Hello,
only to put some more oil into the fire
I do not know how SH3 handles this, but a submarine which is at periscope depth can usually not be detected in reality, for the following reasons:
Hydrophones: the surface noise of waves, even when it is almost calm, prevents a detection, if you do not crash a hammer against the hull. Running silent with up to 150 propshaft rpm will not give away your position. Just do not stick the periscope out too long ...
Early sonar/Asdic: the detection cone in front of a destroyer will not find a sub at periscope depth, because even its upper face has a downward angle which will pass below the boat. A boat that is so near as to be hit by the impulses, will 1st most probably be rammed in the next second with or without detection, and 2nd a hitting impuls would not be directly reflected to its source as long as the destroyer moves.
Only possibility is a thermal or density layer below the sub that would reflect the sonar impulse upward again, and then "touch" the boat. Even then the physical situation has to be in a way that those impulses are directly reflected.
Both statements are from a book "Submarine hunt and detection" from the 1960 ies, and even in that time with developped hydrophones and sonar etc. it was virtually impossible to find a sub at PD.
The only "classic" sub game with Diesel/electric boats i know of, that represented that behaviour, was "Wolfpack". If SH3 can be tweaked to represent this i would like to see it. Speaking of oil again lol ...
Greetings,
Catfish