Thanks For the input! Will revise.
I think ballast could be a one per fleet crew but not have Limit.
Like: You may only have one of this crew per fleet. You may eliminate this crew to give this ship an extra S to this ship’s base move for the rest of the game. (You have to include this crew into the build total.)
If it’s a named crew, the “one per fleet” part is covered by the no-duplicates rule.
Whether it can be duplicated or not is much less of a concern than the granting of a permanent, untouchable effect through elimination. If the goal is just to allow it to stack with a Helmsman, it’s going to be much simpler and far less problematic to just state that directly in the ability text, like, “This ship gets +S to her base move (this ability can stack with a Helmsman).” You can then easily add other effects, like taking up no cargo space or being unable to be unloaded.
There’s also the big question mark about what it represents thematically as a crew that’s being eliminated to gain the effect. It almost seems to imply that bodies are being shoved down into the lower hull to weight it down, when there are numerous less-problematic options for that.
Making it Equipment instead of crew would make it seem far more like a physical upgrade to the ship, and would keep it safe from most cancellers without taking it out of play entirely.