The Ultramarine movie got me thinking about this and I have not been able to find further information on the topic. In the Ultramarine movie, the corrupted captain says "This is my ship". I would assume that Imperial Navy fly the ships, So my question would be are those naval personal considered part of the chapter cult, or are they on loan from the Navy and under the Command of the Chapter Master? Or is there marines that are actually flying the ships who are masters of stellar navigation and what not? Would appreciate any insight into this.