Weber's Honorverse is very much based on the British navy of the early 19th Century, which is, I think, why Manticore is a monarchy with a hereditary aristocracy. Haven is France, with the People's Republic being quite blatantly based on the French Revolution (Rob S. Pierre? Seriously?)
Several other SF books and series are based on the same era, often indirectly, through CS Forester's Hornblower series. David Drake's RCN series and David Feintuch's Seafort Saga spring to mind. (I think you have to be called David.
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I'd imagine a lot of the Baen military SF is based on historical wars and actions, and will reflect the social structures of the historical setting.
I'd have thought there were more Emperors.