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Last game my party debated over setting off a fireball in the cargo hold of a derelict cog. I figured under the circumstances it would basically be a roll on the item saving throw table to see whether the hull was breached. Sadly they didn’t choose this course of action. But it did get me wondering how a fireball or lightning bolt used as artillery might work. I couldn’t find anything official in the book so I wanted to see how other DMs adjudicate this.
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I would go with this from page 276:
"Combat with giant sea beasts is adjudicated
normally, treating every 4 hp damage inflicted as
1 hull point."
I would also do an saving throw to see what happens to the stuff in the hold as well as the ship itself.
So...Class 9 save vs Thick Wood which even has hull parenthetically in it. :-)
I only know this since my group was attacked by some great white sharks and one rolled a x2 damage crit for 59 points of damage. I didn't remember the exact rule I quoted above and had it only do like 7 hull points instead of 14 or 15. The ship has only 14 hull points.
However, the group and I know the correct ruling for future encounters.
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This question definitely came up as we have a shaman, a cryomancer and a pyromancer in the party and until recently we did a lot of seafaring.
We decided that a fireball could be aimed at something on the deck, like the hull or an NPC or at the hull of the ship itself. cast at the hull of the ship itself, we'd use the normal damage rule described above. if the fireball targeted something on the deck, like an NPC or the mast, this would not damage the hull points of the ship but could do all sorts of other things like knock down the mast, bring down the rigging, set the hull or cargo aflame, etc.
Our party was particularly proud of an encounter with a pirate junk in the Lemurian Remnant where the unknowing pirate ship was hammered with a lighting bolt at the mast and a fireball at the crew.