That's a bearing rate computer not used for deriving target speed based on target length. It must be used along with the bearing rate charts not even part of Silent Hunter 4.
Basically it estimates speed based on the change of bearing change per unit time. Just as a thought experiment, you can understand that a target moving at right angles to your viewing angle (AoB 90º), the further away from you it is, the slower the bearing changes.
Now visualize an attack where you are at 90º from the target track but it is approaching from five miles away. At first the rate of bearing rate will be agonizingly slow, perhaps a degree in several minutes. But as it gets to a bearing of 350 or 10, depending on which direction it is coming from it will change one degree in a second or very few seconds.
From the Submarine Torpedo Fire Control Manual:
It was a very fussy and error prone procedure with lots of provisos in there which would warn the sensible to avoid such a procedure!
Here's how you plot your data on the maneuvering board that Silent Hunter somehow totally left out of the game:
And here's the bearing rate data form that also got left out of the Silent Hunter series:
Regardless of what you understand or do not understand as a result of this tour of reality, none of this has any relationship to calculating target speed by timing the length by the wire. Maybe we could talk about using a Ouija board?
