What Is the Plimsoll Line? The Load Line Mark That Stops Overloading

What is the Plimsoll line? It is the load line mark painted amidships on both sides of a merchant ship's hull. The mark shows the deepest the vessel may legally sit in the water when loaded. If the relevant line goes under, the ship is overloaded. She then lacks the reserve buoyancy the law requires. The mark is named after Samuel Plimsoll, the British member of Parliament whose campaign forced it into law.

It is governed today by the International Convention on Load Lines, signed in London in 1966. The USCG, the UK MCA and AMSA all check it during port state control.

How to read the Plimsoll line and its letters

The core symbol is a circle 300 millimeters (about 12 inches) in diameter with a horizontal line through its center. That center line marks the summer load line: the maximum draft permitted in summer zone seawater, which the convention takes as water with a density of 1.025 tonnes per cubic meter. Letters beside the circle identify the authority that assigned the freeboard, such as LR for Lloyd's Register or AB for the American Bureau of Shipping.

Forward of the circle sits a vertical comb of short horizontal lines, each marking a legal limit for different water and seasons:

• TF - tropical fresh water, the deepest mark, since fresh warm water gives the least buoyancy allowance concern on arrival at sea
• F - fresh water
• T - tropical seawater
• S - summer seawater, level with the line through the circle
• W - winter seawater
• WNA - winter North Atlantic, the shallowest mark, applied to smaller ships crossing the North Atlantic in winter where conditions are hardest

Fresh water marks sit higher than salt water marks because fresh water is less dense, so the ship floats deeper in it. A ship loading in a river port may legally submerge its seawater mark by the fresh water allowance, because she will rise to the correct mark on reaching the sea.

Why freeboard is a ship's survival margin

Freeboard is the height from the waterline to the freeboard deck, and it represents reserve buoyancy: the watertight volume above the water that keeps a ship afloat when waves board her or a compartment floods. Load the ship deeper and that margin shrinks. Seas sweep the deck more often, hatches and vents take more punishment, and the hull girder carries stresses it was not assigned to carry.

This is why the load line is a safety device rather than a commercial formality. The assigned freeboard reflects the ship's length, type, hatch strength and stability, calculated under the convention and verified at survey. The annual load line survey checks that the marks remain accurate and that the ship's weathertight closures still justify them.

Samuel Plimsoll and the coffin ships

The mark has a hard history behind it. In the 1870s, heavily insured and heavily overloaded British ships were being lost with their crews at a rate that scandalized the public, and sailors called them coffin ships. Samuel Plimsoll (1824 to 1898) campaigned in Parliament against the practice, and the resulting legislation compelled British ships to carry a load mark. His name attached itself to the symbol permanently.

International rules followed. The first International Load Line Convention was signed in London in 1930 by around 30 maritime nations. The current regime dates from the 1966 convention, adopted under the International Maritime Organization (IMO) and later amended by the 1988 Protocol, which aligned its surveys and certificates with the other major conventions.

Load line zones: the map behind the marks

The convention divides the world's oceans into zones and seasonal areas: tropical, summer, winter and the winter North Atlantic area. The applicable mark depends on where the ship is and the time of year, and the master must plan loading so that the correct mark is never submerged at any point of the voyage, not just at departure.

That planning point catches people out. A ship loading to her summer mark before sailing into a winter zone must allow for it, burning down bunkers or loading less so she crosses the zone boundary at the legal draft. Charterers push for every tonne of cargo, and the load line is the fixed limit that settles the argument.

For quick definitions of freeboard, draft, reserve buoyancy and related terms, see the Glossary on Marine Insight 360.

The mark fails long before a ship is visibly deep. Port state control officers compare the marks against the load line certificate, and the findings that follow are rarely overloading. They are corroded hatch cover gaskets, missing cleats, ventilator and air pipe closures that no longer close, and freeboard deck openings that cannot be secured. Several of those are detainable.

Overloading itself usually begins as arithmetic. Dock water density is measured with a hydrometer and used to work out how much deeper a ship may load in a river berth. Take a bad sample and she sails past her marks the moment she reaches salt water. Corroded air pipe heads have also flooded forward spaces on bulk carriers, admitting water the assigned freeboard never allowed for.

Sources and further reading

• Wikipedia: Load line (watercraft)
• Wikipedia: International Convention on Load Lines
• GOTEC Draft Surveys: Summer Draft and the Plimsoll Line - Load Lines Explained

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