A quick look through the "Allied Merchants Hit" section of
www.uboat.net gives the following:
Medium Merchant (5590 tons):
Quote:
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torpedo struck on the port side just forward of the poop deck. The explosion opened a crack three feet wide across the deck, destroyed the interior compartments and flooded the engine room. The stern dropped 30°, apparently held in place by the shaft.
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http://www.uboat.net/allies/merchants/ships/2220.html
15,000-ton liner took 1 torpedo to stop, but 3 more to sink, plus some shellfire.
http://www.uboat.net/allies/merchants/ships/739.html
A 10,000-ton tanker that also took 4 torpedoes to sink.
http://www.uboat.net/allies/merchants/ships/1843.html
15,500-ton liner sank after only one torpedo.
http://www.uboat.net/allies/merchants/ships/406.html
I know that sunk ships usually can't give proper damage reports, but I thought this would illustrate the variety of sinkings. I have info on a Japanese merchant that sank after being hit by a dud torpedo, and another that took 6 torpedoes and 10 hours of work by the submarine to finally put down.