Hydrophones - Press H to manually use the Hydrphone station. As already said, this is an underwater microphone. Max range approx 30 - 40km(in GWX).
Silent Running - Pressing Z puts your boat in Silent Running. If you are at flank then you speed will be dropped to about 2-3kts and all extra 'noise' is shutdown.
Repairs - These can only be carried out when NOT in silent running. Press Z again.
BTW - Some of the single missions are alot harder than the campaign. Just so you know.
EDIT - We've all jumped over this one. Oh well, we've all said pretty much the same thing so it must be true.
