Load Line

The load line is the visible expression of one of the oldest safety principles in shipping: a ship must not be loaded so deep that it has too little reserve buoyancy and freeboard to survive at sea. Named after Samuel Plimsoll, the nineteenth-century reformer who campaigned against dangerously overloaded ships, the mark is painted amidships on both sides of the hull. It centres on a circle bisected by a horizontal line, and loading must never bring the water above the appropriate line for the voyage.

Because a ship floats at different depths depending on the density of the water and the risks of the season, the load line is not a single mark but a set of lines. A ship floats deeper in warm, less dense water and in fresh water than in cold, dense salt water, and it faces greater danger in stormy winter seas than in calm tropical conditions. So alongside the main mark is a grid of lines for tropical, summer, winter, and winter North Atlantic conditions, plus separate fresh-water allowances. The mariner loads to the line that corresponds to the zone and season the voyage will pass through, ensuring the ship retains adequate freeboard and stability throughout.

The load line is governed by the International Convention on Load Lines of 1966, which sets how the marks are assigned and surveyed and defines the seasonal zones and areas around the world. The freeboard assigned to a ship (the distance from the waterline to the main deck) reflects its size, type and subdivision, and is calculated so that a properly loaded ship has enough buoyancy in reserve. Compliance is certified and checked by flag states and Port State Control, and loading beyond the marks is a serious offence, because an overloaded ship is directly at risk of foundering.