First of all, I refer you to
https://en.wikipedia.org/wiki/Sinkin...rt_proceedings wherein we read:
"Captain Turner gave evidence in England and now gave a more spirited defence of his actions. He argued that up until the time of the sinking he had no reason to think that zig-zagging in a fast ship would help. Indeed, that he had since commanded another ship which was sunk while zig-zagging. His position was supported by evidence from other captains, who said that prior to the sinking of the Lusitania no merchant ships zig-zagged. Turner had argued that maintaining a steady course for 30 minutes was necessary to take a four-point bearing and precisely confirm the ship's position, but on this point he received less support, with
other captains arguing a two-point bearing could have been taken in five minutes and would have been sufficiently accurate."
A two-bearing method is not as precise as a three- or four-bearing method. It is, however, good enough to get a u-boat into a solid firing position where last-minute updates and visual inspection can permit the final adjustments necessary to get the job done.
With nothing more than a hydrophone, a ship's approximate bearing and range can be known. This range is not that accurate, but I generally put a 200 meter circle around the ship's location to represent that the ship is "around there somewhere." This circle can show up differently on different zooms and will not be exactly on the ship depending on the zoom you use. Accordingly, it's necessary to use the right zoom and this is more an art than a science. Some practice will be required.
Some 5-10 minutes later a line can be drawn from the ship's old position (the center of the circle) through the ship's new position. Your sub can move during the procedure. The longer you permit between the two measurements, the more accurate the result will be. For example, you may find that the ship seems to be moving at 60º (ENE) or thereabouts. This is usually accurate give or take 2-3º either way.
When you are within 5000 meters of the ship, you can detect the ship using the periscope (weather permitting). Your TC will drop to 1 (depending on your settings). Pre-radar I just move forward at 2 knots with the scope up at TC 64 and I know the ship is 5,000 meters away when it gets spotted. From there I can draw the ship's course through its location (shown by map contacts) and go scope down for 6:29. In real life this probably would not have been necessary because I could have just ordered my hydrophone operator to count the propeller rotations and tell me the speed. I believe there's some sort of a mod that permits one to do so manually. Still, I have found 6:29 to be "good enough" as it gives me accuracy to within 0.5 knots. Ship's speeds will vary anyway depending on the weather – it's not uncommon to see ships vary between slow/medium depending on wave strength. That will tell you that it's 7.5 knots right off the bat.
After going scope up, if the ship seems to be on the line you drew, then the course is accurate. Alternatively, you can adjust it 1º either way and set up your firing solution. I generally fire a two-torpedo salvo and both hit about 70 percent of the time. Even if you're slightly off, one should still hit and that might be enough or you may have to follow up later with the deck gun. Even if both hit, sometimes one turns without exploding. These things happen.