there is a difference between a superstructure and a conning tower. mostly in the looks!
Heuhen: i forgot to tell you, according to SS she can also work with 4 screws without effecting any other parameter of the report (like speed, strenght and survivability to say some of them..).
That's good to know. I did design her with four, but it looks more correct to have 6. the only down side of 6 shafts is structural integrity.