[TorpX]

Originally Posted by WH4K

I think it's really strange that the escorts don't seem to hear your (very loud!) diesel engines when your're cruising on the surface. Yet, if you submerge and run fast and/or do noisy things (reloading torps etc) within their sonar range, they pick up on it rather quickly.

Of course the above assumes that you have not been eyeballed/caught on radar on the surface.

Am I imagining that, or is there some good reason escorts wouldn't have heard a sub running on the surface?

My comments are based on what I've read about RL, not TMO.
Most of the noise made by a moving sub is from the screws, not the engines, per se. I'm not sure about them hearing you on the surface, but I don't know why they wouldn't be able to. Possibly wave action would make it harder. I think this was one the the reasons why sub captains tried to brake into convoys; once inside the escorts could not distinguish the subs noise from the merchants'.

O'Kane mentions in one of his books that the later fleetboats were much quieter, due to the fact that the screws had been redesigned, to reduce the cavitation which caused most of the noise.

As far as the game AI is concerned, I only know what has been posted by others (mainly Ducimus), but it seems the game sensors/ detection schemes are oversimplified to a great degree, and apt to be rather unrealistic.