This is a common misconception. The ship will take the same amount of time to transit the wire at any angle. You can do this with your finger and a pencil at home. Put your fingertip on the table and run a pencil along it at different angles, and you will see that it takes the same amount of time end to end to run along your finger. The only time it would take less time is if the ship were moving sideways.
That said, the larger the AOB, the easier it is for you to recognize where the bow starts and the stern ends.
Also, if you want to do the math yourself, simply take the target length divided by the number of seconds, times 1.94, or 2 if you want to keep it simple.
I echo John’s sentiments about the scope exposure. Gather your data on the surface while overtaking, and if you need to estimate speed while submerged, use this method with care OR simply eyeball the speed. The rate of its bearing change is a good indication of speed, as is bow wave.
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