1) Roll 1d10; if roll is equal to or lower than firepower value, inflict that many hits on target's hull. At close range, double damage.
2) If enough damage is inflicted to fill a damage box (hits = Burden), inflict a critical hit, based on whatever the unmodified die-roll which caused the critical was:

1 -- Crew; remove one add'l Crew Box.
2 -- Sail; normal effect
3 -- Mast; normal effect
4 -- Rudder; normal effect
5 -- Fire; normal effect
6 -- Leak; normal effect
7+ -- no critical inflicted (the target's in enough trouble already).

If more than one box is crossed off by a single hit, apply multiple instances of the critical (following normal rules for doing so).

Aiming High, or using chain/grape ammo: Apply damage to Crew boxes; ignore any Critical result of 4+.

Raking: Double damage (yes, a close-range rake does *four times* normal damage!).

Musket fire: As for regular damage.

Boarding: Each ship rolls d10s equal to Burden, alternating as per normal rules; damage goes to Crew boxes.

Did I miss anything important? :)