A quick run-down for those unaware of the general details. Grubbian Physics are those physical laws that inform the Spelljammer setting. They include the concept of a gravity plane, a plane through the center of mass of a sufficiently large object which is down for either side of it, and air envelopes, a volume of air that any object in the Wildspace. There is Wildspace which is the temperate vacuum of space contained within a Crystal Sphere, and the Phlogiston, a warm multicolored space of combustible something that Crystal Sphere are suspended in also called the Flow.
An Eberronian Elemental Airship is made of soarwood, which is really light wood, and is powered by either a huge fire or air elemental in the form of a torus around the airship. Despite the description of the airships including both, I have only seen depictions of the flaming torus of a fire elemental; therefore, I will assume that for some reason fire elementals are the default. This also makes a few other considerations easier to think about, air envelopes and the Flow.
The two most important factors regarding the interface of Elemental Airships and Grubbian Physics are the air envelope and the combustible nature of the Phlogiston. The air envelope contains a finite amount of air, and will go stale or foul. The rate at which this happens depends on the size of the envelope, which depends on the size of the body, and the number of creatures consuming air within it. There are also considerations for large fires and other effects. As the bound elemental is bound in the form of a torus encircling the airship while active, I decided to not use the rules for fire and smoke fouling the air, which also simplifies things.
Elementals require sustenance. Something not apparent while being used to power airships within the skies of Ebberon as they draw their sustenance from their surroundings. Air elementals need air, moving air, or they will slowly die like a mortal suffocating or dehydrating. The air within the air envelope of a spelljamming ship is not enough to sustain an elemental indefinitely. This applies in a similar manner to the more common fire elemental. While fire elementals actually derive sustenance from the sun, which is still shining in Wildspace, they are fire and as such consume air. Consider either elemental to consume the air of five regular crew, and the elemental should be considered part of the ship's crew for purposes figuring the maneuverability of the ship.
As the Flow is combustible, highly, any kind of fire makes a fireball three times as intense. This represents a very real problem with using a fire elemental to power a ship through the Flow. Airships use huge (16 HD) or greater elementals. Piloting a ship with an active fire elemental will result in a 48 die explosion with no save by the elemental or the ship. Whether this is 48d8 (or whatever hit die you use for elementals), 48d6 (like a fireball), or three times the max hit points of the particular elemental (similar to how AD&D dragons calculate breath weapon damage) is up to you as the GM. This explosion will likely kill the elemental, possibly removing all motive power.
I recommend that instead of using the 3e/3.5 deck plans for an Eberronian Airship, that either the 4e or the galleon deck plans from Spelljammer be used. This is on account of the description of how large and how much cargo space is written having little bearing the monstrously huge airship plans in 3.5. An airship is described as having less cargo space than a comparable water vessel, such as a galleon or the elemental galleon. If using the deck plans of a SJ galleon omit the traditional rigging. As an elemental airship is built of soarwood, which is as tough as normal wood but lighter I recommend it be treated as light wood for purposes of saving throws.
There is an interesting possible interaction with a spelljamming helm and the elemental powering the airship. Ordinarily, I would rule that the elemental is essentially an exotic helm, like a furnace helm, and as such would not be able to function while another helm is being used and vice versa. I decided that the sight of an airship ringed in fire flying through Wildspace is just too cool to not allow. This also means that I can decide that while it might be possible to get to Wildspace with an elemental airship sans helm, it would not be capable of Spelljamming speeds, instead being restricted to tactical speed with an Spelljamming Rating (SR) of 1.The base Maneuverability Class (MC) of an elemental airship is D or poor, which of course only matter when the elemental is active and it is flying. Adding a helm, any helm, improves the MC to C or average, but only while both are active. This is for the most current generation of elemental airships, which are designed for flight within the sky of a planet and not Wildspace, reflecting its overall better MC when compared to a galleon or other groundling vessels fitted with a helm.
The PCs are hired on as crew for an expedition to the stars. Variations on this are: this is first mission of its kind, this is not the first mission but the PCs and indeed the whole crew do not know this and none of the previous expeditions have come back, or this is a expedition to look for the previous missing expeditions with no idea of why they are missing.
The PCs stumble upon a wrecked spelljamming ship, one of the sort clearly that is clearly a sailing vessel, while also not being one for sailing the seas or of conventional airship design.
Part Six: Air Elementals Don't Clean the Air
Spelljammer Beyond the Moons: for essays and deckplans, covers 2e and 3.x versions
Wildspace: The Spelljamer Fanzine: essays, book reviews, and inspirational art