Cannons onboard a ship

[Asphere]

I am running a game in a few weeks that will partly take place on a ship. During the voyage the player's ship will be attacked by a large vessel and various small vessels. The small vessels will be transporting hostiles to their ship while the large vessel will be firing cannon balls. Are there rules for ships exchanging cannon fire? What about ship movement? If not, what rules would you invent to handle it?

[Kimera757]

I haven't heard of any such rules, certainly not in the core.

You would need to be careful with this type of combat, since it's essentially a mass combat. The abilities of the PCs can be made insignificant.

I would recommend having the cannons and most of the boarders (and actions of friendly NPCs) be narrative. There's some narrative cannon fire, the villains board in large numbers, then "zoom in" on the actions of the PCs versus a small, relatively elite group of villains. (You could have several such encounters against the narrative background.) Eventually the success or failure of the PCs determines whether the villains are driven off or hijack the ship.

[Cheapy]

The most recent adventure path by Paizo, Skull & Shackles, is about pirates and includes ship combat rules.

The free player's guide actually goes over them.