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The Plimsoll mark is the international load-line marking on a ship's side that shows the maximum legal loading depth for the conditions in which she is sailing. It was campaigned for by the British shipping reformer Samuel Plimsoll in the 1870s and made compulsory under the UK Merchant Shipping (Plimsoll's) Act 1876, and internationalised by the 1930 and 1966 Load Line Conventions. The current instrument is the International Convention on Load Lines, 1966, as amended by the 1988 Protocol and subsequent amendments.
The mark is a circle 300 mm in diameter with a horizontal line through its centre, painted on each side of the hull amidships. The line through the centre of the disc is the Summer load line. To the right of the disc is a comb (or "ladder") of additional load lines for tropical, fresh, winter, and (for shorter ships) winter North Atlantic conditions. The letters of the assigning authority (e.g. LR Lloyd's Register, AB ABS, NK ClassNK) are painted either side of the disc.
Maximum draft when the ship loads in tropical fresh water. Highest mark; the ship sits deepest in fresh water (lower density than salt).
Maximum draft in fresh water of any season other than tropical.
Salt-water draft for the Tropical Zone or Seasonal Tropical Areas during the tropical season.
Standard salt-water draft. The horizontal line through the centre of the Plimsoll disc IS the Summer load line. All other lines are referenced relative to S.
Salt-water draft in the Winter Zone during the winter season — slightly less than Summer to allow more reserve buoyancy in heavier seas.
Applies to ships ≤ 100 m in length in the North Atlantic between 1 November and 16 April. Lower than Winter; the most restrictive mark.
Fresh water is approximately 2.5% less dense than salt water. A ship loading to her Summer salt-water mark in fresh water will sit deeper than that mark by an amount called the fresh-water allowance (FWA), which is shown on the load-line certificate. As she steams from fresh into salt water, she will rise back up to her Summer mark. F is therefore set above S by the FWA. Brackish or estuary water gives an intermediate result; the navigator computes a dock water allowance (DWA) for the actual density measured by hydrometer.
The world's waters are divided by the Convention into Zones (Tropical, Summer, Winter), Seasonal Areas, and Seasonal Tropical Areas. A ship's applicable load line at any given time and place is determined by which Zone or Area she is in and the date — for example, the North Atlantic from 50°N to the equator is a Summer zone year-round, but the area between 50°N and 36°N has Summer (16 April – 31 October) and Winter (1 November – 15 April) periods. The IMO publishes the Zone Chart as a colour-coded world map; current chartlets are also embedded in the load-line section of the SOLAS-required navigation publications onboard.
The Summer freeboard is the vertical distance from the freeboard deck to the top of the Summer load line. It is the reserve buoyancy that keeps the deck dry in seaway and provides the safety margin against down-flooding. Freeboards for other marks are computed from Summer using fixed offsets specified in the Convention. The greater the freeboard the safer the ship in heavy weather but the smaller her cargo intake — a constant tension in commercial ship design.
The International Load Line Certificate is issued by the flag administration (or an RO) and is valid for 5 years, with annual surveys. Hold and ballast tank arrangements, hatchcover integrity, ventilator arrangements, and freeing-port areas are key items inspected. The Load Line is a fundamental SOLAS-equivalent certificate — Port State Control will check it as a matter of course, and an overloaded ship in port is a detention case.