How large a target can you cast Heat Metal on?

Question

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:

1. As RAW, the whole metal hull heats up. (As I played it)
2. 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.)
3. The spell fails as the entirety of the object was not in range. (Very Nitpicky)
4. 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.

Thoughts?

Answer 1

Heat Metal works on "a manufactured metal object".

You are the DM. You decide what is an object. If you decide the skiff is an object then it is. If you decide it is an assembly of multiple objects then it is.

Heat Metal gives as examples "a metal weapon or a suit of heavy or medium metal armor". You can use this to inform your decision as to what constitutes an object. For example, you can decide that objects of about that size are objects but big objects aren't objects. Or that things that are now objects are objects even though they were previously multiple objects - chain mail is made of thousands of bits of wire twisted into links after all.

What you are dealing with is similar is a fuzzy concept - some things are clearly objects and some things are clearly not objects and some things might be objects or not objects because the boundary between objects and not objects is imprecise.

As to your examples:

1. Yes, if you like. if you want to treat an aircraft carrier as an object then aircraft carriers are death traps. You can rule that way if you like. You can also rule that say, the Eifel Tower isn't an object because it's individual objects held together by rivets (other objects) while an aircraft carrier hull is welded into a single piece. Or you can rule that an aircraft carrier is not a single object; it's a collection of objects and you decide which object's its made of.

2. No, because armor is specifically called out as a single object in the spell description.

3. No, the rules on spellcasting are that if any part of a target is in range the target is valid even if most of it isn't in range.

4. No, it heats up "an" object. It either is an object in which case it gets heated or it isn't and is therefore not a valid target for the spell.