Star Trek Starship Hulls and Hull Systems

Hulls and Hull Systems
Starships have two hulls, an "outer hull and an "inner" hull. Each one provides up to 3D+1 Resistance, a form of defense which subtracts from any damage which makes it through a ship's shields (such as the effects of attacks which exceed the shields' threshold). Unlike the protection provided by the shields, Hull Resistance does not decrease when it's hit by attacks; it reduces the damage of every attack which hits it. (One exception is ablative armor.) An experienced officer can tell the difference between an attack which hits the hull and one which impacts only the shields. The former is a deeper, more "solid" sensation; it rocks the ship and sometimes drains the IDF.
Outer and Inner Hull Structures
Armor Systems
Structural Integrity Fields
Specialized Hulls



Source
Spacedock - Starship and Construction Manual (pages 16-22)