Hi Rainer,
Many thanks for your very kind and informative reply. I had known that U-boat penetration of the Firth of Forth during WW II was very limited due to the a/s defences but could not quantify it. Your reply both confirms my suspicions and is very informative. With regards to WW I, I am aware there were quite a few U-boat scares, both real and imagined, in the Firth of Forth up until they got the a/s defences sorted out in late 1916 to accommodate the Grand Fleet. In both wars the approaches to the Firth of Forth saw quite a few U-boat related incidents and I am trying to compile a diary type list of these events which include vessels sunk by mines, gunfire torpedoes, etc. I have decided to use an 'approaches arc' for the Firth of Forth, between St. Abbs Head in south-east Scotland to Arbroath, north of the Tay Estuary. Many thanks again.