The merchant men may be the priority but,.....IF you can get in a favourable attack position one eel in the bow of the cruiser might be enough to take him down,use his forward momentum to plough onward to his watery grave so to speak
if you hit the merchant men and cripple them,and the weather is ok,you can surface later and finish them off with the DG even if they are armed,as long as you stay at a respectable distance
Best of luck
