Freeboard
Freeboard is the distance measured vertically at the middle of the ship from the waterline up to the upper edge of the freeboard (main) deck. It represents the reserve buoyancy of the ship — the watertight volume above the water that keeps it afloat and resists flooding in a seaway.
The minimum freeboard a ship must have is set by the load line (Plimsoll) rules under the International Convention on Load Lines. The more cargo loaded, the deeper the ship floats and the smaller the freeboard, so the assigned load line marks the maximum permitted loading — the point at which freeboard reaches its safe minimum.
Freeboard is safety margin made visible. Too little and a ship is overloaded, low in the water and vulnerable to boarding seas and flooding — which is exactly why the load line was made mandatory. It is the direct physical link between how much cargo a ship carries and how safely it floats.
How is freeboard related to the load line?
The load line marks the waterline at maximum permitted loading, which corresponds to the minimum allowed freeboard. Loading beyond it reduces freeboard below the safe minimum and is not permitted.