I never had the time or money to get into Battlefleet Gothic, so 'nautical' lore is one of my weak points in 40K knowledge. Obviously Space Marine voidships are crewed by chapter serfs, helots, and servitors, but are the senior officers Astartes? Or is there some special class of mortal that handles there duties? My players are starting on a series of missions where they will be using a Space Marine Hunter-class destroyer as a base of operations, and I think I should be prepared to deal with some shipboard politics…

They are indeed. Usually captained by a Captain (duh) who reports to the Master of the Fleet (usually the head of the 4th Company) This is the exception to the split between the Imperial Navy and the Astartes

Hm. I would think smaller ships like escorts might be captained by Sergeants or Veterans or Veteran Sergeants. Maybe even some Strike Cruisers having this arrangement. Devoting a Company Captain solely to commanding a single ship is tying up a very valuable and rare resource.

Kshatriya said:

I imagine that, similar to some real-world naval traditions, a vessel's commander is referred to as Captain regardless of his actual rank, as a mark of respect for his authority aboard ship. Actual Company Captains aren't sufficiently numerous to command every ship in a Chapter's fleet in the first place, and while they are expected to be able to command a starship (just as the humble Tactical Marine is expected to be well-versed in every weapon, vehicle, tactic and strategy the Astartes employ, so their officers must be masters of many forms of warfare), some Marines have a greater aptitude for it than others, and it is likely to these that the normal operation of starships will be given - a handful of skilled starship officers aboard every vessel, commanding the chapter serfs and servitors who form the actual crew.

 In the books I've read, Space Marine vessels carry their own captain but said captain answers to the senior  officer of any Astartes Unit onboard. (Similar to the distinction between Master and Commander in the age of sail; The ships Master was it's owner whereas the Captain served the ship's Master by commanding the ship.) The Ship itself is owned by the chapter and thus the Captain will always seek to act in their best interest. While onboard a Space marine officer can (And often will) command the ship himself  if he feels confident doing so. The ship's Normal Captain being relegated to an advisory role similar to the XO.

By the various sources of lore and canon available, I don't think there's a definitive answer on this one. Some ships have Astartes captians, some have mortals. IMO, you should go with whichever one you think suits your game better.

In my game, the watch house is actually a Strike Cruiser with the Watch-Captain being the commander of the vessel. In my game it works well, in yours it may not.

I think most escorts are comfortably captained by high-quality serfs. Strike Cruisers and Battle Barges have to have an Astartes captain.

 also captain of a space marine vessel is a moderately common position given to veteran space marines who may be a bit past there physical prime and or who have suffered injuries that make general marine duties more difficult. It is also a good way for a chapter master to honor incredibly skilled veterans when there are no higher command positions opened.

Cool- thanks for the replies.

All Astartes ships will have a mortal Master of the Ship. They are there to command the ship when the Astartes are fighting a ground war or boarding another ship.

Adeptus-B said:

Also, you can find the PDF for BFG for free on the GW site somewhere - grab it. Even if you never play the game, the book is a great resource for descriptions, pictures and inspiration for stuff aboard ships, astartes or otherwise.