Homebrew
Armored Hull: The airship must take at least 15 damage or its immune to the damage taken. Half the damage taken can be repaired by non magical means as long as a crystal wasn't destroyed. Requiring a DC 15 Carpenters Tool check (The Roll uses Dexterity.) Crystals: If the ship takes 75 damage a Crystal is destroyed reducing the ships max health by 75. The ship needs a minimum of 2 crystals to remain afloat. Crystals cannot be repaired except by magical means.
Anyone on the boat can do one of these actions in place of their own action on their turn. Ballista: +7 to hit, range 120/480, Hit: (16) 4d6 piercing damage (reload 3) Requires an action to reload.