Spelljammer ship stats from the orginal system are somewhat different to the same or similar terms in 5e. Below are the guidelines for converting those stats for use in the new D&D 5e system.
Armor Class
To get the 5e AC of the ships from AD&D Spelljammer, simply subtract the AR from 19. So a hammership that had an AR of 6 now has an AC of 13.
Saving Throws
When it becomes necessary for a ship to make a saving throw use the following chart for the bonus to any appropriate saving throw (a ship cannot make a dex save against fireball. If you do not understand that, I cannot help you).
| Old Saving Throw AS | Saving Throw Bonus |
| Wood, Thin | +2 |
| Wood, Thick | +4 |
| Crystal | +4 |
| Metal | +6 |
| Stone | +8 |
THAC0
To convert the THAC0 of ship weapons to 5e bonus to hit subtract the THAC0 from 20. A medium ballista had a THAC0 of 14. In 5e it is +6 to hit.
Range
The range listed for the ship weapons in the old rules is the normal range. Ship weapons can fire up to 50% farther, rounded down, but at disadvantage.
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