Load line is a special marking positioned amidships which depicts the draft of the vessel and the maximum permitted limit in distinct types of waters to which the ship can be loaded. When the load line is drawn over the output characteristic curve in a graph, it makes contact at a point known as the operating point/ quiescent point or Q-point.
Purpose and Necessity of Load Lines
The Load Line concept emerged in Britain in the 1870s to prevent merchant ships from being overloaded. The fundamental purpose of a Load Line is to allow a maximum legal limit up to which a ship can be loaded by cargo. By prescribing such limits, the risk of having the vessel sailing with inadequate freeboard and buoyancy can be limited.
A vessel should have sufficient freeboard at all times. Any exceptions will result in insufficient stability and excessive stress on the ship’s hull. This is where load-lines play an essential role, detecting whether the vessel is overloaded and its freeboard tremendously effortless.
Lloyd’s Register has established a minimum freeboard requirement for its classed ships to ensure their good reserve buoyancy in heavy seas.
However, since the buoyancy and immersion of the vessel largely depend on the type of water and its density, it is not practical to define a standard freeboard limit for the ship at all times. For this reason, the load line convention has put regulations that divide the world into different geographical zones, each having a separate prescribed load line.
For example, A vessel sailing in Winter on North Atlantic Ocean will have a greater freeboard than on a voyage in Tropical Zones and Freshwaters.
Understanding Load Line Marks And Types
As we have already defined above, the Load Line is a special marking positioned amidships. All vessels of 24 meters and more are required to have this Load line marking at the centre position of the length of the summer load waterline.
- Standard Load Line marking – This applies to all types of vessels.
- Timber Load Line Markings – This applies to vessels carrying timber cargo.
These marks shall be punched on the hull’s surface, making it visible even if the paint on the ship’s side fades out. The marks shall again be painted white or yellow on a dark background/black on a light background. The complete Load line markings consist of 3 vital parts.
- Deck Line is a horizontal line measuring 300mm by 25mm. It passes through the upper surface of the freeboard.
- Load Line Disc is a 300mm diameter and 25mm thick round-shaped disc. A horizontal line intersects it. The upper edge of the horizontal line marks the ‘Summer saltwater line’, also known as the ‘Plimsol Line.
- Load Lines – Load lines are horizontal lines are extending forward and aft from a vertical line placed at a distance of 540mm from the center of the disc. They measure 230mm by 23mm. The upper surfaces of the load lines indicate the maximum depths to which the ships may be submerged in different seasons and circumstances.
S – Summer:- It is the primary freeboard line at the same level as the Plimsoll Line. Other load lines are marked based on this Summer freeboard line.
T – Tropical:- It is 1/48th of the summer draft marked above the Summer load line.
W – Winter:- It is 1/48th of the summer draft marked below the Summer load line.
WNA – Winter North Atlantic:- It is marked 50mm below the Winter load line. It applies to voyages in North Atlantic ( above 36 degrees of latitude) during the winter months.
F – Fresh Water:- It is the summer freshwater load line. The distance between S and F is the Fresh Water Allowance (FWA).
TF – Tropical Fresh Water is the freshwater load line in Tropical. It is marked above the T at an amount equal to FWA.
International Loadline Certification
Every ship surveyed and marked following the present Load line convention is issued an International Load Line Certificate by the authorized administration. The certificate will have a validity of at most five years and will contain all vital information, including the assigned freeboard and freshwater allowance.
It is to be noted that, after completion of a load line survey and issuance of the certificate, no changes shall be made to the superstructure, markings, equipment or arrangements covered under the study. If such changes need to be made, the survey’s authorized authority shall be contacted.
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Source: MarineInsight
