Large steamer is the same as a Liberty Cargo i believe.
Last night on my way back to Lorient, out of eels and ammo for cannon, i stumbled up on a large convoy outside the channel. I almost fell of my chair. It was a convoy with Tankers with no guards. I reported it and left with while i pounded my forhead to my desk *siiiigh*... all that tonnage.
I usually see mixed convoys but troopships are a rare one.
Btw, i'm doing second patrol in Happy days