bulkhead

Bulkhead combines Middle English bulk (a partition, a framework) with head (the front or top of something). The 'bulk' element likely comes from Old Norse bulki (cargo, a partition in a ship), related to the idea of dividing a hold into separate compartments. A bulkhead is a wall that subdivides the interior of a ship, separating compartments from each other.

The purpose of bulkheads is survival. If the hull is breached, water floods into the damaged compartment. Watertight bulkheads prevent the water from spreading to adjacent compartments, keeping the ship afloat even when partially flooded. The principle was understood by Chinese shipbuilders as early as the fifth century CE — Marco Polo described Chinese ships with watertight compartments in the thirteenth century.

The Titanic had fifteen transverse bulkheads, but they did not extend to the top of the hull. Water flooding one compartment could spill over the top of the bulkhead into the next, like filling an ice cube tray. The ship was designed to float with four compartments flooded. The iceberg opened five. The inadequate height of the bulkheads was a contributing factor in the disaster. After 1912, naval architecture required watertight bulkheads to extend to the waterline or above.

The word has extended to architecture (a wall between rooms), aviation (a partition in an aircraft cabin), and computing (a connector that passes through a wall). In every case, a bulkhead is a barrier that separates two spaces. The nautical purpose — preventing catastrophic flooding — remains the word's most serious application.