It's all in the Stars
An Astrological chart was produced. This star sign chart was aimed at U-boat captains. The British would find out when certain U-boats had sunk. Then they spread rumours that the captain of the sunk U-boat had ignored his own personal reading of the star chart, which had shown it was a bad day to set sail.

Sometimes if the British did not want the U-boats to go to sea on a certain day for various reasons, they would try to influence the captains of the U-boats by making sure they got a copy of the bad star chart.
This is the 1st version of the cover
of a fake astrological magazine
This is the 2nd version of the cover of a fake astrological magazine (2 colour)