depends...
of course, the longer one observes its prey, the better the calcs are.
Now at math i suxxorx, but how much difference does a 3 second difference make... how far off will your shot be, if a ship is 50 to 150 metres long, makes 6 kts, and moves for 3 seconds?
guess its not worth the hassle, considering that you have to keep your eye on your prey, gauge the distance accurately too, and do it in not-so-nice weather as well?
if such a long observation period is needed, i suppose the ship is either making high speed, (difficult shot) or far away (difficult shot)
... in both cases, or maybe if both factors are combined, a torp shot is not likely to hit, right? and you do not want to send two or three eels, hoping one hits...
Then you better surface and run ahead of the bugger, at the presumed course.
(...??)
