http://uboat.net/allies/merchants/losses_year.html
You may want to note the tonnage details provided by the site.
Clicking a ship name will reveal the circumstances of the event.
I go for every merchant steamer/tanker - no matter the size.
Fishing boats/trawlers only per 8,8/3,7cm.
Under proper conditions I attack any warship larger than a trawler too.
If you are after renown you should concentrate on larger vessels - as they grant more renown.