A Spelljammer Primer

[Dungeon Master]

A Spelljammer Primer
10/13/02
by Paul Westermeyer <mailto:[e-mail]>


This primer has Paul's own campaign modifications built in to it, so it defers from the basic spelljammer rules, not by much though, and Paul's modifications are pretty decent. So if you don't have or can't find the core spelljammer set, Paul's primer will fit the bill nicely.



1. The Universe

Worlds in SJ are typically planets which are part of larger systems called a "spheres". Sphere systems are literally surrounded by unimaginably huge spheres made up of an unknown material which seems indestructible (called "Cyrstal Spheres"). These spheres in turn bob about in a substance called phlogiston, or the Flow, which is incredibly flammable but seems to be incapable of existing within a sphere. Otherwise it is undectable except for the brilliant, chaotic collors it is composed of. It is odorless and seems to have no effect on breathing. In the Flow planar contact of any sort is impossible, even for the gods/powers.

2. Gravity

Physics operate differently in the SJ universe. Gravity is a constant earth-normal powered force. Every object exerts this, but the direction alters according to shape, and only objects of a certain size (generally about 25' long) exert enough force for a gravity plane to develop. Spherical objects attract objects towards their surfaces uniformly, much as gravity works in our own universe. Objects with a more irregular shape develop a gravitational "plane" which extends along the most convenient axis, generally the longest. This plane works in both direction so that it is possible, for instance, to walk on the bottom of a ship.
This gravitational plane exerts a "field" which extends to the limit of a body's air envelope. When two such fields come into conflict the gravitational field of the larger body dominates. (ie: for those familiar with the debate, I follow the gravity at the air envelope theory rather then the gravity at touch school). This makes it dangerous to be out of pitch or alignment with a larger ship if you enter its gravitational field/air envelope. Specifically, this means that when a 'jammer enters a planetary atmosphere the planet's gravity becomes dominant.(you can walk on the bottom of your ship in space, but in a planet's atmosphere you would fall off and land on your head:))
Like all things in SJ, local conditions may vary. On some worlds the gravity might be more or less powerful then Earth Standard; this is rare however. An active helm does tend to lessen the effects of worlds with extremly low or high gravity but it does not negate them. Sages disagree on the reasons for this.
Also, though gravity fields only extend to the edge of the air envelope anything larger then 10 SJ tons in size produces a gravity "well" which extends 5,000 yrds (at least; some planets have larger wells) out from that body. For those knowledgeable about these wells (and using a proper craft) one can "sail" or "ride" these wells much like a sailing craft uses the wind. This movement is always tactical and thus only practical for movement in dense astroid fields and similar locations.
The more common manifestation of these wells is the way they pull vessels out of spelljamming speeds when entered. Within these wells ships shift instantly to tactical movement. This shift is obvious to the crew as a lurching sensation but causes no damage.

[Dungeon Master]

3. Air

All objects drag air with them whenever they leave an air envelope, this is a function of their gravity field. A typical human, for example, will drag enough fresh air with him/her to breathe for 2-20 turns. After that time runs out the air will turn foul for a like period of time and then become deadly and unbreathable.
For each SJ ton of size a ship drags enough air to support one man sized creature for 4 months. So, a hammership (60 tons) can support 60 crew for four months without needing to refresh the air. If the ship had 120 crew it could only last 2 months. When a vessel reaches its air limit the air becomes fouled; it smells bad and is stale and humid. All attacks and proficiency checks in a fouled atmosphere are at a -2 penalty. Air remains fouled for the same amount of time it remained fresh, once that time wears out it becomes deadly; each turn everyone aboard must save versus poison or pass out. If unconcious they must save versus poison or die.
While important air is relatively easy to replenish, entering a larger air envelope, like that of a planet or asteroid is one of the most popular and cheapest methods. Spells which create fog, like _Wall of Fog_ or _Obscurement_ can also replenish the atmosphere. There are many magical items which also affect air use. Green plants will refresh air, some vessels make great use of these for just this purpose. Many asteroid colonies keep at least half their surface area reserved for plants for this reason as well.

4. Wildspace ecologies

While set in space, the ecology of wildspace most resembles that of the ocean. Deep space, far from celestial bodies is generally devoid of life, but planets and astroids collect an ecology all their own. Creatures resembling sharks, squids, whales, and even giant plants float around these bodies. Most wildspace native creatures cannot survive on bodies larger then size "B"; but this is not a hord and fast rule. Since many of these creatures are carnivores it behooves spelljammers to keep a sharp eye out.

5. Spelljamming

Spelljamming is the province of the helmsman, the individual who sits on the helm and directs the ship's general motion (sails, rigging, and crew provide fine maneuvering). In most cases this is a spellcaster of sorts, but there are a handful of helms which allow nonspellcasters to fill this role.
When spelljamming the helmsman in a sense merges with the ship, he/she feels as if the are personally flying through space, and can percieve the world around the ship as if he were standing on the aft deck. The helmsman perceives damage to the ship as white flashes of pain, but takes no actual, personal damage in most cases. Sometimes, however, the pain is intense enough to cause unconciousness; this is called "spelljammer shock" and is usually a result of a critical hit.
In many ways, helming a vessel is instinctual, because the helmsman feels he/she "merges" with the vessel he/she can generally control the vessel as easily as walking. It's possible, with training and practice of course to become much better (spelljamming proficiency) but any helmsman can handle most functions. It's important to keep in mind, however, that the only time the helmsman controls maneuver on a vessel is when the mininmum crew is listed as 1 (there are few such vessels). Usually, the sailhands control all of the finer aspects of maneuveur, the more skilled they are the better the craft handles. The helmsman controls the speed (and whether such speed is forward or in reverse ).
While spelljamming the helmsman retains his/her normal senses and can hold a conversation with those nearby. In general, spelljamming is no more difficult then walking (except during combat) so that anything a person can reasonably be expected to concentrate on while walking can be done while 'jamming.

[Dungeon Master]

6. Ship Design

Why do Spelljammers look like they do? Why sails and wings, what do they do? Ship design is a complex system of sympathetic magic. Ships that are more maneuverable or speedy _look_ that way, or look like something that is that way (like a mosquito or locust). Why do sails billow when there's no wind? Because sails are supposed to billow when a ship is underway, that's the same reason crew members affect the MC of a ship by altering the sails or wings or whatever. It's all part of the magic of helms. This can produce odd situations, for instance on a galleon or other groundling craft a crewman must man the ship's wheel and turn the rudder as if the vessel were sailing on water when a turn requires it. While the rudder actually pushes on nothing, the action is necessary for the ship to turn in much the same way a wizard must move his hands in the somatic components of a spell if he wishes the spell to work.
Designing a spelljamming craft is a difficult, demanding process that requires long training. Many spelljammer engineers are spellcasters, though not all. The various SJ rules provide at least two systems for designing new ship designs, Leroy Van Camp has produced another on the web. None of these are presented here as they are not essential for play. Instead I am providing the standard ship statistic format with an explanation for each entry.
Name: Ship's name
Type: Ship's design, ie the SILENT STAR is an eelship.
Tonnage: How man SJ tons the ship comprises. 1 SJ ton =100 cubic yds volume
Hull Points: A ship's "hit points"; usually equal to tonnage (not always)
Crew: Min/Max. Min=helmsman+sailhands, Max=# mansized creatures for 4 months air
Maneuverability Class: A maneuverable the ship is, "A" is best.
Landing; Land: Can the ship land on the ground w/o damage
Landing; Water: Can the ship land in the water w/o sinking
AR: the ship's armor rating, how tough it is to hit and cause damage
Saves As: Material the ship saves as on the item saving throw table
Power Type: method the vessel uses to travel through space
Ship's Rating: the ship's speed. 1 SR = 1 hex = 500yds in space
Armament: what weapons the ship carries and where they can fire.
Cargo: How many "SJ" ton of cargo the ship can hold.
Keel Length: longwise length of the ship
Beam Length: crosswise length of the ship

7. Helms

Helms are the primary method of powering spelljamming vessels. Simply put, a helm is a magical device which channels magical energy from a some source into motive force for the ship it's attached to. Some helms operate on slightly different principles but these are few and rarely seen in the Known Spheres.
There are several different types of helms of varying types and abilities, below I list the types most common within the Known spheres and the "standard cost" for such a helm in an active spelljammer port like the Rock of Bral or Refuge. Note that in other places the price quickly becomes what the market will bear and thus can drop or rise considerably. The helms are listed in roughly ascending order of power and rarity.
Minor Furnace Helm
Cost:20,000 gp (SR1) or 50,000 gp (SR4)
Very similar to the larger Furnace helm, this helm propels a vessel by drawing on the power of magical items placed within it's fuel opening. Items placed within are burned (literally) by the fire within, and their dweamor is sucked into the helm to give it power. The size and shape of the furnace is such that only items no greater than 18" on a side can fit within. The item must fit entirely within to begin burning (a thick leather glove is used to protect the hand while adding fuel). Each 100 xp of value provides 12 hours of power. Minor Furnace Helms can helm ships of up to 20 tons in size. They commonly come in two varieties: lifeboat version-SR1, can attain spelljamming speeds, relatively cheap. Fighter version-SR4 but can not attain spelljamming speeds. These helms are used extensively within spheres (they explode in the philogiston, of course) to power fighters, lifeboats, harbor ferries, ect. Most are fueled by scrolls with continual light or some other common spell inscribed upon them. They can even be fueled for an hour or so if a piece of wood or paper with a Magic Mouth, Continual light, or even Nystul's Magic Aura spell has been cast (as oppossed to inscribed) on them. Like a furnace helm they can be helmed by anyone and do not drain spells from spellcasters using them.
Furnace Helm
Cost:100,000 gp
These items function just like the Minor Furnace helm in principle. Specifically, they provide SR 2 and can travel at spelljamming speed for ships upto 100 SJ tons in size. An item with an xp value of 150 provides 1 days worth of power. If two items are sacrificed at the same time SR is boosted to 3 but the helm might explode (25%). Some goblinoids are rumoured to power their furnaces with an incendiary, magical fuel. Some think this fuel might be related to smokepowder in some way but reliable reports are rare.
Minor Helm
Cost:2,000 gp per SJ ton
These are the most common helms in space, drawing their power from the spell energy of their helmsman (draining all memorized spells at a touch). These helms provide an SR of 1 for every 3 levels of the helmsman, rounding down so that at levels 1-5 SR is 1, and then so on. Minor helms can move vessels at spelljamming speeds. These helms are created most commonly by the priesthoods of Ptah and Celestian or by the elven priesthoods. Of course, most elven created helms are employed by the Imperial Elven fleet and they jealously recover those lost when possible. Minor helms are rated according to the size vessel they can power, which typically varies from 18-40 tons, with the lower sizes being by far the most common. Many of these helms are constructed from fine woods.
Arcane Minor Helm
Cost:100,000 gp
Sold only by the Arcane, these helms conform to the specifications for Minor helms listed above except they are all rated for vessels upto 50 tons in size and are all constrcuted of metal.
Major Helm
Cost:4,000 gp per SJ ton
These function just like minor helms but provide an SR equal to the users level divided by 2 and rounded down. A 3rd level helmsman would have SR1, for instance, and one of 5th level SR 2. These helms typically have power ratings of 42-60 tons, though some have been able to power much larger vessels, even as high as 100 tons in size. These helms are much less common then minor helms, they are rarely for sale. The only known, "common" manufacturer of such helms are the elven high wizards and their helms are usually used to power elven man-o-wars. All major helms are constructed of metal.
Arcane Major Helm
Cost:250,000 gp
Sold only by the Arcane, these helms conform to the specifications for Major helms listed above except they are all rated for vessels upto 100 tons in size. These helms are actually more common then thier non-Arcane counterparts among most human spacefaring communities because they Arcane can provide them reasonably regularly. Finding an high powered mage willing to create one, let alone several, is difficult indeed.
Many other types of helms exist, some feed off life, while others use psionic capability as the power source. Others are even more bizarre. This list is just a starting point for "standard" spelljamming cultures, and as always in SJ expect the unexpected.
In my campaigns helms are much more fragile then in the std universe. They make all saves versus magical assaults on a 2, but against purly physical attacks they save according to the material they are constructed of. This means all major helms, for instance, are not created equal. A metal helm is worth more then one made of wood, because of its greater durability. Wooden helms are by far the most common. Helms are also more difficult to fit, to be removed without incident the work crew must be supervised by someone with the shipwright proficiency. This is automatic in a ship yard where all the proper tools are available and plenty of time can be taken, in space (or when in a hurry) it requires a proficiency check. If the check fails, or if the helm is removed by someone without the proper proficiency, then the helm must make a save versus crushing blow. Helms are artifacts and contain very powerful magical forces. If one is destroyed it immediately becomes the center of 1d6 wild magic surges.
One last point, in my game the Arcane are the most predominent (and respected) helm merchants but not the only such suppliers. Used helms are sold by many different people, virtually any SJ shipyard will have a few available. Because of the highly magical nature of helms, however, many of these used helms have acquired curses and side effects similar to those of other artifacts. This might be a simple as a harmless phantom who haunts the helm and appears occasionally, or as deadly as a phantasmal killer which the chair releases to hunt crew members during the night. The Arcane do a brisk buisiness "purging" used helms of these curses, though it's not a hidden art-many high level mages and priests do so as well. Let the buyer beware!

[Dungeon Master]

8. Ship Crew

Ship's crews are rated according to their relative experience: Green, Average, Trained, and crack. For PCs, the crews "rating" is determined by the number of folks handling the rigging who have the appropriate proficiencies. Green: the majority have no rigging experience at all. Average: the majority have at least terrestial seamanship, Trained: the majority have the spacemanship proficiency, crack: all have spacemanship and have sailed together aboard the same vessel for at least 3 months. Green crews penalize the initiative roll by 1, trained provide a +1 bonus, and crack provide a +2 bonus.

9. Long Range Movement

Spelljammers have two main speeds; "Spelljamming Speed" is very fast, 100 million miles per day. Think of it like lightspeed or warpdrive, you can only attain this speed when you are at least 5000 yards from any object of 10 SJ tons or more in size. Conversely, coming within 5000 yards of any object 10 SJ tons or more in size will immediately and uncontrolably slow you to SJ speed. With very rare exceptions all SJ helms move at this speed when "spelljamming".
"Tactical speed" is much, much slower. 17 mph or 500 yards per round _per ship's rating_. Ship's rating is determined by the helm, and the level of the helmsman. Higher level, higher SR. Some spells, proficiencies and magic items can increase it as well. (Example: On the _Flowfish_ all the PC helmsman had an SR of 1 using that ship's Minor Helm.)
Ship's travaling at spelljamming speed are immune to collisions with small objects. When a ship travelling at SJ speed encounters an object of volume less than required to drop it to tactical, that item is "picked up" by the ship's envelope. That object then bounces up and down in the gravity plane until it stabilizes. Once it's resting on the plane, it starts to drift outward towards the edge. When it reaches the edge, it is "let go", and is left behind in the ship's wake.
This protection doesn't work at tactical speed, objects enter the air envelope with their velocity intact. This permits ship to ship combat at this speed (otherwise bolts and catapult shot would be useless) and also makes travel through certain astroid fields and crowded shipping lanes hazardous.
Within most crystal spheres navigation is not difficult, just a more complicated version of terrestial navigation by the stars utilizing three diminsional instruments.
Philogiston Navigation is much more difficult, there are precious few landmarks or navigation aids in the flow. So how does one get from place to place? The easiest method is to follow a flow river, and that is done by determining the predominate colors of the areas flow. There are many major flow rivers through the philogiston, and each has a distinctive color scheme that makes it distinct from the others. Since most crystal spheres remain in relatively stable positions within the flow one can navigate by counting the spheres passed as one follows the river. River intersections provide another means for determing location, as the colors do not intermix (Sages still debate the reasons for this). Travel to sphere's outside a major flow is much more time consuming and difficult: the navigator must first determine his/her exact position within the sphere to be departed, then he/she must determine at what angle from the sphere the destination sphere lies and plot a direct course to it. Obviously this method is very risky, though some navigators are so good they hit everytime.

10. Planets And Other Celestial Bodies

Planets and other celestial bodies are quite varied within spelljammer. Planets can be round, flat, disc shaped, cube shaped, or even stranger. Astroids, stars, nebula and other oddness rounds this out. A quick rating system for various celestial bodies has become standard through out known space. It's origins are much debated but unimportant for our purposes.
Celestial bodies in this system are described by size, shape, and type. These ratings are standard on most charts, planetary almanacs like the legendary Geonomicon, include a substantial discussion on the planets ecology, inhabitents, and weather conditions making them valuable commodities.
There are ten size catagories as follows:
• A= less then 10 miles across
• B= from 10- 100 miles across
• C= from 100- 1000 miles across
• D= from 1000- 4000 miles across
• E= from 4000- 10000 miles across (Earth, Toril, etc.)
• F= from 10000- 40000 miles across
• G= from 40000- 100000 miles across
• H= from 100000- 1000000 miles across
• I= from 1000000- 10000000 miles across
• J= from 10000000+ miles across
Shape catagories are easier as they are descriptive: Amorphous, belt, cluster, spherical, cubic, flatworld, and elliptical are often used though descriptions like "tetrahedon" are used when appropriate. Finally, a planet is described by type:Fire, earth, air, water. The planet is named after the most predominate feature, though this is subjective. Earth is an earth type planet even though 70% of it is covered with water, for instance. Some cartographers include a fifth type, "Live" which indicates worlds join by giant plants.

11. Atmospheric Movement

Travel within planatary atmospheres is a very tricky buisiness, spelljammers are are not really designed to stand up to the stress caused by wind and weather and in many ways landing on a planet is the most dangerous maneuver a spelljammer might routinely attempt.
When a spelljammer is caught in a storm the vessel must make a saving throw versus crushing blow each turn. This save can be modfied in several ways. If all sails are taken down and secured (reducing the ship's MC to "F") then the save is made at +4. Additionally, if the helmsman has the Spelljammer proficiency, he can add +1 to the roll. Regardless, if the roll fails the ship suffers a random critical hit. As some of these can be disasterous for a vessel within a gravitational field (such as spelljammer shock) atmospheric traval is often considered quite dangerous. Stronger storms impose penalties on the save, the DM informs the crew of this as appropriate.
Even when the wind is calm spelljammers are limited to a move of 24 in the atmosphere, the rough equivalent of SR 1. If the ship is pushed beyond this limit the vessel must make a saving throw again for each turn spent beyond SR 1, further more, the roll is penalized by the amount over. A vessel flying at SR 4, for instance, would have a -3 on the save. Positive modifiers can be used here as well when appropriate. Again the ship suffers a random critical hit when the save is failed.
Additionally, spelljammers are inherently less maneuverable within the atmosphere, all MCs are reduced by one level. All spelljammers can hover, however, though they can't turn while hovering unless they are MC "A" within the atmophere.
Note the saving throws for speed and weather are cumulative. If a ship is traveling through a hurricane at SR 6 that vessel would have to make two saves each turn, not one.
Finally, nearly all planetary bodies of size D and larger have a band of very high winds in the upper atmopshere, similar in most respects to the Earth's jet stream. Passing through this band, whether on landing or take off requires a saving throw versus crushing blow as well. Again, if the save is failed a critical hit occurs.
Ship's suffer damage in this way often enough to keep spelljamming interference within most world's affairs to a minimum. All the major campaign worlds, (Oerth, Krynn, Mystara, Toril) possess such a band of high winds. A few planets smaller then size D have it as well.
Beaufort's Wind Scale
Developed in 1805 by Rear Admiral Sir Francis Beaufort, this describes the strength of the wind based on the effect of wind on sailing ship. It was revised as steam became more used. The following table is from the 1957 Worldbook Encyclopedia.
Beaufort Wind Miles per hour
Scale # Designation Wind speed
0 Calm 0-1
1 Light air 1-3
2 Slight breeze 4-7
3 Gentle breeze 8-12
4 Moderate breeze 13-18
5 Fresh breeze 19-24
6 Strong breeze 25-31
7 Moderate gale 32-38
8 Fresh gale 39-46
9 Strong gale 47-54
10 Whole gale 55-63
11 Storm 64-75
12 Hurricnae Above 75

[Dungeon Master]

12. Languages

One of the great mysteries of wildspace is the way so many worlds possess the same languages. Elvish, for instance, is intelligible to the elves of most worlds, the same principle applies to dwarven, gnomish, and most other demihuman and humanoid languages. Dialects, accents, and even a few "odd" languages appear of course, but the widespread use of these languages on worlds whose development was seemingly unrelated is remarkable. The "common tongue" of Toril is almost identical to the common tongue of Oerth and Krynn, as well.
A variety of explanations for this have been advanced by sages, a current popular approach is the idea that each crystal sphere is in fact an alternate prime material plane in which evolution took a different course, and thus the languages just show that each sphere is simply an alternate diminsion. Another suggest that each races vocal cords are structured in a way which makes certain language forming sounds unavoidable. A third says this all comes from the gods. Many other explanations exists but when it comes right down to it, most spacers don't care why it is so they are just glad that it is so. After all, wildspace is a difficult enough place as it is.

13. Religion
Nothing illustrates the limits of the gods better then the philogiston, where priests are completely unable to recover their spells (their ability to man a helm is not effected, however). Moreover, all deities cannot be contacted in all spheres. Unless a diety already has an established band of worshippers (100 seems to be the minimum number) within that sphere a priest must go to extrordinairy lengths to stay in touch. For this reason many dieties highly encourage their priests to establish temples and centers of worship in new spheres, hoping to expand their influence.

14. Magic

In most respects Spelljammer is a typical setting in regards to magic. The only significent, constant difference is the way the phlogiston prevents any spell which contacts extradiminsional space or the planes in any way and the way fire spells tend to cause massive conflagrations in the phlogiston (just like any other source of fire). While magic does work differently in some spheres, in most of the well known spelljamming spheres it follows standard patterns. This is a major difference when compared to Planescape, for instance, where magic tends to change rules from plane to plane.
Note this stability includes the existence of wild dead magic zones within wildspace and the Flow. These are somewhat dangerous as they are difficult to chart, a dead magic zone in space is often called a sargossa as it causes helms to cease functioning and leaves vessels adrift, unable to replenish air. Unless a non-magical means of movement is available these become slow death traps for ships. The most common method of avoiding them is to bank as hard to port or starboard as possible the moment one is encountered, hoping the vessel will drift out with it's incoming velocity.
A final note concerning magic, in wildspace it is somewhat more common then on many worlds, even worlds as rich in magic as Toril, for instance. This is especially true of "household" magic used to make living easier. Artifact level magic is no more common (aside from the helms, if they are seen that way) then elsewhere, however.

15. Economics

Spelljamming economics tend to revolve around high profit goods, though as always this is usually a function of supply and demand. The civilizations of most large worlds are self-sufficient, they tend to supply much of the raw material and basic goods needed by spacefaring civilizations. In return they often are good places to sell rare products from other worlds. Wildspace civilizations require virtually everything, especially staples like food, water, building materials, even air.
Spelljamming is a very expensive buisiness, the cost of the helm alone often takes a ship owner ten years or more to pay off. As a buisiness spelljamming ships require high intial investments and are high risk in addition, they are attractive only to the more adventurerous investor.
Economic conditions within any given sphere are highly variable, it is advisable to find a guide or at least purchase a sphere book before undertaking any trade ventures within or to a unfamiliar sphere.

16. Proficiencies

There are many proficencies which have a direct impact on spelljamming vessels and spelljamming life. Most of these are detailed in the Complete Spacefarer's handbook or the War Captain's Companion. For a variety of reasons I consider the CSHb to be the more authoritative source, so when these to conflict I go with that source. A list of these proficiencies and their slot costs follows.
Weapon Proficiencies:
Ballista 1
Catapult 1
Bombard 1
Greek Fire projector 1
Jettison 1
Note: Nonproficiency penalties apply
when firing crew served weapons.
Nonweapon Proficiencies:
General:
Heraldry(space) 1 slot Int 0
Shipwright 1 slot Dex +1
Note: Spellcraft is also required to design a SJ ship
Signalling 1 slot Int +2
Slow Respiration 1 slot Wis 0
Spacemanship 1 slot Dex +1
Sail Manipulation 1 slot Int 0
Note: This allows a sailmaster to increase MC by one step for 1 rd
Wizard or Priest Group:
Cartography 1 slot Int -2
Note: Also in the Rogue group
Navigation (wildspace) 1 slot Int -2
Navigation (Phlogiston) 1 slot Int -2
Planetology 2 slots Int -1
Spelljamming 2 slots Int -2
Note: The Seamanship proficiancy is sufficient to handle the rigging on most spelljammers, on "groundling" vessels such as a galleon or galley seamanship is identical to spacemanship and provides all the needed knowledge. See the section on crew for how this affects the crew rating.
Also, anyone can attempt to use a proficicency, even if they don't have it, but the NWP score is halved before modifers are added. In some cases, additional modifiers will be used.

17. Ship To Ship Combat

Ship to ship combat is one of the primary features of Spelljammer, two different but very similar systems were developed, the original system in the first SJ boxed set and and "large battle" style system presented in the WCC boxed set. This system is a modfied version of both of these.
First the combat sequence:
Initiative (modified by MC)
Initiative Winner Moves
Initiative Loser Moves
Return to 1 and repeat process
Fire from personal missile weapons, spells, and siege weapons can occur during any of the phases above, but must follow the initiative order within each phase.
Keep in mind that in space, each hex represents 500 yds across. Most "personal" weapons can only be fired when vessels are within the same hex, a few (longbow, heavy crossbow, for example) have a range of one due to the lack of gravity. Spell ranges should be carefully considered. Generally, those with a range of at least 100 yards can be cast at targets within the same hex, those with less then 100 yards range can only be cast at grappled or rammed targets. Some spells only work when the air envelopes are mixed.
Ships move on a hex grid. Beside each vessel counter, the ship will have a 3-d indicator showing the ship's height above or below that combat's "0" level. Each vessel's bow will point to a hex side, not the joint between two hex sides. Speed is determined by SR, how often the ship can turn is determined by SR.
Maneuverability Class:
The helmsman controls velocity and has some very crude maneuverability control, depending on the design of the ship. The crew, working the sails and control surfaces, determine most of a ship's maneuverability. All ships have a Maneuverability Class which determines how far they can turn each round.
MC HFC SC IM RS CSv Notes
A 4 3 -3 3 +8
B 3 3 -2 3 +6
C 2 3 -1 3 +4
D 1 2 0 2 +2
E 1* 2 +1 2 0 *Must move forward 1 hex before turning
F 1* 1 +2 1 -2 *Must move forward 2 hexes before turning
G 0* 1 +3 1 n/a *Cannot turn
MC=Maneuverability class; HFC=Hex Face Change per round; SC=Speed change per round; IM=Initiative modifier; RS=Reverse Speed Max; CSv=Crash save modifier
Note: Officially, vessels must spend an SR to turn the HFC, but in my campaigns I simply make that the "maximum" turn each round and SR is not spent in that way. This makes for a more "fluid" battle where turning and banking are more important.
Shipweapons:
First, in my campaign all non-proficiency penalties apply even though the weapons Thaco is used rather then the wielders. So taking a large weapon proficiency can be a big help. Weapon's have "ranges" which are really the SR of their projectile. The range of a lt catapult is 5, which means its stone moves 2500 yards each round until it hits something or goes off into infinity. In planetary atmospheres the range is reduced to normal earthbound ranges.
Ramming:
Ship's equipped with rams can inflict massive amounts of damage. When ramming the Thaco is equal to the NWP score of the sailmaster, or pilot if a vessel's minimum crew is 1. If a hit is scored damage is: Attackers tonnage/10*SR. SR varies according to situation, for instance, if ramming a fleeing vessel from the rear the SR will be less then if the two vessels were colliding head on.
Ship's which attempt to ram vessels more then three times their size in tonnage, or vessels which ram without having a ram "crash" instead. The crashing ship usually takes a great deal damage, though it might inflict significent damage in return.
Shearing:
Ship's equipped with piercing rams or shearing blades may attempt to shear away the rigging of opposing vessels. This is treated as a ram attack, except no damage is done. Instead, for each successful shear attack the target's MC is reduced 1 step until repaired (usually after the battle).
Grappling:
First, the relative speed difference between vessels cannot be more then 1 SR when attempting to grapple (grappling rams exclude this). If both ship's crews wish to grapple the attempt is automatic, if not the base chance is 10 or less (d20) modified as follows:
Green crew attempting to grapple +1
Average crew attempting to grapple 0
Trained crew attempting to grapple -1
Crack crew attempting to grapple +2
Grappler has 2-1+ advantage in #s -1
Defender has 2-1+ advantage in #s +1
Degrappling is automatic if both vessels desire it, otherwise it's resolved just as a standard grappling roll except the defender ("degrappler") makes the rolls and gains the modifiers listed above.
Critical Hits:
Crewed weapons often have a chance of inflicting a critical hit, this is listed in that weapons statistics. Other events (ramming, some spells, turbulence, failing a save in high winds) can also cause critical hits. Finally, a vessel reduced to 50% of its hull points takes a critical hit. Note, since some critical hits reduce the ship's hull points there can be a short cascading of critical hits in this manner. When a critical hit occurs the result is determined by the following chart:
d20 roll
1 Loss of 5 Hull points
2 Deck crew casualty
3 Interior Crew casualty
4 Ship Shaken
5 Large weapon damaged
6 Deck Crew Casualty
7 Hull holed
8 Maneuverability loss
9 Loss of 10 Hull points
10 Ship Shaken
11 Fire!
12 Loss of SR
13 Deck Crew casualty
14 Large weapon damaged
15 Ship Shaken
16 Hull Holed
17 Maneuverability loss
18 Loss of 10 Hull points
19 Helm hit
20 Spelljammer Shock!
Most if the above is self explanatory. #20 causes the helmsman to save versus spells or go into a coma for 1d4 days.

Notes:

The above contains TSR trademarked material, as well as paraphrased copyrighted material. This use should not be construed as a challenge to that status.

Bibliography

• War Captain's Companion Boxed Set
• Spelljammer Boxed set
• SJR1 Lost Ships accessory
• Lyndon Baugh's "Wind" post to the SPELLJAMMER MPGN list