Tidal changes..

[KL Seestern]

Originally Posted by Floater

Tide was a very important factor in the Scapa raid. The raid was timed to coincide with a spring (or neap, or something - I'm not very good at this stuff) tide, both during entry and exit from the harbour.

From http://www.u47.org/english/u47_sca.asp :

'The date set (for) what became known as Operation Order North Sea No. 16 was the night of 13/14 October, when both periods of slack water would take place during the hours of darkness. U-47 was set to leave port on the preceding Sunday, 8 October. The selection of the dates was not lost on Endrass [Prien's 1WO at the time] - as well as casting off on a Sunday, the mission had been timed for Friday the thirteenth. If anything was to test that age-old superstition, this was going to be it.'