Ship-to-ship combat will work on a scaling system.
A ship-mounted weapon will deal 10 times the value of damage stated in the book for it’s man-portable variant.
A man-portable Heavy Crossbow would deal 1d10 damage.
A ship mounted Heavy Ballista would deal 1d10 damage (x10).
However, the ship’s Hull HP value works in a similar way. 1 Hull HP is equal to 10 person’s HP.
Therefore, only man-portable weapons able to deal 10+ damage could inflict damage on a ship’s Hull, and the value rolled on a ship mounted weapon corresponds exactly with the amount of damage dealt to the Hull.