If target is not yet visible hydrophon will be the only mean available but you can't have precise data. Once your target is visible yes it's strictly optical approach like it was in real i think.
The first thing is to get target's course: I have a formula which work well, the only unknown thing is AoB and it's easy to determinate from a visual sighting (with training of course!)
Ship's course= uboot course + periscop deviation + 180 +/- AoB*
*If AoB is left you add it
*If AoB is right then you minus it
Sometimes it will be a great number, example uboot course 330, periscop deviation 318 and left AoB 100. So ship's course=330+318+180+100=928. A circle is 360° so real ship's course is 208° (208+360+360) You can adjust the formula if you are at ease: the periscop deviation can be written +318 or -42 and instead adding 180 you can minus 180 due to trigonometric proprieties. So ship's course= 330-42-180+100= 208°
Even if it's not the real course of your target you have a good idea of the target's direction.
I will post a screenshot, more understandable i think...
Then estimate distance, speed...
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