@blackswan40 - thanx for the reply. While comparing your F7 picture with my own F7 screen, I pressed 'I' and noticed a tool tip that an external torpedo was being moved. So I time compressed until that torpedo got loaded into Tube 1 and then everything was good to go. Higher speeds were then available.
So the explanation for my u-boat's sudden speed limitation is that higher speeds are not allowed while moving external torpedoes around. Makes sense but I wish that there was a more obvious indicator when torpedoes get moved. Us newbies need all the help we can get.
Anyway, appreciate the quick response and here is a pic of my active mods courtesy of JSGME, just in case:
