How U-47 passed Scapa Flow's blockship defenses

How did U-47 get past the blockships guarding Scapa Flow?

It used a route where old blockships had decayed, shifted, or been removed, leaving a gap in the harbor defenses.

The approach routes were supposed to be obstructed, but several World War I blockships no longer protected their assigned gaps. Storm damage and decay had broken some apart, while others had been removed because they threatened navigation. Admiral Forbes regarded Scapa as unsafe and received permission to disperse much of the fleet, although Royal Oak remained. U-47 entered through a passage where the planned defense no longer matched the physical harbor.