What is a load line?
A load line, also called the Plimsoll mark, is the set of marks on a ship’s side showing the maximum draught, the minimum freeboard, to which she may legally be loaded in a given water density and season. The marks are assigned under the International Convention on Load Lines, 1966.
The load line is one of the oldest and most visible safety regulations on a merchant ship. It exists to stop a vessel being loaded so deep that she has too little reserve buoyancy and freeboard to ride out heavy weather. The campaign to make the mark compulsory in British ships is named for the reformer Samuel Plimsoll, which is why the disc is still called the Plimsoll mark in everyday use. The modern regime is set by the International Convention on Load Lines, 1966, as amended by the 1988 Protocol, administered through the International Maritime Organization (IMO).
The marks are not painted on by the owner at will. A recognised classification society, acting as the load-line assigning authority on behalf of the flag state, calculates the assigned freeboard from the ship’s structure, subdivision and weathertight integrity, then surveys and certifies the marks. For a dry bulk carrier the load line is the regulatory ceiling that fixes how deep she can sit, which in turn caps the cargo weight she can lift. The rest of this page works through the marks themselves, how they differ from the draught scale they are routinely confused with, and how the permitted draught shifts by zone and season.
Freeboard and the load line marks
Freeboard is the vertical distance from the waterline up to the freeboard deck, and the load line marks fix the minimum freeboard the ship must keep. The disc, the deck line and the comb of seasonal marks make up the assigned set.
| Element | Meaning | Notes | Reference |
|---|---|---|---|
| Freeboard | Vertical distance from the waterline to the upper edge of the freeboard deck at the ship's side, amidships | Minimum freeboard is what the load line fixes; more freeboard means more reserve buoyancy and a shallower permitted draught | International Convention on Load Lines, 1966, as amended by the 1988 Protocol |
| Deck line | A horizontal line marking the level of the freeboard deck on the hull side | Freeboard is measured down from this line to the relevant load line mark | Load Lines Convention 1966 / 1988 Protocol |
| Plimsoll disc | A ring with a horizontal line through its centre, marking the summer load line | The centre of the disc corresponds to the summer freeboard in salt water | Load Lines Convention 1966 / 1988 Protocol |
| Assigning-authority initials | The initials of the society that assigned the marks, set beside the disc (for example LR for Lloyd's Register) | Identifies the classification society acting as load-line assigning authority for the flag state | Load Lines Convention 1966 / 1988 Protocol |
| Seasonal and zonal marks | The comb of lines abreast of the disc: S, W, T, F, TF and WNA | Each mark sets the permitted draught for a season or zone; see the zone table below | Load Lines Convention 1966 / 1988 Protocol |
| Fresh-water lines (F, TF) | The fresh-water and tropical-fresh marks, set above the corresponding salt-water marks | A ship floats deeper in fresh water, so she may load to a greater draught before reaching the salt-water mark | Load Lines Convention 1966 / 1988 Protocol |
The set reads as a vertical scale. The disc and its summer line sit in the middle; the winter mark (W) sits below summer, the tropical mark (T) above summer, and the two fresh-water marks (F and TF) sit higher still, reflecting that the ship floats deeper in less dense water. The Winter North Atlantic mark (WNA) is the most restrictive of the salt-water marks for the vessels it applies to. The exact vertical spacing between the marks is calculated by the assigning authority from the ship’s summer freeboard and length, so the relationships in the rows above are the standard direction of each mark, not fixed distances.
Load line vs draught marks
A common error is to treat the load line and the draught marks as the same thing. They are different scales doing different jobs, and the contrast is the cleanest way to understand each.
| Load line (Plimsoll mark) | Draught marks | |
|---|---|---|
| What it is | A fixed regulatory mark for the maximum permitted draught | A measuring scale that reads the ship's actual draught |
| Where it sits | Amidships, on both sides of the hull | At the bow, amidships and stern, on both sides |
| What it tells you | The deepest she may legally load in a given zone and season | How deep she is floating right now |
| Who sets it | The classification society as load-line assigning authority | Standardised draught scale referenced to the keel or baseline |
| Changes voyage to voyage? | No, the marks are fixed once assigned | Yes, the reading changes with how much she has loaded |
In practice the master and the loadport surveyor read the draught marks during loading and stop the cargo before the relevant load line mark goes under. The load line is the limit; the draught marks are how you check you are inside it. The gap between the two, the freeboard, is what the regulation protects. A vessel read down to her marks at the load may need to slow loading or shift cargo if she is trimmed by the head or stern, because the controlling figure is the draught amidships against the applicable load line.
Where the load line sits on the hull
The marks are positioned amidships on both sides of the hull, where the relationship between draught and freeboard is most representative of the ship’s loaded condition. The horizontal deck line is set first, level with the freeboard deck. Below it sits the Plimsoll disc, a ring with a horizontal line through its centre at the summer load line, with the assigning society’s initials set on either side of the ring. Abreast of the disc, on a short vertical bar, is the comb of seasonal and zonal marks: the tropical-fresh (TF) and fresh (F) lines highest, then summer (S) level with the disc line, then tropical (T), winter (W) and Winter North Atlantic (WNA) below. A surveyor checks loading against whichever of these marks applies to the ship’s current zone and season.
Load line zones and seasonal marks
The Convention divides the world’s oceans into permanent and seasonal zones and areas, set out on a worldwide load line chart. The zone a ship is in, and the calendar date, together decide which mark she must load to.
- Permanent zones apply the same mark year-round. The tropical belt around the equator is a permanent tropical zone for most of its extent, and certain high-latitude areas are permanent summer or winter zones.
- Seasonal zones and areas switch between summer, tropical or winter at fixed calendar dates set out in the Convention. A vessel must comply with the mark for the zone she is in at the time she is there, including while crossing a zone boundary on passage.
- The Winter North Atlantic area is a defined area of the North Atlantic where, in the winter months, vessels below a certain length must keep additional freeboard and load no deeper than the WNA mark.
The logic is straightforward: heavy weather is more likely and more severe in winter and in exposed high-latitude waters, so the ship is required to keep more freeboard and may load less deeply there. In calmer tropical and summer conditions she may load deeper, lifting more cargo, because the reserve buoyancy needed to ride out a storm is lower. For a dry bulk carrier this directly affects how much cargo she can lift on a given lane and date, which feeds straight into the voyage estimate. The interaction with the ship’s overall carrying capacity is covered on the deadweight tonnage page, and the dimensional context sits on vessel dimensions.
How the load line varies by seasonal zone
Each mark permits a different maximum draught, and therefore a different freeboard, for the same ship. The table below reads the marks against the summer load line, which is the reference point from which the others are calculated.
| Zone / mark | What it permits | Relative freeboard / draught vs summer | Where / when it applies |
|---|---|---|---|
| Summer (S) | Loading to the summer load line, the disc line, in salt water | Reference mark; standard summer freeboard | Summer zones and seasonal zones during their summer period |
| Winter (W) | A shallower draught than summer, keeping more freeboard | Less draught, more freeboard than S | Seasonal winter zones during their winter period |
| Tropical (T) | A deeper draught than summer in salt water | More draught, less freeboard than S | Permanent and seasonal tropical zones |
| Fresh (F) | Loading deeper than summer to allow for the ship sitting lower in fresh water | More draught than S in fresh water; the fresh-water allowance | When loading in fresh water at summer conditions |
| Tropical Fresh (TF) | The deepest of the standard marks: tropical loading in fresh water | Greatest draught, least freeboard, combining the tropical and fresh-water gains | When loading in fresh water in a tropical zone |
| Winter North Atlantic (WNA) | The most restrictive salt-water mark for the vessels it covers, with the most freeboard | Least draught, most freeboard of the salt-water marks | The defined Winter North Atlantic area in the winter months, for vessels below the length threshold |
The fresh and tropical-fresh marks reflect a physical fact rather than a weather rule. Fresh water is less dense than salt water, so the same ship floats deeper in it; the F and TF lines let her load to a deeper fresh-water draught knowing that she will rise to the salt-water mark once she reaches the sea. This is the fresh-water allowance. For a bulk carrier loading grain at a river port and sailing into salt water, the difference between the fresh-water and summer marks can be worth meaningful extra cargo, which is why brokers and operators factor it into the stem. On a large class such as a Capesize, where the summer draught already runs deep, a few centimetres of fresh-water allowance translates into thousands of tonnes of additional loadable cargo. The size of that allowance depends on the ship’s summer displacement and the water density at the berth, so it is calculated per vessel and per port rather than being a fixed figure.
Common confusions about load lines
There are four recurring confusions worth separating out.
- Plimsoll mark vs draught marks. The Plimsoll mark is the fixed legal limit amidships; the draught marks are the changing scale you read to see where the ship actually floats. They are different markings serving different purposes, as the comparison above sets out.
- The fresh-water allowance. Because a ship floats deeper in fresh water, the F and TF marks sit above the corresponding salt-water marks (S and T), not below. Read carelessly, this looks backwards; it is correct, and it lets a ship load deeper in fresh water knowing she will rise on reaching the sea.
- A load line is not a single line. It is a set of zonal and seasonal marks, of which the summer line through the disc is only one. Referring to “the load line” usually means the applicable mark for the ship’s current zone and season, not a single universal line.
- Overloading and port-state control. Loading past the applicable mark is a load-line breach. A port-state-control inspection that finds the relevant mark submerged can detain the vessel until cargo is discharged to bring her back inside her marks, so the limit is enforced operationally, not just on paper.
Scope and what this page does not cover
This page explains the load line as a vessel attribute: what the Plimsoll mark and the seasonal and zonal marks mean, how freeboard is defined, and how the permitted draught varies by zone and season. It does not reproduce the worldwide load line zone chart or the Convention’s exact zone boundaries and seasonal dates, calculate assigned freeboard or the fresh-water allowance for a specific ship, or interpret flag-state or port-state enforcement in a particular jurisdiction. For the governing detail, work from the International Convention on Load Lines, 1966, as amended by the 1988 Protocol, the ship’s load-line certificate, and the assigning classification society.