I've heard this story before and, as already stated in one of the previous posts, the story is in all probability a fabrication.
However, like all the best myths there is probably a grain of truth in it.
While the Royal Navy at the time, and since, have been prepared to admit that the Royal Oak was sunk by Prien there is a particular complication to his story.
Prien maintained that as well as the Royal Oak he had also torpedoed and damaged/sunk another warship. Later German claims stated that the other ship had been HMS Repulse. I also believe that even after the war several U Boat men who'd been with Prien stuck with this story about the other 'two funnelled warship' that had been attacked.
The Royal Navy, to this day, catagorically state that no other ship beside HMS Royal Oak was attacked in Scapa Flow that night, let alone sunk.
Furhtermore it is now a matter of proof that HMS Repulse was not anchored in the Flow that night but was at Rosyth.
Repulse had been anchored in the Flow the previous day and, interestingly enough, in roughly the position where Prien had reported the mystery ship. The relative positions of Royal Oak and Repulse, and for that matter other ships in the Flow, would have been taken from German aerial reconnaisance, carried out the day before the attack.
Another fact to take into consideration is that Royal Oak had been taking on stores the day before. These stores had been loaded from the dockside by civilian labour. They had stcked crates and boxes in gangways deep inside the ship. My point is that there was opportunity to get explosive devices onboard the ship so sabotage can not be ruled out completely.
Hope it's been of interest
"There are no flowers on a sailors grave"