The Plimsoll line is the mark painted amidships on a ship’s side that shows how deep she may legally be loaded. It is a circle crossed by a horizontal line, with a set of load lines beside it for different seasons and waters.
This guide explains what the Plimsoll line is, how to read the marks, and why it carries Samuel Plimsoll’s name. The rules behind it are covered in our guide to the Load Line Convention.
What Is the Plimsoll Line?
The Plimsoll line is the popular name for the load line mark. The horizontal line through the ring shows the summer load line in seawater, the deepest a ship may float in that condition.
Its job is to protect reserve buoyancy. A ship loaded beyond her marks has too little hull above water to stay afloat after taking a heavy sea or minor flooding.
The Plimsoll line is not the same as a ship’s waterline. The waterline is wherever the sea meets the hull at that moment. The Plimsoll line is a fixed legal limit that the waterline must not pass.
Where Is the Plimsoll Line on a Ship?
It is marked amidships on both the port and starboard sides, below the deck line. The two sides carry identical marks, so the draft can be checked from either quay.
The marks are cut or welded into the plating and then painted. They stay visible if the paint wears off. Draft marks at the bow and stern are a separate system, explained in our guide to ship hull markings.
How Do You Read the Plimsoll Line Marks?
Read the marks against the water surface. The ship may load until the water reaches the upper edge of the line for her current zone, season and water density, and no further.
A full set of load line marks has three parts:
- Deck line: a horizontal line 300 mm long and 25 mm wide, marking the level of the freeboard deck.
- Load line disc: a ring 300 mm in outside diameter and 25 mm wide, crossed by a horizontal line. The upper edge of that line is the summer load line.
- Load lines: horizontal lines 230 mm long and 25 mm wide, running from a vertical line 540 mm forward of the ring’s centre.
The distance from the deck line down to the summer line is the summer freeboard. It is worked out for each ship, so no two designs carry identical marks.
What Do the Letters on the Plimsoll Line Mean?
| Letters | Meaning | When the ship uses it |
|---|---|---|
| TF | Tropical fresh water | Loading in fresh water in the Tropical Zone |
| F | Fresh water | Loading in fresh water elsewhere |
| T | Tropical | Seawater in the Tropical Zone |
| S | Summer | Seawater in the Summer Zones; level with the ring |
| W | Winter | Winter seasonal zones in winter |
| WNA | Winter North Atlantic | Ships of 100 m or less in the North Atlantic in winter |
The two letters either side of the ring identify the authority that assigned the load line. “LR” is Lloyd’s Register, for example, and other classification societies use their own initials.
Each line is offset from the summer line. Tropical and winter lines move by 1/48 of the summer draft, and fresh water lines by the fresh water allowance.
Why Do Some Ships Have Two Sets of Plimsoll Marks?
Ships that carry timber on deck may have a second set of marks aft of the ring, prefixed “L” for lumber. A secured timber stow adds buoyancy, so these lines sit lower and allow deeper loading.
The timber marks only apply when the deck cargo meets the stowage rules. See intact stability criteria for timber deck cargo for the LS, LW and LT lines.
Why Is It Called the Plimsoll Line?
It is named after Samuel Plimsoll (1824–1898), a British Member of Parliament for Derby. He campaigned against overloaded, unseaworthy ships, and the Merchant Shipping Act 1876 made the load line mark compulsory on British ships.
Plimsoll began investigating ship losses in 1870. In 1873 he published Our Seamen, an attack on owners who sent overloaded and poorly repaired ships to sea. His campaign led to a Royal Commission and then to the 1876 Act.
His supporters called him “the sailor’s friend”. In the 1920s, rubber-soled canvas shoes became known as plimsolls, because the band around the sole looked like his line.
What Were Coffin Ships?
Coffin ships were 19th-century merchant ships so overloaded or poorly repaired that they were likely to sink.
Seamen who refused to sail in them could be jailed for desertion. According to Royal Museums Greenwich, sailors petitioned Queen Victoria in 1855 after being convicted for complaining about dangerous ships.
The same source records a prison inspector’s report from south-west England. Nine of the twelve prisoners he examined were sailors, jailed for 12 weeks for refusing to sail in ships they considered unseaworthy.
How Did the Plimsoll Line Become International Law?
The mark moved from British law to a worldwide treaty in about 90 years. British rules came first, then the 1930 international convention, then the 1966 Load Line Convention that applies today.
| Year | Milestone |
|---|---|
| 1835 | Lloyd’s Register introduces freeboard tables, but only for ships it classes |
| 1873 | Plimsoll publishes Our Seamen; a Royal Commission on unseaworthy ships follows |
| 1876 | Merchant Shipping Act makes the load line mark compulsory on British ships |
| 1894 | Merchant Shipping Act fixes the position of the disc by Board of Trade rules |
| 1906 | Foreign ships calling at British ports must also carry a load line |
| 1930 | First International Load Line Convention |
| 1966 | International Convention on Load Lines adopted; in force 21 July 1968 |
| 1988 | Protocol aligns load line surveys with SOLAS and MARPOL; in force 3 February 2000 |
The 1876 Act had a weakness. The disc marked the depth to which the owner intended to load, so an owner could still set it too deep. The 1894 Act closed that gap by tying the position to official freeboard rules.
Why Does the Plimsoll Line Change With Water Density?
A ship floats when the water she displaces weighs the same as she does. Fresh water is less dense than seawater, so the same ship sinks deeper in a river port than at sea.
That is why the fresh water lines sit above the seawater lines. A ship can load to her F mark in fresh water and rise to her S mark as she reaches the sea. The physics is explained in why ships float higher in salt water.
For brackish dock or river water, officers apply a dock water allowance between the S and F marks. A ship loading in a river port may also load slightly deeper to cover the fuel and water she will burn before reaching the sea.
How Is the Plimsoll Line Used When Loading a Ship?
The officer works out the deepest permitted draft for the whole voyage, not only the loading port. The mark for the most restrictive zone on the route sets the limit.
A ship loading in the tropics for a winter destination must arrive at the winter zone no deeper than her W mark. Fuel and water burned on passage are counted in that plan, which becomes part of the ship’s cargo plan.
On completion, the chief officer reads the draft on both sides and at the Plimsoll mark. In my experience, the offshore side is the one to check, because the quay side is often in calm, sheltered water.
What Do Inspectors Check on the Plimsoll Marks?
Surveyors and port state inspectors check that the marks are visible and correctly placed on both sides. They must match the freeboards on the International Load Line Certificate.
- Marks permanently cut or welded, not only painted.
- Deck line to summer line distance equal to the certified summer freeboard.
- Authority letters matching the organization named on the certificate.
- Draft at departure not above the line for the zone, after density correction.
Marks repainted in the wrong place after a drydock are a real finding. Hull paint crews sometimes cover the cut lines, and the new paint ends up a few centimetres off.
The wider survey covers hatch covers, closures and freeing ports as well. Those checks, and what happens to an overloaded ship, are covered in the Load Line Convention guide linked above.
Does Every Ship Have a Plimsoll Line?
No. The Load Line Convention covers ships of 24 m and over on international voyages. Warships, fishing vessels and pleasure yachts not in trade are excluded.
Many smaller and domestic commercial vessels still carry load line marks under national rules. Their marks may look different from the international pattern.
Related Topics
Frequently Asked Questions
What is the Plimsoll line on a ship?
It is the load line mark amidships on the hull: a ring crossed by a horizontal line, with lines for different zones and waters. The ship must not be loaded so that the relevant line is submerged.
Why is it called the Plimsoll line?
It is named after Samuel Plimsoll, the British MP who campaigned against overloaded ships. The Merchant Shipping Act 1876 made the mark compulsory on British ships.
What is the difference between the Plimsoll line and the load line?
They are the same marking. Plimsoll line is the popular name; load line is the term used in the Load Line Convention and on the ship’s certificate.
What do the letters TF, F, T, S, W and WNA mean?
Tropical fresh water, fresh water, tropical, summer, winter and winter North Atlantic. Each line shows the deepest permitted draft in that zone, season or water type.
What do the letters beside the Plimsoll ring mean?
They identify the authority that assigned the load line, usually a classification society. LR stands for Lloyd’s Register, for example.
Why are there marks on both sides of the ship?
The marks are required on both port and starboard sides amidships. The ship may list slightly, so drafts are checked on both sides.
Do warships have a Plimsoll line?
Warships are excluded from the Load Line Convention, so they do not need one. Navies set their own loading limits.
Is the Plimsoll line still used today?
Yes. Ships of 24 m and over on international voyages must carry load line marks under the 1966 Load Line Convention. Surveyors and port state control check them.
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