They do relate to the torpedo speed but not like you're thinking.
If you are not doing your firing calculations via trigonometry or manual TDC, then do this:
You should see a 1 2 and 3 on both the torpedo track and your target's projected course.
Right now think of them to be 3 different times, "1 minute, 2 minutes, 3 minutes" - they don't exactly but this will help you understand.
If you were to launch a torpedo, the torpedo will travel from your sub to "1". At the same time your target travels from it's current position it it's "1" on it's course line. Note: In order to see the target's course line I think you have to "lock" onto it with your periscope.
Same goes for when the torpedo reached it's "2", the target has now reached it's "2".
When your torpedo passes it's 3, the ship will pass it's 3.
If you were to increase the torpedo's speed, your numbers on the torpedo track will change their distance from your ship, but the target's will not change unless it actually changes speed itself.
So in order to aim, you line up your torpedo solution and the target's course so that they reach their 1,2, or 3 at the same time.
The numbers get really close or criscross completely, you fire, the torpedo travels from your ship to "1", then "2", and finally "3" as your target does the same on it's "1", "2" and "3" and if you lined them up right - BOOM!
Note you don't HAVE to line up directly on a number for this to work. The positions just have to be relatively equal. If you line up your torpedo to intercept the target at 2.25 on your torpedo guideline, then you need to shoot when the target is also around 2.25 on it's projected course.
Hope that covers it!
~GDF
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