[joegrundman]

Well as the war went on, i think they got better at it,

and the british made it a key point from the beginning, but they had the advntage that most of the world's merchant marine engines were built in Britain in the years before the war started.

Anyway, even with what you have, you can do pretty well:

A merchant moving at medium speed according to your hydrophone operator is either travelling at 8 or 9 knots

Now that piece of information helps you a lot, and if you are close, 1 knots difference will not stop your fish hitting.

there are other factors too: if you can see the target isn't too far away, but your hydrophone operator is having trouble hearing him, chances are it's very slow, say 5 knots or less

joe