As jimbuna says, the tonnage got sinking a DD is little. Just avoid them.
I remember playing the stock SH3 1.3 for the first time, the best position to attack a DD is 90º from Port or starboard and set the pistol to the magnetic position and (if i remember well) put the torpedo deep to 3 - 3,5 meters, depending the type of the DD.
If you have to choice between a DD and a C3 cargo.... the C3 cargo taste like the sweet honey
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