I always go for the biggest targets that I can get. I came accross a fast small convoy in the carribean, IT had 2 large tankers, 4 medium tankers and a large merchant, Fired two torps at a medium tanker that was in range (the convoy had 4 buckley destroyer escorts, so I didn't want to be counter attacked by them in water lass than 100m deep) Hiting it with one torpedo, which slowed the convoy down, allowing me to fire two torpedo's at a large tanker that was at the maximum range tor T2 torpedo's. One hit near the bown causing her to sink 10 minutes later.
The escort never got a trace on me, due to a high sea state. Not bad for a daylight attack in heavy seas at long range. I usualy try to get within 1500m to 1000m of the target before fireing, but I was caught out of positon, and at the speed they were going I wouldn't have caught up. Plus the fact that there were aircraft in the area due to me sinking half a dozen tankers in the area
