There is a separate category stream for slave ships (the voyages) that already exists. All you need to create the ship category is information about the voyage (where it left from and where it went to. And what date--at least the year)
We also have ships by name (since most ships had multiple voyages)
I created a ship category for the Esther and added it to John's profile, but I would need the voyage information to add it to the slavery ship stream.