When 2 fleets engage in combat, a circlular area is drawn on the sea delimiting the combat zone. In obligatory combats (story missions/bosses), being outside of the circle for 15 seconds will trigger a loss and you must reload. For optional combats (voluntary engagements with roaming fleets/bounties), leaving the area for 15 seconds will put an end to the combat and the ships from each fleet will reform their sailing line and continue on their current course.
A ship can be defeated in 2 ways :
During a battle, the currently active broadside has a white cone drawn that is your area of "auto-targeting"; any ship within the cone when you fire will have all guns fired at it (with an attempt to lead the ship based on its current course) regardless of where you or the enemy sail.
Once a broadside is fired, the cone turns red and the cannons on that side are reloaded. Because each side is reloaded separately, you can fire from both sides of your ship independently (however only one side can be active at a time).
The cone's length and arc is determined by the cannon's installed on a ship however, the length of the cone is not the true range of your guns.
Each ship occupies space. who would have thought. Also consequently, your ships can block your shots.
Consequently, firing cannons at a ship will be more likely to strike larger ships and less likely to strike smaller ships, based on both the length of the hull, but also its height (I think I've seen cannonballs pass over a ship? is this just an animation and it's simply related to accuracy?). On top of a turning boat or a boat sailing perpendicular to a broadside presenting a smaller target, sudden changes in speed will throw off the lead necessary to hit a ship and so turning into or away from a boat before it fires will greatly reduce incoming damage.
The bow and stern modifications are your supplement to broadsides.
These cannons can be broadly categorized by their function :
Hand-to-hand combat occurs when 2 ships are in proximity to one another (within the white circle drawn around a ship). While the ships remain in proximity, 1 crew/second will be subtracted from each ship, then additional effects may occur, eg. aftercastles will deal an additional X crew damage/second if the modification is present.
While in proximity, the ships will "build up" boarding (is it a building meter, ie. a fixed time per ship-ship interaction or is it a random chance to board, ie. variable time for an identical pairing).
If the ships leave proximity, is their a decay? reset?. Whereas if they remain in proximity long enough, boarding of the enemy ship will occur and the screen changes to a view of the deck.
Deck size is determined by the size of the enemy ship(?), with spaces being filled on it according to captain, assigned companions (I assume hired crew members that are assigned go here?), unassigned companions, default sailors.
Each participating crew member will fight an enemy sailor with (what stats?) being used to determine the winner. A shield is granted to each side when one of their crew members wins. Each shield will inflict X damage against the other ship's Morale and Crew count. If a ship is reduced to 0 sailors after combat, the ship surrenders and you are returned to the sea. Depending on the assigned Boatswain and which side won more shields, a duel may be initiated between captains (your Boatswain replaces your captain if one is assigned).