I normally shoot the nose off of C2's and C3's, takes about 30 rounds from the deck gun usually. Course you have to shoot the deck gun off the C2/3 first, or you'll incur hull damage due to their nasty habit of shooting back. For a Cargo with a front mounted deck gun, imagine a vertical line from the cargo's deck gun downward, and shoot forward of that. I'm playing SH3 with patch 1.4b (i think) and using a VIIB with the 8.8cm deck gun. No way i'd try ramming one though if i expected to finish my patrol
I've also been rammed by destroyers, while going backward at back emergency trying to blow them to bits before they get to me, and failing lol, and lost a few careers by not submerging quick enough.
I've found that even at back emergency, once they get closer than about 900 metres it's too late if they're doing 35 knots straight at you.
Oh by the way, i'm at November 7th 1942 patrolling out of brest, and when i wandered round to merthyr docks i found a passenger liner in it and en route to AL14 i went via, erm, is it Eyre loch? the one below and to the left of Scapa flow, and found an Illustrious class carrier, a Bogue class carrier, and a fji light cruiser, plus a T3 tanker in there. Thought it was my birthday lol.
Happy hunting fellas