You can change the models using a good 3D modeling app such as Wings3D, 3D Studio MAX, or Maya. 3DSMax was the tool used by the SH3 devs at Ubisoft, if I recall correctly.
In the 'SilentHunterIII\data\Objects' folder are all the files for the "turms", or conning towers, of the U-boats. The models are in the .dat files.
Use Pack3D or skwas' S3D (Silent 3ditor) to unpack the models into .obj files for editing.
And you now know as much as I do on the subject.
Good luck and let us know what you learned so others can learn, too!
