Waterline

Source: Wikipedia, the free encyclopedia.
"Plimsoll line" redirects here. For the use of the Plimsoll symbol in chemistry, see Standard state.
For other uses, see Waterline (disambiguation).
Load line mark and lines on the hull of a ship

The waterline is the line where the

amidships), 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,[1] particularly with regard to the hazard of waves 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
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 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 Z axis of an X × Y × Z coordinate system, the other two axes being the fuselage station (X) and buttock line (Y).[citation needed]

Purpose

Load line markings on a cereal carrier, certified by Bureau Veritas.

The purpose of a load line is to ensure that a ship has sufficient

classification society and that society issues the relevant certificates. This marking was invented in 1876 by Samuel Plimsoll
.

History

Samuel Plimsoll

In the Middle Ages, the

Venetian Republic,[3] 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,[4][5] and of Genoa three horizontal lines.[6]

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".

In the 1860s, after increased loss of ships due to overloading, a British MP,

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 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.[8]

Standard load line marks

Lloyd’s Register (“LR”) load line mark and lines and timber load line mark and lines for power-driven merchant vessels
Lloyd’s Register (“LR”) load line mark and lines for commercial sailing vessels

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

Registro Italiano Navale. These letters are approximately 115 millimetres in height and 75 millimetres in width (4.5 by 3.0 in).[9]
The load line length is referred to during and following load line calculations.

The letters on the load line marks have the following meanings:

  • TF – tropical fresh water
  • F – fresh water
  • T – tropical seawater
  • S – summer temperate seawater
  • W – winter temperate seawater
  • WNA – winter North Atlantic

For the purposes of loadline marks, freshwater is considered to have a density of 1,000 kg/m3 (62 lb/cu ft) and typical seawater 1,025 kg/m3 (64 lb/cu ft). 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 Δ/4T 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 100 m (330 ft) in length when in certain areas of the North Atlantic Ocean during the winter period. When assigned it is 50 mm (2 in) below the winter mark.[1]

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

timber
as deck cargo, but with the facility to jettison this cargo.

The letters on the timber load line marks have the following meanings:

  • LTF – timber tropical fresh water
  • LF – timber fresh water
  • LT – timber tropical seawater
  • LS – timber summer seawater
  • LW – timber winter seawater
  • LWNA – timber winter North Atlantic

The summer timber load line is arrived at from the appropriate tables in the load line rules.[10]

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.[1]

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.[11]

Subdivision load line marks
Passenger vessel with no allowed subdivision load line
Passenger vessel with one allowed subdivision load line
Passenger vessel with two allowed subdivision load lines

See also

References

  1. ^
    ISBN 0-85309-040-8
  2. ^ Statutory Instruments 1998 No. 2241 The Merchant Shipping (Load Line) Regulations 1998 Sections 5(1) and 5(3)
  3. ISBN 978-0-415-38163-5
    . Retrieved 20 July 2013. At Venice, an official mark was placed on the outside of the hull, and the ship was inspected before she sailed. If the mark was found to be more than a specified depth below the water-line, the excess cargo was removed by the authorities and the owners heavily fined....Thus, we have here a foreshadowing not only of Plimsoll's Mark but of the classification of shipping.
  4. ISBN 978-2-86413-020-8
    . Retrieved 20 July 2013. The very first regulations appeared in Venice in 1255. They made it illegal to exceed the draught, marked on each ship by a cross. Similar provisions were to be found in Cagliari and Pisa at the same period, and also in Barcelona, in the decree issued by Iago de Aragon in 1258, and in the maritime statutes of Marseilles in 1284. The most elaborate regulations appeared in the 14th-century Genoese statutes.
  5. ^ Augustin Jal (1848). Glossaire nautique: Répertoire polyglotte de termes de marine anciens et modernes. Bottega d'Erasmo. pp. 508–.
  6. ^ Augustin Jal (1840). Archéologie navale. A. Bertrand. pp. 262–.
  7. ISBN 0-316-72612-5
    .
  8. ^ "History of the Load Line". International Maritime Organization. Archived from the original on 2015-05-11. Retrieved 2012-09-05.
  9. ^ Statutory Instruments 1998 No. 2241 The Merchant Shipping (Load Line) Regulations 1998
  10. ^ The Merchant Shipping (Load Line) Regulations 1998, as amended by the Merchant Shipping (Load Line) (Amendment) Regulations 2000
  11. ^ Merchant Shipping (Instructions to Surveyors) (Passenger Ships) Regulations (Hong Kong)

External links

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