Dunno how close to history Travally has stuck, but the info here might be useful,
http://en.wikipedia.org/wiki/Operation_Hartmut
Operation Hartmut was the U-boat part of
Weserübung
Preparations began in early March.
On 4 March 1940
Karl Dönitz received orders from Berlin:
Further U-boat sailings are to be stopped. U-boats which have already sailed are not to operate off the Norwegian coast.
At this point, commanders and crews were simply ordered into the North Sea. A sealed envelope aboard each of the U-boats held their official orders. The envelope was to be opened upon receiving the radio code word "Hartmut" (hence the attributed name Operation Hartmut).
On 6 April, the codeword "Hartmut" was transmitted and German submarines began their designated operations.