A broadside is the side of a ship; the battery of cannon on one side of a warship; or their simultaneous (or near simultaneous) fire in naval warfare.

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. An 18th century man of war like HMS Victory had cannon that were only accurate at short range. The penetrating power of naval guns was mediocre, which meant that the thick hull of a well-built wooden ship 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 fire power which can be delivered upon a single target, due to the fact that 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 carry a main armament of nine 16-inch main guns in turrets which can all be trained to a single broadside. Each 16-inch shell weighs 2,700 pounds, 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 has a broadside of 12 short tons (11.0 tonnes), the weight of shells that she can theoretically land on a target in a single firing.

See list of broadsides of major World War II ships for a comparison.