My method is even easier, and if the contact is in fact on a bearing bringing you closer to your boat (sonar man says "closing!") it is virtually foolproof, even if you don't get an exact AOB (until a peek thru the scope when it is close confirms the AOB).
Each time you get a reading draw a line from your boat to the contact's heading.
When the contact is first acquired, drive straight towards it to guage whether it moving to the starboard or port. Once you confirm that then angle your boat about 60 or so degrees off that heading to set up an intercept course. [the heading you choose depends on the speeds of both your boat and his ship and a few other things which are more intuitive than hard-and-fast]
Get periodic readings of his headings at regular intervals. What you want to do is keep the bearing of the contact in a constant direction (such as 60 as stated above). If he starts to close the bearing down to 0 then he is outrunning you to the interception point, in which case you may need to speed up and/or alter your course to an optimal heading (which in a non-XXI probably would involve surfacing and running at high speed).
If the bearing starts to get larger then you are outrunning him and will beat him to the interception point. If the bearing remains steady you will both arrive at that point simultaneously-naturally since you want to take a shot then you need to allow about 1,000 meters or so of room for you to shoot.
If you do it right and keep on steaming fearlessly ahead, you'll actually collide, which almost happened to me last night with a Norwegian freighter (which was neutral at the time and thus I couldn't have shot it anyway)-had to hurriedly order a crash dive and retract my scope and I just barely avoided his hull!
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