Age of Sail
In the Age of Sail (and in the early years of steam) ships had long rows of guns set in each side of the hull which could only fire to the one side: firing all guns on one side of the ship was known as a "broadside"; firing all guns on both sides was a double broadside. The cannons of 18th century men of war were accurate only at short range, and their penetrating power mediocre, entailing that the thick hulls of wooden ships could only be pierced at short ranges. These wooden ships sailed closer and closer towards each other until cannon fire would be effective. Each tried to be the first to fire a broadside, often giving one party a decisive headstart in the battle when it crippled the other ship.
As a measurement
Additionally, the term broadside is a measurement of a vessel's maximum simultaneous firepower which can be delivered upon a single target, because this concentration is usually obtained by firing a broadside. This is calculated by multiplying the shell weight of the ship's main armament shells times the number of barrels that can be brought to bear. If some turrets are incapable of firing to either side of the vessel, only the maximum number of barrels which can fire to one side or the other are counted. For example, the American Iowa-class battleships carried a main armament of nine 16-inch (410 mm) main guns in turrets which could all be trained to a single broadside. Each 16-inch shell weighed 2,700 pounds (1,200 kg), which when multiplied by nine (the total number of barrels in all three turrets) equals a total of 24,300 pounds (11,022 kg). Thus, an Iowa-class battleship had a broadside of 12 short tons (11.0 tonnes), the weight of shells that she could theoretically land on a target in a single firing.
See list of broadsides of major World War II ships for a comparison.
- George Dorsey, "When a U.S. Battleship Fires a Broadside," The New York Times Magazine, 30 December 1917.