[AS]

Historically they really eyeballed most of their targets. Speed could be estimated by the sonar man who could count the propulsion (pretty much the simple version of how they do it in high tech subs today). Apart from those theoretical things it was just experince. And we shouldn´t underestimate the "real world" factor: Just imagine you were on a real U-Boat tower with a huge merchant in front of you - how difficult would it be to hit such a vessel? It would be more simple than in SH3, I´m sure, because you were actually THERE and with some experience you would learn to judge the right data instinctively. Sadly, this cannot be simulated in any subsim I guess...