Thank you Reece but i finally found it.
So, in order to eliminate the fog from the Uboat's interior use S3D and open NSS_Uboat7(or 9)_CR.dat which is located in the data/Interior/NSS_Uboat7(or 9) folder. Look for the node "Interior root-COMMAND ROOM" and expand it. Then, look for the entry "FogParameters ", expand it and finally click on the entry "FogParameters". Change the value of "FarDistance" to 30 -at least. That is it.
Edit: Don't forget to click on "Save" too.