[Magic] Portable Area - proposed 15% rule

[Gef]

In multiple campaigns, as a GM I've been asked to rule on Area spells on a moving object such as a ship? Would the ship keep right on moving out from under the spell? Or the latest manifestation in present campaign, using path magic, can a Ward be placed on a rug? If not, do you have some kind of worldlaw where the planet itself isn't hurtling through space?

So my proposed rule is that an Area spell can be cast on a ship, but not the whole ship, just up to 15% of its area (a seventh of its hexes). For the rug, if it were 5'×8', a section 2'×3' could be protected with a Ward, enough for a man to stand comfortably, especially if a design on the rug marks the boundary.

What do you think of the idea? Will it settle arguments in your campaigns? And the fraction's arbitrary; would you change it?

[David Johnston2]

I don't really think it's a issue. You usually cast an area spell on a surface. If the surface moves then the spell moves with it. If you cast it on a volume of air or water it would move with the currents.

[Anaraxes]

Kromm's suggestion (from 2008) would allow the spell to move with an object on which it's cast (such as a flying carpet). His limit was 50% of the area of the object.

And why not, after all? Area spells cast on the ground are still cast on a small fraction of a moving object (even in a lot of fantasy cosmologies in place of realistic ones).

Originally Posted by Kromm

I'd allow an Area spell to be cast on a surface and move when the surface moves if the spell's area were no larger than 50% of the area of the mobile surface. For figures on a map, I'd use size in hexes -- not skin surface area or some equally bizarre concept. For circles, this amounts to "no greater than 70% of the target surface's radius"; e.g., you'd be out of luck for a 1-yard-radius rug, could manage a 1-yard Area spell on a 2-yard-radius rug, and could park a 3-yard Area spell on a 5-yard-radius rug. For other shapes, estimate or count hexes. I'd also impose a limit that the spell can't exceed the surface's boundaries; thus, a 1 hex × 50 hex red carpet might theoretically allow a 25-hex effect, which is just over a 3-hex radius, but actually be limited to a 1-hex effect because a larger radius would fall outside it.

For all of the above, I'd exclude the world's surface and the surface of anything fixed to the world, like a building. I see no reason not to allow spells to fill rooms or forest glades to the boundary, since neither a room nor a glade moves relative to the world. And yes, this privileges the world as a surface . . . and I'm fine with that. It feels more magical to me.

As for surface tilt, I see no reason to favor an orientation. If all other conditions are met, why not allow Area spells on walls or ceilings, or on sloping floors?

[Dalin]

I asked a similar question last year in a thread in the DFRPG forum (Area spells on boats?). Kromm weighed in with a somewhat simpler ruling: the entire spell must be able to fit on the surface in theory, even if you leave some hexes out. See post #7 in the thread.

[Gef]

Thanks for those replies, look like folks are thinking along the same lines but more generously on the fraction.

[RyanW]

Originally Posted by Dalin

I asked a similar question last year in a thread in the DFRPG forum (Area spells on boats?). Kromm weighed in with a somewhat simpler ruling: the entire spell must be able to fit on the surface in theory, even if you leave some hexes out. See post #7 in the thread.

Which means smart wizards buy circular rugs.

[Anaraxes]

Originally Posted by RyanW

Which means smart wizards buy circular rugs.

Which the muggles all confuse with flying saucers. But at least the Secret is kept.

[Fred Brackin]

Originally Posted by Gef

In multiple campaigns, as a GM I've been asked to rule on Area spells on a moving object such as a ship?

There are several Spells where this is explicitly the case. See Waves, Current, Tide and Wind from Magic p.194-195.

[Dalin]

Originally Posted by RyanW

Which means smart wizards buy circular rugs.

And coracles over canoes.