Stat queries

Hi all, i love the game by the way. Well worth the asking price, i have a question however. I'm playing a pretty successful run at the minute, just purchased a nuke cannon a few turns back, im wondering, what exactly does the 'hull' stat mean on the nuke cannon? Not the be confused with hull damage. What does this add? It doesnt explain it anywhere and i am reluctant to spend my points upgrading it until i know exactly what it is. 

Thanks in advance.