With the recent question about medieval ships, I’ve wondered about the feasibility of medieval cities or kingdoms maintaining large navies in peacetime for the purposes of fighting pirates and smugglers, as well as guarding their merchant shipping. Would it be possible for such a navy to keep a region of water almost entirely safe from pirates, and charge high (but still preferable to having an entire boat’s shipping stolen) fees upon the merchant ships using it? Would this setup be profitable?

You’re in luck; I did some research into this a while back. Historically speakin, both medieval kingdoms and cities maintained navies for that very purpose. As to whether you’d consider them large, that’s tricky – wooden fleets have a relatively limited lifespan (14 years on avg, if I recall correctly), so fleet sizes tended to wax and wane rather dramatically as you’d have a big build-up and then that batch of ships would have to be decommissioned all at the same time. 

Now, “almost entirely safe from pirates” is a bit more complicated. The prevelance of piracy is driven by a number of factors: the relative power balance of different navies (if navies are weak in a given area because ships are needed elsewhere, it’s easier for pirates to operate), the extent to which political authority over a given body of water is fractured (i.e, the classic case of the Caribbean, where you had many different countries with competing colonies, which made policing difficult because pirates could cross jurisdictions easily; or modern-day Somalia, where the collapse of government authority on the mainland has given pirates a safe harbor to operate on), and the involvement of governments in piracy (in the case of the Spanish Main, when Spain and Portugal divided the New World between themselves, the English, Dutch, French, etc. went into the state sponsorship of piracy in a huge way, as well as claiming their own competing colonies). 

Geography and coordination also play a big role – it’s easier to deal with a contained body of water or a given choke-point, and it’s much harder to deal with wide open areas where pirates can simply keep running from you, as the Romans found out when they tried to suppress piracy in Mare Nostrum. At first the Romans tried to attack particular pirate strongholds – like the Antonine expedition against the Cretan pirates – but pirates adapted by just moving elsewhere in the Mediterranean, which led to a futile game of whack-a-mole. It wasn’t until Pompey’s extraordinary commission that the Romans attacked the program by dividing up the whole sea into districts and then making a concerted west-to-east push with their navy that swept the pirates in front of them and penned them in to ever-smaller areas until they were wiped out.