Bosun

There are four kinds of people you must fear and respect on board of any ship. In order, first, the chef. You want to eat slop, piss 'em off at your own risk. Second, the medic. If you don't understand why, you're in need of their services to get your head checked out. Third, not the captain, no, the bosun. A captain can maroon or brig you. But the bosun, they can make your life a living hell.

Alternative name(s)

Boatswain, deck boss

Rank

Second

Responsibilities

The best way to explain what a bosun's responsibilities are, is to first explain that of the captain. The captain is responsible for navigation, and space battle. In other words, they decide where the ship goes. The bosun or boatswain, however, is responsible for making sure the ship can actually get there. They are in charge of maintenance and safety, commanding every crewmember involved in maintaining the ship.
The only ones not under their command, are those that clearly fall outside that jurisdiction. And even then, a boatswain will easily take action against anyone getting in the way of public safety. As such, most stormfarers know better than to piss off their boatswain.
On most ships, the boatswain also is in charge of shore leave. Or they are in charge of the one in charge of that. Even more the reason not to piss them off, if you want to ever go to a bar.

Origin

Boatswains already were around when navies first started becoming professional. As people started venturing into the Hypersea, the rank made a comeback. The reason was quite simple: Management should be competent. A captain knows how to sail, but tends to be too busy to properly order the crew around. This causes problems to be missed, until they start wreaking havoc. So as a proper manager, they delegate that, rather than micromanaging the crew.
Enter the boatswain. Letting the captain focus on managing the navigation, the boatswain keeps the ship in working condition. To be able to do that, they need to know the basics of all involved departments, so they can recognise problems in time. Pity is the fool that believes all a bosun has to do is carry a big stick and shout loudly.
... Though the stick does help.