[Nisgeis]

You haven't given enough information - are you playing stock, or a super mod or any other mods that may affect the game? Generally, if submerged, show a narrow profile to the nearest escort and go slow and quiet. If on the surface show a narrow profile if possible, keep your speed down or increase your range to avoid detection. But it all depends on what mods what year and what the japanese force you are attacking is. Whether they have radar late in the war etcetera. What you can 'get away with' changes throughout tha war and also heavily depends on the mods you have installed.

This would be better posted in the Silent Hunter 4: Wolves of the Pacific forum, instead of here. You'll get a faster response if you post in the right section, as some people don't read both forums (that often).