Firing all cannon at once on all decks in a broadside would wreck the ship firing such a broadside. That much sideways force would not only violently push the ship away from the ship it was firing at but it would snap masts knocking sailors down, and generally screwing up the ability of the rudder to steer the ship. Here is a short article talking about how, in the age of sail, a broadside was not a "simultaneous" all decks fire broadside: http://www.military-history.org/fact...-broadside.htm