TANKERS!
They're like C2's that refuse to die. Spotted a large convoy in BF 17 just now, I sank all the small stuff leaving just the T2 Tanker, 1 immobile C2 and an armed trawler without any arms (tonage too low for me to waste torps on :P )
So what are their weakspots? I hit the rear building, then the front one.. then the middle section then the frontal section.. and he's still afloat. Are these things made out of rubber or what?