How to use the Stowage attribute of a ship in mechanical terms?
What the number means?
Do I count the total Encumbrance of the items against the stowage value or the idea is something else? What happens when you go above the limits?
Stowage entries are provided for those with limited space: if the value is a “—”, the amount of Stowage is so large, or so well-distributed throughout the ship, that it has no effect on the ship’s ability to operate. Generally, any vessel with the Ship Quality has enough storage to accommodate any reasonable amount of cargo.
This is better explained in the core rulebook (p137).
Generally, it is assumed that a mount or ship can transport any character (up to passenger limit) with their equipment. The Encumbrance of any additional items not equipped to the characters counts towards the Stowage capacity. If the Encumbrance goes above the Stowage capacity, the GM should impose penalties that make sense, e.g. increase the Difficulty of certain tests.
