In the closing hour of Friday 13th, U-47's final approach to Scapa Flow began. By 2307 hours it had made its way to Rose Ness, at which it was forced to submerge on spotting a merchant vessel. The problem having passed, the U-Boat resurfaced twenty-four minutes later at 2331 hours. The main defences consisted of booms - large defensive barriers - and the small Kirk Sound to the north of the little island of Lamb Holm was protected by a number of sunken wrecks or "blockships", strategically placed to prevent a possible submarine attack. The Kirk Sound itself was little more than half a kilometre in width; while the intelligence reports gathered before the raid had suggested that the route was not completely impenetrable, it didn't fail to mention the obvious dangers. It was through this small stretch of water that Prien was to guide U-47.
Using the Northern Lights to his advantage, Prien then ordered a course with a bearing of 320 degrees that followed the tide. He carefully scanned the coastline. Spotting a half-sunken wreck he assumed to be one of the blockships guarding the Kirk Sound, Prien ordered a course of 270. The announcement was made that U-47 was now in the Kirk Sound, but something was not quite right. According to the reconnaissance report, the Kirk Sound had been guarded by three blockships, yet Prien's scanning the projected route revealed only one. The mistake was noticed at the same time by the navigator when he noticed a sudden decrease in the gap under the keel. They were heading straight into Skerry Sound! Spahr desperately bellowed the order to change course and the boat was swung hard to starboard onto a corrective course of 30 degrees - turning at almost a right-angle.
Prien cursed at his mistake. But still the panic was not over, as the distance between the keel and the sandy surface of the sea bed began to drastically decrease. 2 metres... 1 metre... 0.50 metres... until there was the sound of the keel scraping against the bottom. Prien was undaunted, for there was no turning back now. He gave the order for both engines to be powered to full speed ahead. For a brief moment Prien feared the engines packing up, but Wessels had indeed done a fine job. The diesel motors roared loudly as the little boat pulled itself forward. After what seemed like an eternity, Maschinen-Hauptgefreiter Erwin Hölzer in the control room could finally report that there was clear water under the keel. Prien scanned the area once more and there were the three expected blockships. They were at the mouth of the Kirk Sound.
A diagram showing Prien's view of the blockships at the entrance to Scapa Flow. Not only were the channels narrow, there was also the matter of the fast-running current. (Image credits: After the Battle, No. 72)
With so little room to play with and under constant fear of the unpredictable currents, the path through the Kirk Sound was a perilous one; having studied the situation concerning both the tide and the position of the three partly-visible blockships, Prien instructed his helmsman Wilhelm Spahr to take the narrow channel between two of them, the Seriano and Numidian, knowing that there will be little space to spare below the keel.
Taking a course of 260 and running with the current, Prien's boat came within fifteen metres of the Numidian, and there was a particularly heart-stopping moment as the U-boat found itself momentarily grounded as its hull brushed the exposed anchor cable of the Seriano. After blowing the ballast tanks U-47 managed to free itself and rejoin the current, only to come so close to the shore that it found itself illuminated by the headlights of a passing taxi cab in the vicinity of St. Mary's Village. Prien and the others on the conning tower were convinced they had been rumbled; ship's joker von Varendorff's mocking comment that the driver should know better about blackout rules went down like the proverbial lead balloon. But the alarm didn't come.
A detailed illustration showing the path taken by U-47 through Kirk Sound to enter and exit Scapa Flow. When making its entry into the Flow, U-47 actually ran aground after having brushed itself against the anchor cable of the blockship Soriano.
By just after midnight on the morning of 14 October the U-boat had finally made its way into the harbour. This fact was duly noted in Prien's log book at 0027 with the famous words "Wir sind in Scapa Flow!" ("We are in Scapa Flow!") After the nerve-jangling prelude, the scene was now set for one of the most dramatic incidents of the war.