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Spelljaming is the conversion of magical energy directly into motive force for ships in space. There are a vast myriad of ways in which spelljaming can be accomplished, however the typical method involves the use of standard Spelljammer Helms, such as the minor and major helms.
Spelljamming ships have two speeds, tactical and spelljamming. Tactical speed is a function of SR (Ship's Rating), which is in turn determined by the type of locomotion it has and the power of its helmsman. 1 SR translates into 500 yards per round (about 17 mph or 400 miles per day) which compared to most ground movement is a fantastic pace. It is not, however, a particularly impressive speed when measured against the vast distances of space. That is where spelljamming speed comes in.
When a ship is travelling at spelljamming speed, regardless of it's tactical SR, it can travel 100 million miles per day, 4 million mph. For those mathematically inclined, this is roughly 0.00596465972 times the speed of light. This speed can only be maintained in a straight line, meaning spelljamming ships must decelerate to tactical speed in order to turn.
Despite this massive speed, spelljamming ships are in no danger of collision with other objects. Objects of less than 10 tons in volume lose relative velocity to the spelljamming vessel as soon as they touch its air envelope, bobbing across its gravity plane until they come to rest and are slowly expelled from the envelope. If the spelljamming vessel comes within 12,500 yards of an object of greater than 10 tons volume, it automatically and instantly decelerates to tactical speed.