@Dani: Thanks for your helpful comments. I am currently thinking about making the response time shorter (-> less lost messages) for the time until end of 1942 and using the standard response time (more lost messages) for 1943 and later.
-> in accordance to urfisch's input:
- early war: Uboats have no problem relaying messages
- late war: Uboats forced to dive more often - messages can get lost.