@rob42 I'm not completely sure if this is the way to go here, we're requiring implementations to fill an array with (new) objects following the schema in vessel.json, `additionalProperties` implies (by name, I haven't checked this up in the jsonschema spec yet) that the `"vessels"` object can have additional properties on top of the ones already in the `"vessels"` schema. 
I do agree with the type change, an Array would make no sense since we're associating vessel objects by name, not by index.