May I immodestly suggest the post linked
here?
Perhaps what is confusing is the question of when target's AOB is 90. If you are facing perpendicular to target's true course, AOB is only 90 when target is directly in front of you, at bearing 0, or right behind you at bearing 180. Waiting until then to set up TDC isn't much use to you. For any other bearing, AOB differs from 90 by the amount bearing differs from 0 or 180. For instance, if target is moving from left to right on a course perpendicular to yours, then when target is at bearing 340 degrees, AOB is 70.
A) As long as you really are facing perpendicular to target's true course, a modification of the method in my link above can also give you true current AOB. Replace steps 8 to 11 with :
8. Rotate scope until you are pointing at target.
9. Lock scope on to target.
AOB readout will be correct and will be correctly updated automatically by the TDC, unless you alter course, until you unlock scope from target. To fire correctly:
10. Open tube doors.
11. Fire when gyro angle reaches 0.
B) Note that the above method A) aims the torpedo at the centre of the ship. To aim at a point aft of the centre, replace step 11 with:
11. When gyro angle reaches 0, unlock scope from target.
12. Fire when desired aiming point crosses vertical line in centre of scope.
C) To fire at an aiming point forward of the centre of the target, replace step 11 and 12 with:
11. When gyro angle is about 5 degrees from 0 (355 degrees if target is moving L to R, 5 degrees when target is moving R to L), unlock scope from target
12. Rotate scope in direction of target movement (i.e. away from target) until gyro angle reaches 0.
13. Fire when desired aiming point crosses vertical line in centre of scope.
D) To go to full TDC firing for any target bearing angle, as long as your course is perpendicular to target's course, replace steps 11 and after with:
11. Get a range reading
12. Immediately decouple TDC from scope to allow manual input.
13. Immediately set range on the TDC
14. Immediately couple scope to TDC.
15. Fire immediately, rather than waiting for gyro angle 0.
E) Method D) also fires at the centre of the target. To fire from any bearing angle at a point other than the centre of the target, replace step 15 in D) with:
15. Immediately unlock scope from target,
16. Immediately rotate scope to desired aiming point.
17. Fire immediately.
F) Finally, to fire at any part of target at any bearing angle when you are not headed perpendicular to target's true course, but you know target's true course and speed, follow these steps:
1. Lock periscope on target.
2. Decouple periscope from TDC to allow manual input.
3. Enter target speed on TDC.
4. Read bearing from periscope.
5. Instantly calculate AOB from your course, target's course and bearing to target.
6. Instantly enter AOB in TDC
7. Instantly couple TDC to periscope.
8. Get a range reading
9. Instantly decouple periscope from TDC to allow manual input.
10. Instantly enter range on TDC
11. Instantly couple scope to TDC
12. Instantly unlock scope from target
13. Instantly rotate scope to desired aiming point.
14. Fire immediately
If you don't know target's course and speed, you are going to need to first measure target speed, at least. If this doesn't also give you course, you will need to eyeball AOB or take careful measurements to determine course and then caculate AOB.
Since steps 5 to 7 and 9 to 14 all need to happen instantly, but this isn't really possible, you may need to adjust the values that you enter to take into account any change in AOB from the time you allow manual input to the time you couple the TDC to the scope, and any change in range between the time you measure it and the time you fire. For typical merchant speeds the changes will be negligable as long as you do not hesitate. To avoid hesitating while calculating AOB from two courses and a bearing, do most of the calculation in advance, and then just plug in the bearing to do the final step in the calculation.