Hmmm, Le Bug. Version 1.3 often did that, I thought it was fixed in 1.4. Still got the bug where you start with an aft gun you got an aft gun forevermore, there's no changing it between patrols. At the very start of a career it works fine though in 1.4, just moves the gun crew to empty slots.
What I always did for 1.3;
Increase renown,
Move the gunners out of the crew slots for the aft deck gun so it don't have a crew.
Spend some renown to slide a fore gun to replace the aft gun.
Check that the slot now says fore deck gun, slide the scurvy swabs back in.
Gots fore deck gun so I can shoot where I'm going instead of where I been.
Possibly that Narwhal is gumming up the works somehow, if I'm reading your previous posts correctly it uses some parts from the Porpoise? If so it might be using something from Sargo as well, and screwing up the base model it's taking parts from. Where did you get the Narwhal?