Shadow the convoy for a few hours at night and monitor the escorts. over time you might notice a pattern of when they break away from the group and execute a zig zag search pattern.
try to time it so that you are closing in on the convoy (on the surface) from either the 10:00 or 2:00 position just as one of the flanking escorts is at the outer limit of his search pattern.
when you feel you have established a comfortable firing position, order ahead slow and submerge to periscope depth and commence your attack.
for evasion i recommend going under the convoy. the confusion and collision hazards above you will sometimes prevent the escorts from responding with an effective or organized attack.
once you have evaded attack, head back to the left or right side of the convoy just at the limit of visual range and follow them again... repeat the process for a few days until you have managed to chip away at the merchants sufficiently.
good luck!
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