I hope that helps
So, yes there are only two electric and two diesel engines.
Let me explain.
NSS_Porpoise.zon defines the compartments. There are 4 compartments of type 21,22,55,56. These are the links to zone properties in zones.cfg.
increasing armor will make the engines take no damage at all when the bombs cannot penetrate it.
increasing hp will cause less damage but will still damage both.
What you can do:
1. you can have 4 engines. by creating 4 new zones and zoneproperty-entries.
2. you can make the zones smaller and wider appart so its harder to hit all engines simultaneously.
Adding new zones is hexwork, you can just copy existing zones and change their coordinates. At same time you can make the displacements. When adding new property entries ones must be carefull. You may not just add new zones to the zonelist. There can only be 210 zones and they are all used. So what you can do is replace an existing zone. You must find zones that are basicly the same and combine them to make a free zonenumber that you can use.