Yes Jim, indeed, I don't think it has a sweet spot, they're hard to sink with one fish anyway. I don't shoot at those anymore unless there really isn't anything else worth shooting at. that includes any other merchant of equal tonnage
As to large cargo's what the others said, one fish only Mr. Borodin, one fish only (bit of red october there sorry
)
same goes for large merchants, Cunard Liners and any warship under 10000 tonnes, although you have to hit the Dido's in the engines I find