my recommendation would be to save the eels for something bigger... and preferably a merchant... DDs blow up nicely but between you and me, they arent worth much renown OR tonnage.
Use your eels defensively if you have to.
assuming this is 1939 or early 1940, I would work my way south along the eastern British Coastal waters in hopes of running across a merchant or two on the way home. avoid run ins with planes and escorts.
return to base physically (ie dont hit escape and select return to base until you are within about 20 miles of home port) and there will be an extra 100 renown points in it for you.
Good luck!
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