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The waterline is the line where the hull of a ship meets the surface of the
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned
amidships This glossary of nautical terms is an alphabetical listing of terms and expressions connected with ships, shipping, seamanship and navigation on water (mostly though not necessarily on the sea). Some remain current, while many date from the 17t ...
), that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain
buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
, particularly with regard to the hazard of
waves Waves most often refers to: * Waves, oscillations accompanied by a transfer of energy that travel through space or mass. * Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (ban ...
that may arise. 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 Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
with a similar lessening effect upon buoyancy. For vessels with displacement hulls, the hull speed is defined by, among other things, the waterline length. In a
sailing Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the ''water'' (sailing ship, sailboat, raft, windsurfer, or kitesurfer), on ''ice'' (iceboat) or on ''land'' ( land yacht) over a chose ...
boat, the waterline length can change significantly as the boat heels, and can dynamically affect the speed of the boat. A waterline can also refer to any line on a ship's hull that is parallel to the water's surface when the ship is afloat in a level trimmed position. Hence, waterlines are a class of "ships lines" used to denote the shape of a hull in naval architecture lines plans. In aircraft design, the term "waterline" refers to the vertical location of items on the aircraft. This is (normally) the axis of an coordinate system, the other two axes being the fuselage station () and buttock line ().


Purpose

The purpose of a load line is to ensure that a ship has sufficient freeboard (the height from the water line to the main deck) and thus sufficient reserve
buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
. 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

In the Middle Ages, the
Venetian Republic The Republic of Venice ( vec, Repùblega de Venèsia) or Venetian Republic ( vec, Repùblega Vèneta, links=no), traditionally known as La Serenissima ( en, Most Serene Republic of Venice, italics=yes; vec, Serenìsima Repùblega de Venèsia ...
, the
Republic of Genoa The Republic of Genoa ( lij, Repúbrica de Zêna ; it, Repubblica di Genova; la, Res Publica Ianuensis) was a medieval and early modern maritime republic from the 11th century to 1797 in Liguria on the northwestern Italian coast. During the La ...
and the
Hanseatic League The Hanseatic League (; gml, Hanse, , ; german: label= Modern German, Deutsche Hanse) was a medieval commercial and defensive confederation of merchant guilds and market towns in Central and Northern Europe. Growing from a few North German to ...
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. The first 19th-century loading recommendations were introduced by
Lloyd's Register Lloyd's Register Group Limited (LR) is a technical and professional services organisation and a maritime classification society, wholly owned by the Lloyd’s Register Foundation, a UK charity dedicated to research and education in science and ...
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, ). These recommendations, used extensively until 1880, became known as "Lloyd's Rule". In the 1860s, after increased loss of ships due to overloading, a British MP, Samuel Plimsoll, took up the load line cause against strong opposition. A Royal Commission on unseaworthy ships was established in 1872, and in 1876 the United Kingdom
Merchant Shipping Act Merchant Shipping Act (with its variations) is a stock short title used in Malaysia and the United Kingdom for legislation relating to merchant shipping. Merchant shipping acts and regulations also exist as well in other countries, and they ar ...
made the load line mark compulsory, although the positioning of the mark was not fixed by law until 1894. 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 The International Convention on Load Lines (CLL), was signed in London on 5 April 1966, amended by the 1988 Protocol and further revised in 2003. The convention pertains specifically to a ship's load line (also referred to as the " waterline") ...
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 AB for the
American Bureau of Shipping American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the " United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, p ...
, BV for , VL for DNV GL, IR for the Indian Register of Shipping, LR for
Lloyd's Register Lloyd's Register Group Limited (LR) is a technical and professional services organisation and a maritime classification society, wholly owned by the Lloyd’s Register Foundation, a UK charity dedicated to research and education in science and ...
, NK for and RI for the . These letters are approximately 115 millimetres in height and 75 millimetres in width (). The load line length is referred to during and following load line calculations. The letters on the load line marks have the following meanings: * – tropical fresh water * – fresh water * – tropical seawater * – summer temperate seawater * – winter temperate seawater * – winter North Atlantic For the purposes of loadline marks, freshwater is considered to have a density of and typical seawater . Freshwater marks make allowance for the fact that the ship will float deeper in freshwater than saltwater. A ship loaded to her fresh water mark in fresh water will float at her summer mark once she has passed into seawater at the same displacement. Similarly, if loaded to her tropical freshwater mark she will float at her tropical seawater mark once she passes into seawater. *The ''summer load line'' is the primary load line and it is from this mark that all other marks are derived. The position of the summer load line is calculated from the load line rules and depends on many factors such as length of ship, type of ship, type and number of superstructures, amount of sheer, and bow height. The horizontal line through the circle of the Plimsoll mark is at the same level as the summer load line. *The ''winter load line'' is one forty-eighth of the summer load draft below the summer load line. *The ''tropical load line'' is one forty-eighth of the summer load draft above the summer load line. *The ''fresh water load line'' is an amount equal to millimetres above the summer load line where Δ is the displacement in tonnes at the summer load draft and ''T'' is the tonnes per centimetre immersion at that draft. In any case where Δ cannot be ascertained, the freshwater load line is at the same level as the tropical load line. The position of the tropical fresh load line relative to the tropical load line is found in the same way as the freshwater load line is to the summer load line. *The ''winter North Atlantic load line'' is used by vessels not exceeding in length when in certain areas of the North
Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
during the winter period. When assigned it is below the winter mark.Notes on Cargo Work: Kemp and Young:


Timber load line marks

Certain vessels are assigned timber freeboards, but before these can be assigned, certain additional conditions have to be met. One of these conditions is that the vessel must have a
forecastle The forecastle ( ; contracted as fo'c'sle or fo'c's'le) is the upper deck of a sailing ship forward of the foremast, or, historically, the forward part of a ship with the sailors' living quarters. Related to the latter meaning is the phrase " ...
of at least 0.07 the length of the vessel and of not less than standard height, which is for a vessel or less in length and for a vessel or more in length with intermediate heights for intermediate lengths. A poop or raised quarter deck is also required if the length is less than . The letter ''L'' prefixes the load line marks to indicate a timber load line. Except for the timber winter North Atlantic freeboard, the other freeboards are less than the standard freeboards. This allows these ships to carry additional timber as deck cargo, but with the facility to jettison this cargo. The letters on the timber load line marks have the following meanings: * – timber tropical fresh water * – timber fresh water * – timber tropical seawater * – timber summer seawater * – timber winter seawater * – timber winter North Atlantic The ''summer timber load line'' is arrived at from the appropriate tables in the load line rules. The ''winter timber load line'' is one thirty-sixth of the summer timber load draft below the summer timber load line. The ''tropical timber load line'' is one forty-eighth of the summer timber load draft above the summer timber load line. The ''timber fresh'' and the ''tropical timber fresh'' load lines are calculated in a similar way to the freshwater and tropical freshwater load lines, except that the displacement used in the formula is that of the vessel at her summer timber load draft. If this cannot be ascertained, then these marks will be one forty-eighth of the timber summer draft above the timber summer and timber tropical marks, respectively. The ''timber winter North Atlantic'' load line is at the same level as the winter North Atlantic load line.


Subdivision load line marks

Passenger ships having spaces which are adapted for the accommodation of passengers and the carriage of cargo alternatively may have one or more additional load line marks corresponding to the subdivision drafts approved for the alternative conditions. These marks show ''P1'' for the principal passenger condition, and ''P2'', ''P3'', etc., for the alternative conditions; however, in no case is any subdivision load line mark placed above the deepest load line in saltwater.Merchant Shipping (Instructions to Surveyors) (Passenger Ships) Regulations (Hong Kong)


See also

*
Ballast tank A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list ...
* Coffin ship (insurance) * Plimsoll shoe * Sailing ballast * SS ''London'' – sank in 1866 and stimulated Parliament to introduce the Plimsoll line *
Stability conditions (watercraft) The stability conditions of watercraft are the various standard loading configurations to which a ship, boat, or offshore platform may be subjected. They are recognized by classification societies such as Det Norske Veritas, Lloyd's Register and ...
* '' The Onedin Line'' (episode " Danger Level") * Waterline length


References


External links


Historical background to the adoption of the Load Lines convention



IACS Unified Requirement L: Subdivision, Stability and Load Lines
{{Ship measurements Buoyancy Ship measurements