Ahoy and welcome.
I have yet to encounter a big warship but even without them, convoy attacks are nerve-wracking! My first one also involved me getting right in between two columns - took out two large merchants and then dove for all I was worth and got away clean. I was lucky that my two chosen targets were in the first 2-3 rows so I just turned 90 degrees to port and headed straight back underneath the rest of the convoy - by the time they had all passed overhead I was running silent at 130m and the escorts never picked up on my position at all.
I still can't believe I pulled it off.