Hmmm, this could easily have been avoided if you had followed proper procedure, that early in the war you're supposed to surface, order the merchies to heave to and have their papers inspected, now If they're carrying war-materiels you sink them, if not, leave them alone.
Since they were unescorted that's what you should have done
Now, I realise that they would've steamed on regardless, which would be your excuse to sink a number with eels and then take the rest on with your deckgun, that would also have avoided the unfortunate collision :rotfl:
I'm sure your flotillenchef was not amused :p U-boats don't grow on trees you know...