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By the way, does anyone know how range from the deck was normally estimated? Did they go down and use the scope or can it be done with the UZOs?
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It was estimated with the MKI eyeball, with reticles in the binoculars, and sometimes (rarely, not a standard item in a U-Boat, but sometimes issued to them) with a hand-held rangefinder device.
I have a transcription of an interview with Ace Georg Lassen, where he explains that the estimates were practiced a lot. In the naval academy they went to sea several times per day for the sole purpose of practicing estimates of range, AOB and speed.
Then, they also had some aids which we don't. For example, they had a realistically
curved earth, with a distance to horizon known (8.5 km from the Uboat bridge). So, any ship with a fully visible hull was closer than 8.5 km, for starters, and coming close in the estimate also was aided with the binocular. Knowing the linear field of view at 1000 metres, you knew that if a ship of such length covers the full FOV it is at 1000 metres, if only one half at 2000, if only 1/4 at 4000 and so on.
Reinhard Suhren (Who had a specially abled eye for that) in his memoirs describes how he once made a "master shot" in U-48, while surfaced. He estimated the range by naked eye as 5000 metres, and according to the measured torpedo run he was off just by some tenths of metres.
But in any case, the plot made by german Uboat crews was quite different to what their US counterparts did. They relied on a direct comparison of uboat and target course and exact bearings, with only rough estimates of distance. Target course and speed were estimated more by comparison with own uboat (Which the Kaleun tried to put on paralell course to the target and overtake it at constant distance), and the plot served more to determine the zig-zagging patter than anything else. By noting the uboat zigzags as ordered by the Kaptain (Who in turn just mirrored what the target did), and seeing how the bearing changed as the uboat overtook the target, the navigator was able to made an accurate estimate of target speed and course.
Hope that explanation helps
sorry to write such a wall of text
