Question

[Mittelwaechter]

I'm playing DID since my first career and I start every career in '39.
The allies install radar onto escorts in '41 (iirc - in game) and it's resolution is quite bad. There seem to be several steps in radar quality until they are able to detect the snorkel in '44 or '45.
In calm seas it's a good idea after reading incoming/passive radar signals to run decks awash while closing into a convoy to get into weapons distance - or better - visual range. If you stay surfaced an escort will soon head your way to check the radar echo. With decks awash this happens significantly later.
I guess the game radar "detects" you in dependance of the reflecting surface you show above the waterline. This corelates with the better radar resolution of later models. In '43 more escorts are equiped with better apparatus and in '44 and '45 they pick you up anyway while surfaced.

But in '42 it's still a good idea to close in decks awash.