Improving every year
I am not sure, but correct me if wrong.
First time they start to picking you up with radars the range hangs around 10 kilometers, then improving every year to 25 km (1944)
I was getting pickked with snorkel up from 10 km later days of war.
Periscope detected, seems to happen very easy. Some help did come when the periscope get covering layer uppgrade. I could drive with my XXI boat inside the convoy and carefully keeping periscope very low and was not detected.
I just love that XXI with all uppgraded, it is like stealth machine. (And fast like devil.)
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