U-boats had inflicted enormous damage on Allied shipping earlier in the war. They attacked supply convoys as well as naval vessels. The Gut Holz, 67m (220ft) long, was nearly brand new.

The SM UC-55 U-boat had been laying mines between Shetland and Orkney when it was forced to surface. A World War One German U-boat has been identified by divers off the coast of Shetland.