Scapa Flow was the natural deep water base for the British Home Fleet. As the anchorage is surrounded by islands with only narrow channels between them it appeared to be easy to secure, especially when ‘block ships’ were sunk in these channel to further obstruct them. However as the base for many capital ships it was a natural target for German attention, whatever the potential dangers.
From the Log of Gunther Prien, commander of U-47:
E. of Orkney Islands. Wind NNE 3-4, light clouds, very clear night, Northern Lights on entire horizon. At 0437 lying submerged in 90 meters of water. Rest period for crew. At 1600 general stand-to. After breakfast at 1700, preparations for attack on Scapa Flow. Two torpedoes are placed in rapid loading position before tubes 1 and 2.
Explosives brought out in case of necessity of scuttling. Crew’s morale splendid. Surfaced at 1915. After warm supper for entire crew, set course for Holm Sound. Everything goes according to plan until 2307, when it is necessary to submerge on sighting a merchant ship just before Rose Ness. I cannot make out the ship in either of the periscopes, in spite of the very clear night and the bright lights. At 2331, surfaced again and entered Holm Sound. Following tide. On nearer approach, the sunken blockship in Skerry Sound is clearly visible, so that at first I believe myself to be already in Kirk Sound, and prepare for work. But the navigator, by means of dead reckoning, states that the preparations are premature, while I at the same time realize the mistake, for there is only one sunken ship in the straits. By altering course hard to starboard, the imminent danger is averted. A few minutes later, Kirk Sound is clearly visible.
It is a very eerie sight. On land everything is dark, high in the sky are the flickering Northern Lights, so that the bay, surrounded by English mountains, is directly lit up from above. The blockships lie in the sound, ghostly as the wings of a theatre. I am now repaid for having learnt the chart beforehand, for the penetration proceeds with unbelievable speed. In the meantime I had decided to pass the blockships on the Northern side. On a course of 270 I pass the two-masted schooner, which is lying on a bearing of 315 in front of the real boom, with 15 meters to spare. In the next minute the boat is turned by the current to starboard. At the same time I recognize the cable of the northern blockship at an angle of 45 degrees ahead. Port engine stopped, starboard engine slow ahead, and rudder hard to port, the boat slowly touches bottom. The stern still touches the cable, the boat becomes free, it is pulled round to port, and brought on to course again with difficult rapid maneuvering, but; we are in Scapa Flow.”