Its always bothered me that the only real way to travel through space requires magic users and priests in abundance. While I agree that the SJ setting is more magic-heavy than is "typical" for a AD&D setting, the existing system makes it so that you can't swing a dead cat for hitting some kind of magic-tosser. The following additions are my own inventions for Spelljammer, and should exist side-by-side with helms and what-not.
ManaSails are a new product of the Arcane, and represent the latest in Spelljamming technology. The ManaSail converts ambient magical energy into motive force in a manner far more efficient than a standard helm. Of added benefit is the abolition of the "100 ton limit" in Spelljamming craft.
Each ManaSail generates enough motive force to propel 20 spacial tons of mass at an SR of 1. Only 1 ManaSail may be attached to a given mast. This system works so that if a 30 ton ship with 3 masts is equipped with ManaSails, it would have an SR of 2 (60 tons of motive power divided by 30 tons to move). ManaSails cost 5000gp each.
Ships may be constructed with "3-dimensional" rigging array (such as for wind-propelled dirigibles and whatnot) that allow for more masts, and hence more ManaSails.
The principle disadvantage to ManaSails is that they are easily targetted for attack, and are as vulnerable as ordinary cloth to receiving damage. The advantage, outside of no longer needing a mage or priest, is that Spelljamming speed is multiplied by the SR the ManaSails give the ship. Theoretically, ManaSails can allow even faster travel between the spheres.
Controlling ManaSails and maneuvering requires that the crew be retrained. Moving in space is fairly straightforward, but "station-keeping" or "hovering" in a gravity field is considerably more complex, and can only be done by an experienced crew, although it is easier if the sails are connected to "wings" which can be turned "downward" to resist gravity. For game-play purposes, treat this as a new proficiency, which defaults to existing Spacemanship -4.
GravWood is taken from the Integral Trees of Nivanspace. The Integral Trees are giant trees that spin around the gas torus in the center of the sphere (from a distance, they look like a spinning integral sign from calculus, hence the name). Any object made from this substance is unaffected by any other gravity field. For example, a ball made of GravWood will float in mid-air wherever it is left, if thrown it will continue in a straight line until it literally runs into something. Its most practical use is in ship construction, especially when combined with ManaSails (eliminating the hovering problem explained above.)
A GravWood vessel will support any amount of weight placed on it, as long as the amount of GravWood (displacement wise) is more than 50% of the total volume of the object. In practical terms this means a ship made from GravWood has no significant encumbrance limit beyond what can normally fit in its hold. However, it does mean that a single piece of GravWood bolted onto the keel is not going to lift the ship. GM's discretion as to what works and what doesn't.
GravWood costs 2x normal hardwood costs, and is treated as softwood for purposes of armor. Any ship made of GravWood will get a +1 bonus to its MC and a +1 bonus to its SR (this doesn't boost the SJ speed of a ship with ManaSails though!). Base MC is for softwood (so its 1 better than that!) These costs are for ships built/bought in NivenSpace, they will go up the further the wood has to be transported (obviously.) Whether Integral Trees can be grown on other air-worlds is up to the GM.
