Spanish Steam San Carlos was a 223 tons vessel:
"At 19.50 hours on 16 Dec, 1940,
U-37 missed the
San Carlos with one torpedo off Cape Juby and decided to attack the vessel at 20.00 hours from a distance of 800 metres with the deck gun and the 37mm AA gun, but the deck gun broke down after 21 shots. The 15 crew members and 13 passengers abandoned ship in two lifeboats, but one man was lost. The Germans rowed in a dinghi to the ship and placed scuttling charges on the vessel that later sank the ship.
Doing so they noticed that the fired rounds had not been able to penetrate the 20cm strong wooden hull of the ship. "
From:
http://www.uboat.net/allies/merchants/722.html