I was running a game and the party was trying to flee from a flying Illithid raiding skiff. It was made of metal and had two Illithid crew members on the deck. The bard in the party cast Heat Metal on the hull of the skiff. I had no idea how to run it so, to keep the game moving, I ran it as close to RAW as possible. It was a manufactured metal object in range, so everyone on the deck had to take damage and try to make the save. If I had more time I'm not sure how I would have ruled it.
As it stands I can think of a few possible alternatives and want some external input:
- As RAW, the whole metal hull heats up. (As I played it)
- The spell targets one plate of the hull, and heats that up, anyone in contact with that plate is affected by the spell. (Each plate is manufactured, then assembled, kinda nitpicky.)
- The spell fails as the entirety of the object was not in range. (Very Nitpicky)
- the spell heats only a certain mass or volume, perhaps a 10 ft by 10 ft region of the hull. (Assuming that the armor of a huge giant is only large spread around)
Option one has the implications that you could cast it on an Aircraft Carrier and doom them all.
Option two has the implication that only one piece of a full plate suit of armor is targeted by the spell and the heat is conducted to the rest.
Option three has the implication that casting a targeting spell while only only having a creature's hand in range would fail.
Option four has the implication that its heat is limited in scope.