Hull plating is an external armor component of a starship's hull which allows for greater resistance against space debris and enemy weapons fire. Prior to Earth's development of shields, Starfleet equipped their vessels with the ability to polarize their hull plating, which allowed for increased hull strength in emergency situations. (Star Trek: Enterprise) However, even into the 24th century some escorts were equipped with ablative hull plating to allow for increased survivability when shields are lost.
The starboard hull plating of the Chaffee was slightly damaged when it entered the energy barrier of a L-class planet in a rescue attempt of Lisa Cusak of the USS Olympia. (DS9: "The Sound of Her Voice")