As long as the wind speed is at or above the StormConditions setting for your U-boat you won't be able to man the guns. It might look fairly calm out there at 7 m/s winds, especially after a few days of 15 m/s, but if you have the following line in your (for example) NSS_Uboat7b.cfg file...
StormConditions=7,0.1;max wind speed [m/s], max rain intensity [0,1]
... and the wind is 7 m/s or higher, the guns aren't usable.
GWX raises this to 9 m/s for the II, 11 m/s for the VII and IX and 14 m/s for the XXI. Mostly.
As a side note, the fatigue model assumes storm conditions at 7 or higher no matter what this setting.
Hook