The load line, also known as Plimsoll line, indicates the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves. The load line is a waterline that corresponds to the maximum draft of the ship, thus yet another name, load waterline. Varying water temperatures will affect a ship's draft, because warm water is less dense than cold water, providing less buoyancy. In the same way, fresh water is less dense than salinated or seawater, with a similar lessening effect upon buoyancy. The rules for international load lines are defined by the International Convention on Load Lines from 1966. For inland water transport regional, national or local rules apply.
Load lines are indicated by special markings on the hull. The marking for the main load line, the summer load line, is called load line mark or Plimsoll line (positioned amidships), the marks for other conditions are named after the condition suffixed with "load line" (e.g. winter load line).
Purpose
The purpose of a load line is to ensure that a ship has sufficient freeboard (the height from the waterline to the main deck) and thus sufficient reserve buoyancy. The freeboard of commercial vessels is measured between the lowest point of the uppermost continuous deck at side and the waterline and this must not be less than the freeboard marked on the load line certificate issued to that ship. All commercial ships, other than in exceptional circumstances, have a load line symbol painted amidships on each side of the ship. This symbol is also permanently marked, so that if the paint wears off it remains visible. The load line makes it easy for anyone to determine if a ship has been overloaded. The exact location of the load line is calculated and verified by a classification society and that society issues the relevant certificates. This marking was invented in 1876 by Samuel Plimsoll.
History
Early history In the Middle Ages, the Venetian Republic, the Republic of Genoa and the Hanseatic League required ships to show a load line. In the case of Venice this was a cross marked on the side of the ship, and of Genoa three horizontal lines.
Reform and standardisation The first 19th-century loading recommendations were introduced by Lloyd's Register in 1835, following discussions among shipowners, shippers and underwriters. Lloyd's recommended freeboards as a function of the depth of the hold (three inches per foot of depth, 250 mm/m). These recommendations, used extensively until 1880, became known as "Lloyd's Rule". The creators of the 'rule' did not enjoy the prominence it gained, knowing it was an oversimplification that did not take into account depth, sheer, and other form factors relating to the size and shape of the hull. In 1869, after increased loss of ships due to overloading, a bill was introduced to Parliament that was amended to include the provision that the draught of water when a vessel was leaving port should be recorded. The bill expired, failing to be passed before the end of the parliamentary session. The law was re-committed the following year and passed as the Merchant Shipping Act 1871, requiring a scale in feet on the stem and stern of a ship in six-inch high letters to coincide with the draught line. In 1873, Samuel Plimsoll took up the load line cause against strong opposition with his book Our Seamen. The Royal Commission on Unseaworthy Ships was established in 1872, resulting in a new law, the Merchant Shipping Act 1876 (39 & 40 Vict. c. 80) was passed making the load line mark compulsory for all ships over 80 GRT.
The Merchant Shipping Act 1876 described the load line for the first time in law.
[The ship] shall be permanently and conspicuously marked with lines of not less than twelve inches in length and one inch in breadth, painted longitudinally on each side amidships, or as near thereto as is practicable, and indicating the position of each deck which is above water [and a] mark upon each of her sides amidships, or as near thereto as is practicable, in white or yellow on a dark ground, or in black on a light ground, a circular disc twelve inches in diameter, with a horizontal line eighteen inches in length drawn through its centre. The centre of this disc shall indicate the maximum load-line in salt water to which the owner intends to load the ship for that voyage.
The more controversial vertical positioning of the mark was fixed by the Merchant Shipping Act 1894, amending the previous law.
The centre of this disc shall be placed at such level as may be approved by the Board of Trade below the deck-line marked under this Act and specified in the certificate given there-, under, and shall indicate the maximum load-line in salt water to which it shall be lawful to load the ship. Lloyd's Register were continuing to improve their own freeboard calculations, and by 1882 had settled on Freeboard Tables that took more hull form factors into account. These were formally adopted by the Board of Trade in 1894 and referenced by the act to determine the position of the load line disc.
Wider adoption In 1906, laws were passed requiring foreign ships visiting British ports to be marked with a load line. It was not until 1930 (the 1930 Load Line Convention) that there was international agreement for universal application of load line regulations. In 1966 the International Convention on Load Lines was concluded in London which re-examined and amended the 1930 rules. The 1966 convention has since seen amendments in 1971, 1975, 1979, 1983, 1995 and 2003, none of which has entered into force.
Standard load line marks The original "Plimsoll mark" was a circle with a horizontal line through it to show the maximum draft of a ship. Additional marks have been added over the years, allowing for different water densities and expected sea conditions.
Letters may also appear to the sides of the mark indicating the classification society that has surveyed the vessel's load line. The initials used include:
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