Welcome to my very amateur take on ranking ships capable of piracy! Ships are ranked in their capacity to be quick (in & out), agile (slippery), hold cargo (room below deck), manage limpet controllers (the crew) and as a secondary measure, their ability to remain cold to slip in and out of black markets. The tiers are a reflection of a ship's overall ability to perform relative to other tiers and their ship size.
In the S tier, we see ships capable of quick getaways while also packing some impressive firepower. The slower ships in this tier may not be as quick but make up for it with extra cannons on deck. Ships here can hold a decent to generous amount of rum in the hold relative to their size and all are capable of sporting a crew of limpets.
In the A tier, these ships are capable pirate vessels but are outclassed by the above tier in speed, firepower, or cargo capacity. Still very decent pirate ships and would be proud to be stuck at sea with any of these!
In the B tier, some vessels are starting to suffer from larger shortcomings than the tier above. They could be an easy target due to their size or lack of agility. Or they might have the speed but lack the firepower to make that trade ship walk the plank. Capable ships in the hands of an experienced captain but there are better choices.
In the C tier, these ships will need to push the absolute limit to be sea-worthy pirates. They might be able to hold endless barrels of rum but would take forever to set sail. Or they're too highly specialised in other roles like sea combat and have no room for anything else. Not to mention, some of these ships run hot!
In the D tier, I'd rather be in Davy Jones' locker than be stuck inside any of these vessels. Too hot, too slow, no room for cargo, no room for limpets or ship crew! Or maybe, not enough cannons! You call yourself a pirate in any of these? You're a braver sailor than anyone I know!