[Kromm]

Area spells can be anchored to anything that has a surface large enough to contain the entire spell. Spell diameter is (2 × cost multiplier) - 1 yards, so the smallest dimension must be no smaller than this. Put another way, the cost multiplier cannot exceed (smallest dimension in yards + 1)/2.

Example: For a ship's deck that's 7 yards wide and 25 yards long, width is the smallest dimension, and thus the limiting factor. That's enough to fit an Area spell cast at (7 + 1)/2 = 4 times base cost. As the diagram on p. 12 of Spells shows, that gives exactly 7 yards of radius, which is small enough not to exceed width.

You cannot cast a larger spell to cover the entire vehicle area and "leave out" parts of the spell just because Area spells let you leave holes. To anchor a spell to a mobile surface, it must be theoretically possible to anchor the entire spell.

In the example, you couldn't cast at 13 times base cost to get 25 yards of radius and then just leave out everything beyond the width of the deck to encompass the entire deck from stem to stern.

Nor can you cast a larger spell to cover the entire vehicle area and not "leave out" anything, to enable a small vehicle to cart around a large Area spell. Again, the entire spell must fit atop the vehicle.

In the example, you couldn't cast at greater than 4 times base cost to get 9 or more yards of diameter, giving a portable area effect that extends beyond the deck to either side (and possibly ahead and astern).

I realize this is less fun for those seeking to exploit magic shamelessly, but it keeps things under control.