Tater,
The Patrick J. Hurley 10,600 tons took 30 shells to sink.
http://www.subsim.com/radioroom/showthread.php?t=146217
The 6,900 ton Merchant Louisiane took gunfire for 10 minutes and sank in 10 more.
The 210 ton steam trawler Leukos took one hit and sank!
http://www.uboat.net/allies/merchants/288.html
The 6,834 ton Alcoa guide sunk with 27 shells.
http://www.uboat.net/allies/merchants/ships/1546.html
The 1,885 ton Catherine sunk with 12 in high seas U47 fired 113 rounds but only hit the ship 12 times. Range was not given.
http://uboat.net/allies/merchants/382.html
The 1,018 ton Vistula sank after 14 shells.
http://www.uboat.net/allies/merchants/59.html
The above list is from shelling only. No torpedoes.
Tater,
I went in the sub school and sank the 10,000 ton tanker in convoy with 30 shells. On the 30 th hit she exploded.
The campaign game would be a better way to test this I think.