Welcome!
Making new ships involves the following:
1-Taking an existing unit
2-Cloning it using the tool S3d
3-3d modeling your unit in blender/wings3d or whatever
4-Importing the modified hull and other ship parts (3d obj files) into the new cloned ship file
5-Editing zon, sim, eqp files so that your new ship has the right durability, speed and armament.
You say you are working on a new turret, if so it will need to have its own entry in:
C:\Program Files (x86)\Ubisoft\SilentHunterIII\data\Library\guns.da t
C:\Program Files (x86)\Ubisoft\SilentHunterIII\data\Library\guns.si m
C:\Program Files (x86)\Ubisoft\SilentHunterIII\data\Library\guns.zo n
Then you need to add an entry in the ship's eqp file so it equips the new turret. Here is an example for the V/W destroyer, note how it shows the date the gun is equipped
[Equipment 1]
NodeName=M01
LinkName=4InchSingle_BR
StartDate=19380101
EndDate=19451231
I recommend you make sure your SIlent 3d editor is the latest and read this thread closely (link below) - I can send you a copy of the latest Silent 3d editor if you need it. Subsim members VonDos and KptLehmann are also good sources of info.
Also try just playing around with Silent 3d editor on "throwaway" files so you learn what does what.