User blog comment:David Whitaker/Me Ship Ideas!!!!/@comment-68.207.92.224-20120101050312/@comment-1140008-20120101063106

Choose one of the 3 upgrade tiers, Reinforced, Cargo, or Streamlined, upgrade them to Rank III, after that you can choose a special broadside hull to upgrade to. (Reinforced tier gives Explosive, Fury, and Firebrand 1, Cargo gives Fury, Firebrand 2, and Firebrand 1, Streamlined gives firebrand 2, firebrand 1, and thunderbolt.)