This comes into play in an ongoing campaign, when the activities of many characters need to be coordinated. Time is the amount of time needed to build, repair, or modify a ship.

Some special costs are keyed to tonnage (such as rigging and any ram fitted). Materials used in the construction of the ship's hull determine its cost (which determines Armor Class). Tonnage determines ship size (by limiting the size of the air envelope), minimum crew size (a function of minimum rigging needed), maximum crew size (the amount of air available), the type of spelljamming helm that can be fitted, the number and type of ship-mounted weapons carried, and how much cargo space is available. Either way, the basic factors of ship design are tonnage, cost, and time. The players and DM can experiment with new spelljamming ship designs themselves, or they might handle ship design as campaign activity undertaken by the player characters.