Converting vessel sizes is relatively straightforward.  Each Spelljammer ton is equal to 100 cubic yards.  This translates into 5.5 displacement tons.

Thin Wood (2" [5 cm]) = AV1
Thick Wood (6" [15 cm]) = AV3
Metal (1 cm) = AV2

Building a Spelljammer ship is very simple:  Design the shell, add in primitive mechanical controls and add the helm.  No special consideration need be given to life support, workstations, or accommodations.  Crew equals one (the helmsman) plus whatever deckhands and personnel required to man the ship's weapons.

Up to half the available tonnage can be devoted to cargo carriage.  The rest goes to weaponry and crew amenities.

Gravity:  All objects in space possess a 1G gravity field.  In large objects, such as planets and stars, this field is toward the center of the object.  Smaller objects retain the gravity field from which they originated, or the last gravity field they were in.  Asteroids, being remnants of exploded planets, have a definite up and down based upon their original orientation in the now defunct planet.  Ships built in planetary or asteroidal shipyards have their alignment based upon the gravity field of the planet or asteroid.  Objects within the gravity field of larger objects temporarily lose their own gravitational field, as to larger object's gravitational field overwhelms the smaller object's field.

Air:  All objects in space can maintain an air envelope, though not all do.  The envelope extends out one diameter in all directions.  Spelljamming vessels with a functional helm are constantly replenishing this air envelope, so need not worry about an air supply.  A vessel can carry up to 14 kilograms of individual per displacement ton without taxing the air supply.  If the helm ceases to function or more than the maximum number of people are being carried, however, the air will gradually become increasingly foul, and run out completely in 2D6 days.  A helm is considered functional even if no Empath is on it at the time, as long as the helm itself is not damaged or destroyed.  Small craft often go overloaded as they are used only for short duration flights.  In this case, the ship can carry up to 140 kg of individuals per displacement ton for up to 12 hours.  Dipping into the atmosphere of a planet or asteroid automatically replenishes the air supply.

Non-spelljamming settlements rely upon plants to replenish the air supply.  Each inhabitant (human, monster or animal) requires 0.25m² of plants per kilogram of inhabitant mass to maintain this air supply.  If only moss or lichens are available, multiply the requirements by 10.  If crops are planted, they fulfill not only the air supply requirement, but also provide enough food to satisfy the supply requirement of the population.  Such gardens may also be installed in spelljamming vessels at the rate of 1 kiloliter per kilogram of individual supported.

Supplies, including food, water, and sundries mass 50 kg per person per week, have a volume of 0.05 cubic meters and costs Cr50.

Humans mass about 80 kilograms.

Temperature: The temperatures within an atmospheric envelope will vary widely.  As long as a vessel has an active helm and is not within the atmospheric envelope of a larger object, such as a planet or asteroid, the temperature within the envelope will be whatever is comfortable to the Empath using the helm. 

Crew requirements:  All spelljamming vessels require a helmsman and a number of deck hands equal to the ship's size in displacement tons divided by 50 and rounded to the nearest whole number, but never less than one.  Each individual carried requires at least 0.1m³ of living space per kilogram of mass.  This space includes bunk space, access ways, and communal facilities.  Additional space can be allotted.  Passengers can be charged Cr30 per kiloliter allotted to them.  Long range expeditions would do well to allot more than the minimum space for each crewmember, too.  For each 0.1m³/kilogram allotted, the crew will be able to function at an optimum level for one month without a port call.  After that time period, all crew tasks become one level more difficult for each additional month, and crew morale suffers (-1 to morale rolls per additional month).

The chief method of moving through space is through spelljamming.  Spelljamming helms are the easiest way to get a ship moving, but not the only one. Helms can be acquired through a number of means, the most usual being discovery in old ruins or raiding an enemy ship for its helm.  They may be researched and built by powerful Empaths at great expense.  Or they may be purchased.

Spelljamming helms are magic items; they are extra-dimensional entities inhabiting throne-like forms in this dimension.  They are relatively unintelligent and not given to domination.  They have a modified form of sorcerous flight that allows them to travel rapidly through space.  They also have a special form of Change Environment, allowing them to maintain atmosphere and temperature within a limited area (the air envelope).

Two types of helms are generally available in civilized space, the major and minor helms.

	Minor Helm	2 kl	250 kg	Cr9,000	Maximum Ship Size = 300 tons
	Major Helm	2 kl	250 kg	Cr22,500	Maximum Ship Size = 600 tons

Spelljamming vessels have three movement rates:  Interstellar, Space and Tactical.  

Interstellar movement rates are equal to one light year per day.  Vessels automatically go to interstellar speed when twice as far from a system's primary as the orbit of the most distant planet.  As an example, the cutoff point for the Sol system is 78 AU.

The space movement rate is 1 AU (150 million kilometers, or 500 light seconds) per day, regardless of the Ship Rating (SR) of the vessel.  This translates into a speed of about 1736 kilometers per second.

Ships automatically drop to tactical speed when within 100 diameters of another spelljamming ship.  The diameter is based on the largest dimension of the largest ship.  Non-spelljamming objects, such as asteroids and planets, will force a spelljamming vessel to drop to tactical speeds within one-tenth of the diameter of non-spelljamming object if the object is larger (in its greatest dimension) than the vessel, or within 100 diameters of the spelljammer if the vessel is equal to or larger than the object. 

Tactical movement should be carried out using a hex grid, with each hex representing 10 meters (identical to the 10-meter grid squares used in GDW's House Rules system).  Each point of SR translates into a tactical speed of approximately 27 kilometers per hour.  This is its maximum speed.  Spelljamming vessels do not worry about cruising speeds; however their NOE speed may not exceed 40 kph.  Spelljamming vessels are treated as lift vehicles for determining movement, so an SR of 1 translates into a combat move of 4 10-meter hexes.

Any Empathically-endowed character may operate a spelljamming helm.  Note, an Empath need not be at the helm at all times.  If, for any reason, the Empath using the helm has to leave the helm, the vessel will continue along its current vector until an Empath mounts the helm and changes the direction or speed.  The tactical speed of the vessel is based upon its size and the type of helm mounted:

	Ballista
	Catapult, Heavy
	Catapult, Light
	Light Bombard

Squid Ship

Lizard Men (AD&D)

Both the original Spelljammer game and the GURPS conversion use the old "standard" system for measurements.  In keeping with the standards of GDW's House Rules, these values have been converted to the metric system.  Besides, who's to say a fantasy milieu would use either system?

The "magic" system used in this conversion is pulled directly from Dark Conspiracy's Empathy rules and Traveller:  The New Era's psionic rules.  No direct conversions are possible between the TSR and GDW systems, so the closest approximations have been made.  A list of current conversions can be found on the Advanced Dungeons & Dragons conversion page.

The space topography is based upon that given in the Polyhedron update.  The crystal shells are gone and the whole universe (which, for our purposes, is actually a proto-dimension) operates under the same "laws of physics."  This necessitates a third spelljamming speed, which will be referred to in this conversion as "interstellar speed."

A note on tech levels.  The original Spelljammer rules postulated a Medieval fantasy setting (TL 1).  This conversion has "updated" that setting to an Enlightenment-era type setting, more in keeping with a "Pirates of the Caribbean" and "Treasure Planet" feel.  The resulting tech level is 2 to 2M; problems with gunpowder have been pretty much alleviated through tweaking of the formula, so cannons and primitive firearms are fairly common.  However, because of the slow nature of reloading, melee weapons are still fairly common.