I think you're right. This book mentions the attack on SS
Cathrine by Gunther Prien and U-47. The claim is that it took 113 rounds of 8.8cm to put the merchant down. There doesn't seem to be any indication of how many shells actually hit the target, which means that it may have taken a lot less than that number to sink the ship.
http://books.google.com/books?id=gDP...veness&f=false
There is a lot of good information in that article.
On the other hand a quick look at U-boat.net's article on the attack says that U-47 fired 113 rounds for a total of 12 hits. That means that 101 rounds missed. It also means that it only took 12 shells to sink Cathrine, which only displaced 1885 tons.
http://uboat.net/allies/merchants/ships/382.html