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 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 Inertial Dampening Field.
To acquire the Ship's total Hull rating, simply add the Base Hull from its Size and the Resistance ratings from the Outer and Inner Hulls together.
Starship Size
Armor Systems
Specialized Hulls



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