For the next session we roll, my players will be faced with pirates. My question is how would you determine what kind of damage chained cannonballs inflict? (By chained cannonballs I mean two cannonballs that are chained to one another, and then shot out of a cannon. Kind of like a rocket powered bolas) When an individual is really hit by chained cannonballs, their body is generally destroyed in a very rapid and gore-filled fashion. So would being hit by something this devastating fall in to the inescapable death category? (Given players dont have anything to mend their bodies)
Historically, chainshot (the technical term) was very much less effective against people than grapeshot (canister of musket balls) or even than roundshot (single ball) precisely because the latter tore up large impaling splinters, and indirect casulaties were heavier than direct. Chainshot was normally fired at masts and sails, in the hope of bringing down important parts of the rigging (which might of course land on the characters).
What this leads to is that you will need to abstract some of the details. If the party were in a battle between two armies you would presumably not roll for every shot aimed at their unit, but use battle rules to obtain a casualty rate. Similarly, make a roll (perhaps modifed by the relative tactical skills of the captains) to see how much damage is done before the pirates board. If it is high, the characters make a saving throw to find out whether they are knocked unconscious or just entangled in a fallen sail for a few rounds.
Chained shot would not be used in an antipersonnel role. Instead you'd have grapeshot or deck guns being used to hit the crew of a ship.
This doesn't mean that your characters aren't going to be hit by falling tackle, spars, masts, sails and other sailing bits and bobs though...
I'd suggest having the characters take damage from shrapnel, deck shot, grape shot and falling debris. Cannonballs however, will definitely hit with the chunky-salsa rule in full effect if you decide to include them. Ain't no personal armour protecting from that!
If you want the added danger of an instakill, roll for damage on every attack. It might be that a perfect shot coupled with several failing rolls would lead to them being hit by a cannonball. That way, if they're really unlucky they are going to get instakilled by a direct hit, else they are knocked out of the way by shrapnel or debris (roll for damage) or are incredibly lucky.
If you want to model the cannonballs realistically, a direct hit is most definitely an instant kill. But if you want to give your players a chance to survive a cannonball attack, you can have an attack "hit" without making the big ball of metal smash the character's face or the chain splitting him in two.
For one, they can take second-hand damage caused by a cannonball hit. Have one of the balls hit the ground they're standing on, making them fall through the deck and take fall damage. Or have a big splinter shoot out and stick in a limb of your choice.
Another way to model "damage" is through battle fatigue. Maybe the character is able to dodge the ball by jumping away with all he's got. He can't reasonably jump around for dear life for too long. Eventually his body won't be able to keep up, or he'll slip, and the attack will hit its mark. It all works well, both mechanically and in roleplay, but if your players are unfamiliar and/or unwilling to count fatigue as damage, this might not go over well at the table.
Whichever way of dealing damage you choose, you can take the damage of familiar monsters to help you gauge the difficulty of the encounter. If you notice during the encounter that your initial assessment was wrong, feel free to modify the encounter live. Have a canon fail if it's too hard, or bring out the flaming chained cannonballs if the characters are going through swimmingly.