Yebbut....
First, you would have to be headed exactly 180 relative, because small angles would introduce an error proportional to your range from the target. Any deviation from 0/180 relative would cause 10x greater error at 10 km than at 1 km.
Second, the angle subtended by the target at 10 km would be 1/10th that at 1 km, so the transit time would be 1/10th as long. If the target transited the wire in, say, 30 seconds at 1 km, it would transit in 3 seconds at 10 km. A 1-second error in measuring the transit time (not an unlikely amount) would cause a 3% error in the measured speed at 1 km, and a 33% error at 10 km. So, if the target were moving at 10 kts, you might measure the speed as 10.3 kts at 1 km, but as 13 kts at 10 km. Using a normal angle to the track and a 30-kt Typ II electric torpedo, the 3-kt speed error would result in a 6-degree error in the firing angle. (24 degrees of deflection versus 18 degrees for a correct solution. At 1 km, a ship 150 m long subtends an arc of 8 degrees. Aim dead amidship, and your torpedoes would pass well astern.
So, yes, you could measure the speed of a merchant from 10 km using the fixed-wire method, but your possible errors at that range would make the resulting measurement untrustworthy for a fire-control solution. On the other hand, if all you were using it for was to plan an end-around, then the estimate would probably suffice. Just allow for that greater error range when trying to reacquire the target at the other end.
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