Hi,
Usually you’ll need 3 torpedoes to sink for sure a capital war ship.
But sometimes only 2 can make it... just don’t leave the area before he definitely sink otherwise it will not credit you.
To settle the right depth, you need to identify the ship with the ID book, then send the data to TDC. Up left in periscope view, you’ll see the ship’s hull shape, and you can adjust the right depth to not pass it underneath
Be aware that in rough sea above 10m/s, even if you set the right depth, your torpedo can pass underneath because of waves elevating the ship.