IMO: staying at PD is good cover because of the angle of the ASDIC array (as said earlier). However this depends critically on the year (later you are jacked because they don't have the same blindspot) and the weather. In stormy or rough seas, you can run even at 2/3, loading torpedoes, and be fine at PD. But if you are deeper, you're burned. The surface is a noise area, so if you're below it you stand out. If it's totally still, they hear everything for like 2-3km around - if an escort is approaching you, turn everything off and shut up. In perfectly still water, even early in the war (and GWX 3 since i started anew!) I've been detected by an escort passing at about 6-700m. This won't happen in stormy weather. I had one run right over me (almost hit me) in high seas, and i was all schtop and he just steamed on.
On another note - sadly, torpedo firing makes no sound in SH3. Escorts never yell 'Torpedo in the water!'. If you sit in the center of the convoy and fire 5 wake-less torpedoes and none of them hit anything, the convoy treats it as if nothing happened. Worse, premature detonations also have no effect on convoy (or anybody's) alert status. What a bummer. Again, UBI made mistakes that no one can fix.
Alas, I still get killed pretty reliably in '44
