Freeboard

Freeboard is the vertical distance from the waterline to the upper edge of the deck at the ship's side. It represents the reserve buoyancy available to cope with waves and load changes without water entering the superstructure. On ships, freeboard is a central parameter in naval architecture and is regulated under the International Convention on Load Lines. Each vessel is assigned a freeboard value based on hull form, deck arrangements, occupancy, and intended service. The freeboard is indicated by load line marks on the hull, with different values corresponding to seasonal and water type conditions (for example summer, winter, tropical, freshwater). A greater freeboard indicates more reserve buoyancy and improved protection against deck immersion in rough seas, but can reduce cargo capacity and influence stability margins.

In civil engineering and flood risk management, freeboard also refers to the margin above the expected water level that a structure or channel must maintain to prevent overtopping. This is used in dam safety, reservoir design, and floodplain regulation.

Freeboard should not be confused with draft (the depth of a vessel below the waterline) or keel-to-deck height; together with draft, freeboard helps define the hull’s position and safety margins in operation.