Piecewise, how would the gameplay of Mainspring work? Would it use an Oro-type mission system, or an abstracted MMWW system? Or maybe it would use a detailed everyone-acts system, but groups of players are assigned to ships and act as if the ship is their char?
Here's my suggestion: the ship is just going to go from port to port, without a final destination. A small group of PCs will board it, acting as both crew and passengers. Each PC has some sort of reason for being there, that will determine what sort of trouble their presence brings, and also their destination. So what happens is, each PC will get off at a predetermined destination, and the current crew will select someone from the waitlist, keeping in mind that they bring their inventory, skills and also the troubles with them. That Flesher could come in handy, but bear in mind they're transporting a creature whose calls attract the minions of the Wyrm. That assassin can easily dispatch unwelcome boarders, who definitely will be coming, because they're on the run after their latest hit. That captain? They aren't too well liked by merchants, who'll jack up the prices while they're here.
PCs will only be on the ship for a short amount of time. What that means is they won't level up, because by the time it happens, they'd have reached their destination to continue with their own lives, their stats no longer having any in-game significance as they've retired from being PCs. There'll be no character progression, instead, the ship itself will progress. Over time, people will upgrade it, people will modify it. It'll get its battle scars and its odd quirks. It'll get enormous cannons, and then maybe a quick passageway from the kitchen to them, because as it turns out it, they overheat too damn quickly, rendering it necessary to pour beverages over them to keep them cool in the midst of battle. It will also attract people with more interesting lifestyles, that is, higher level PCs.
So your character will not themselves level up, but by keeping the ship going, subsequent characters will automatically be higher level. It's like a succession fortress of sorts, with everyone progressively altering the ship, until it's a terrifyingly powerful but utterly unholy mess of clockwork and flesh, with a cannon that for whatever reason fires straight into the mess hall, wiring that goes nowhere, and a gimpy mainspring that despite already being replaced by another, has been left there as a decoy for would-be saboteurs, because the cash needed to uninstall it got blown on the maze of decorative statues on the upper deck. And because PCs will retire quickly, there's no need for the waitlisters to wish death on the current batch.