Basic Rule for what to do when research fails ~ concentrate on the goal at hand.
If we take some time to look on line to learn what we can about siege weapons, it becomes apparent in short order that: a) no one can agree on what anything is; b) no one can agree on how it works; c) there are no legitimate statistics for weight of machine, range, missile, effect or durability; and d) there are hundreds of little groups making their own siege engines with 2x4s, creating noise that goofs up any real scholarship on the internet.
I've been down this road a couple of times before, only to come up short. So ... we dispense with research, remember a game world isn't an accurate simulation and come up with a solution that pays no attention to accuracy.
What we need is a sliding scale of ship's weapons, the most important thing being that they differ from one another not only in power but in specific benefit. To that end, I've created this table:
It doesn't matter what any of these things are; only what they are in relationship to each other. Generally, I'm treating a mangonel as a light counterbalanced catapult firing a small ball that is good for distance and momentum but relatively poor against crew/rigging; it also requires a big ship. The catapults are torque weapons, fair against the hull and rigging, with the light catapult being a cheap yet effective weapon against crew. The heavy ballista is light, has good range, can fire at a shorter range and again, a good anti-crew weapon. The light ballista is lighter still.
I'll put up to other tables here that will help explain the damage, but first let me discuss hardpoints and the limit to ship size. A hardpoint is a location on a ship where a siege engine can be emplaced. On the table above, where it addresses damage to hardpoint, it means to the siege engine that has been put on that hardpoint ~ mangonel, catapult or ballista.
It takes a bigger ship to mount a mangonel, with the tonnage shown being cumulative. Two mangonels would require a ship of 120 tons; two heavy catapults, a ship of 80 tons. An Indian carrack, at 1,005 tons, has 20 hardpoints and could mount 16 mangonels ... which could quickly destroy ships at +1 hull damage (see below). This is unlikely, however, as that would also mean sacrificing 54 tons of cargo space to heavy weapons. A merchant ship (and most of them would be) would do with less firepower, though a warship would load up.
When firing a siege weapon, the artillery crew decides whether to fire at the hull or the rigging. Whichever is aimed at, rolls are made for hits on the tables below. We'll start the with hull, the more interesting one.
Yes, I know. I dislike hit location as well. But firing at a moving ship with a ballistic weapon is not like firing a bow at a combatant, so considerations have to be made. All siege weapons use the above table. The artillerist pointing the weapon aims according to his or her THAC0, and if aiming at the hull must hit AC 3. A natural 20 indicates a critical hit. Ordinary hits roll only on the left table, applying modifiers to hull/rigging damage as shown on the siege engine table. Damage to crew is resolved as normal D&D. A destroyed hardpoint can't be repaired; a wrecked hardpoint can be rebuilt and restored in 24 hours, a damaged hardpoint can be rigged to be impaired, which than then be restored in an hour and an offset hardpoint can be reset in half an hour.
When rolling a critical hit, both tables are rolled upon and both always apply. For example, a 20 on the left and a 12 on the right would mean that the enemy ship lost hull and rigging and its crew was forced to make a morale check. I haven't made morale rules yet for shipboard combat, but that's coming.
Hits to the crew select a random crew member; if the player designates a specific point of the ship being fired at, then hits to a crew member will be a random determination of crew nearest that point. Most of the time, it will be difficult to identify and target the captain, who will be concealed by other crew members, housings, masts and whatever else might be in the way. Still, it's possible.
Which hardpoint is hit is also rolled randomly in the same fashion.
I suppose I shall have to create a table identifying the effect of magic spells against rigging and hull.
Here is the targeting rigging table:
This is still, more or less, following a framework created by Wooden Ships & Iron Men, but of course it is much more gritty and detailed as befitting D&D.
I have more to do to nail down a lot of the details on this subject, particularly how long it takes to load a siege engine, the size of its crew and I think probably that more crew means faster reloads to a point. I admit, however, that my premise is that ships should be very hard to sink with ship's weapons alone. At best, smashing the rigging or getting lucky with a critical hit will reduce a ship's yare, so that it will move slower and make it dangerous to turn into the wind.
I also expect that some sea battles will simply be two to three rounds of getting turned around before the faster ship simply sails away. The players at low level are likely to have very fast ships, so that they shouldn't be concerned about a big pirate ship, unless encountering it in the fog or some such. If the players eventually get a big ship, they'll have to decide for themselves how best to use it if they want to chase down victims. There might be spells such as haste that can be used to let one brilliant crewman act more quickly, enabling the ship to catch the wind a little better ... and there's always sage abilities that could improve a ship's crew to the point where they could improve on the ship's base yare. I'll get to those rules too.
Still a lot to do, but I feel like I'm over a hump. I just hope noisms doesn't mind that I'm writing all this prescriptive content on how to run ship combats.
Relax. That's just a joke.