Where Are The Big Ships?

[Wigabee]

I have got my new campaign off to a pretty good start, sinking seven ships on my first war patrol. The thing is, all of the ships were under 4,500 GRT and my total tonnage for that patrol was 14,000. Even though that is pretty good, I feel the need to hunt the big ships like C2 Cagroes, Large Cargoes and big tankers. I was wondering if anybody could tell me where these big ships lurk and if there are any tricks I can use to find them. I am not a fan of raiding harbors though just because it isn't realistic. As a side note, I received a message from BDU an my last patrol to not attack warships because it might humiliate the enemy. I was wondering if it would hinder me if I attacked a warship that came my way or if that message is just a nice touch of realism in the game because I would hate to pass up sinking a cruiser or battleship if I crossed paths with one. Any help is appreciated and good hunting!