[Pisces]

First of all, I'm not using SH5. Still Sh3 or 4. But that's all the same physics.

I either plot on the map, or do as I explained if I am in a hurry. The fixed wire method should not be done when he is showing an AOB that is rather small (near 0 degrees) or very big (near 180), since then you can't see his bow or aft clearly because of his deck-structures. If might even be obscured by the side of his bow if he's wide enough.

The idea with the fixed wire method is that when the periscope/uzo is along the bow/aft line that you project an imaginary fixed wall in the sea. Your speed is exactly along it. The target passes through it in the same time no matter what course he has, or your distance to him. His length is fixed so the speed is now known also, length over time. Mix in a conversion factor of m/s to knots of 2 (for simplicity) and you're done. You just need sharp timing with the stopwatch, and a steady course. Submerge if there is wave-action.

I've seen a website of a worker at a German Uboat museum which explained they had a real device to do that. A vertical line in the optics moving left /right was synchronised with the movements of the gyrocompass.