Convoy Strategy
 

I seem to be coming across a lot of troop transport/passenger ships escorted by 4 destroyers, and was wondering what other captain's preferred strategies were:

The typical scenario is two or more merchants with a lead destroyer, one destroyer on each side, and one pulling up the rear. If I can catch the convoy at a 90 degree AOB, I usually try to close to 2,000 feet at 1/3 speed and silent running, launch 2 torps a piece at the merchants, and turn in the direction the convoy is heading. This puts my stern tubes in line with the outside destroyer which is now about 1,000 feet behind me and closing fast. As soon as the torps hit, or the escort closes to within 800 feet, I launch a single torpedo at the escort from the stern at shallow depth, dive to 200 feet and turn again, facing away from convoy (180 degrees from my attack position). This lets me finish off any wounded merchants, or sink any destroyers if they get too close. Once below the thermal layer, I'll slip away at 2 knots while the escorts pound the waves.

I hate to waste torpedoes on escorts, but I seem to have even less luck if I engage the merchants from +4,000 feet as they seem to detect the torpedoes and change course and speed. (even when running the torps a little deeper)

Any thoughts?

i use my radar and contacts, use the ruler to draw up an aproxamate course.
staying far away i then race ahead and go to peri depth using the ruler once again to estimate the distance i will then be from the convoy once they arrive, silent and stopped

destroyers never spot me in stock unless i move over 1/3 speed or do stupid stuff such as come out of slient mode...