Have a look at BdU's war diary 23.10.1943:
http://www.uboatarchive.net/BDU/BDUKTB30332.htm
When shooting from the bow they needed to crash dive to avoid the torpedo and there were speed and sea state restrictions.
Personally I have only used acoustic torpedoes for a very short time before I got sunk. I do remember that sinking destroyers was kind of easy but survival was not because the buddies of the sunken destroyer were quick to gathwr around to depth charge me.
I also remember using an acoustic torpedo in a regular attack. Depth 3 meters perpendicular attack, impact pistol. Sank a merchant that way.