Well, flensing weapons SHOULDN’T be for piracy, their role is to assist in the aid of killing zee-monsters, and the current dynamic is to choose between torpedoes or flensing weapons depending on whether or not you want to make your Frigate/Corvette/Dreadnought specialized in either fighting other ships or zee monsters. With the one exception being the single extra foward weapon cannon that does equal damage to both types of targets at the cost of not being super effective in either direction.
Currently, this system works well for the way combat works, but it (as I’m seeing so many have reported) sucks for trying to be a pirate captain, something the game simply doesn’t have real implementation for.
The most obvious method to get piracy going would be a several step process, requiring a number of different storylets and items to be added to the game, as well as changing a lot of AI behavior and spawning, and probably needing at least one new underlying system (that the game should probably have for other reasons) - reputation.
The first step is in needing an item - likely something like "Boarding Hooks" as an Auxiliary piece of equipment. Possibly a Forward weapon of "Harpoon Gun" would also be useful in this task too, to grapple onto and reel in a target to prevent escape (and possibly useful for catching Zee-Monsters of larger sizes if they had an escape AI protocol when their health got low).
I think like others have said, that this type of item would be best if it enabled boarding actions via ramming another ship to start a boarding storylet rather than any other method. This would work best with the other systems in the game already without necessitating a bunch of new mini-games or something like that. The importance of having the item should be obvious - it acts as a trigger to let this storylet type get enabled in the game when ramming other ships, so that not equipping it would allow for normal ramming hits to function as they currently do and not cause boarding actions. Obviously, all "Pirate" style ships would be assumed to have this item equipped, so if they ram the player, they start a boarding action but in reverse. This would require new AI protocols for these types of ships so that they’re generally attempting to do at least some damage to the player’s boat, then ramming them to board (or attempting to ram the player’s boat straight away if the player’s hull is < 50% integrity.
Ramming should be essential to pirating. It should probably act as a unique type of hit that doesn’t do much hull damage, but can knock crew members into the zee at a much higher rate (at least knocking 1-2 off the boat if less than 50% hull integrity while doing minimal hull damage around 1-5). The other application to altering ramming in this way would to be to have another new forward weapon: a built in ship ram that increases the weight of the ship, but does extra hull and or crew damage when ramming opponents (or zee monsters). Perhaps even of multiple types for differing levels of specialization (like a big metal forward ram that does extra hull damage, or a giant spear harpoon that does lots of damage to zee monsters).
Then there’s the boarding storylet itself. Whether defending or attacking, it’s pretty self explanatory. There would be an initial storylet starter about swinging to the enemy ship via a veils check (or trying to prevent the swinging onto your ship via a counter check probably based on veils or iron). Then a series of repeating checks based off of your Iron score and the number of your crew versus the number of the enemy crew, with each check having a number of different potentialities - sometimes you can kill a bunch of them on a success, sometimes you trade, sometimes you get defeated. Iron would be the big skill check here, but hearts might also be important when defending (giving a speech to your crew before the enemy boards to inspire them for example). The first side to diminish the enemy crew to zero (or some other low number) wins the action, and then you get to results. Boarding another ship would give larger rewards in fuel and supplies, as well as potentially stuff found in their holds at better drop rates or in higher amounts. You could get other choices too, like trying to capture enemies as prisoners or having mercy or being cruel to the enemy captain. Losing a boarding action (or yielding during a fight, which should always be an option) would result in different outcomes too, likely causing large losses in cargo as well as fuel and supplies, but not necessarily leading to a game over if you can retain enough crew to hobble back to London afterwards. There would also have to be an extra bit of AI writing here so that once you get boarded and yield or lose, the enemy ship (or ships) back off-screen to temporarily despawn and give the player a chance to hobble home, or you’ll get a lot of feedback saying that you should never yield as pirates will just shoot and sink you after they rob you.
This would then lead into new spawning behavior. Obviously, if piracy were implemented, it would require a lot of new spawns for neutral or even potentially friendly ships so the player could then run around robbing everyone on the high zees. It would also require some new and improved reputation systems, especially for Gaider’s Mourn (and potentially Polythreme). If you join the Gaider’s pirating groups, it’d be especially nice if that meant you’d stop getting harassed by them, and could start treating the Mourn as a home port rather than London (potentially adding residence functions and/or new family/romance options unique to the Mourn), which you’d hope would recognize that you’re now a fugitive in some capacity and lock you out unless you figure out alternate methods of entry (also required, a new method of sneaking into London for those that are loose-cannon brigands but still need to smuggle in goods or trade in illicit cargoes with their contacts there).
All in all, piracy would be an immense amount of work to do properly. Some of the stuff is conceptually easy - item to trigger boarding actions and a simple repeatable storylet to cover how they work (with lots of unique extra variations depending on the Captain’s experience and items available). Other stuff would be much harder, like changing how the AIs work to make it fully realized or creating a whole new system for those that choose the pirate life for them. I’d say it’d all be totally worth it to include the option and make it realized, but perhaps more as a DLC rather than merely an update, since it could potentially be VERY game changing and require almost as much work as making half of Sunless Sea already.
Still, it’d be nice, wouldn’t it?