). You can draw a line out on that bearing and then check again in a few moments to get an idea of the direction of travel.
Checking every 2 hours or so yourself on the hydrophone can add micromanagement but will get you the occasional contact that the AI hydrophone operator would miss.
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