Merchant ships have a marking on their hull know as the Plimsoll line or the Plimsoll mark, which indicates the limit until which ships can be loaded with enough cargo, internationally, the Plimsoll line on a ship is officially referred to as the international load line. Every type of ship has a different level of floating and the Plimsoll line on a ship generally varies from one vessel to another.
All vessels of 24 meters and more are required to have this Load line marking at the centre position of the length of summer load water line.
There are two types of Load line markings:-
These marks shall be punched on the surface of the hull making it visible even if the ship side paint fades out. The marks shall again be painted with white or yellow colour on a dark background/black on a light background.
- Standard Load Line marking – This is applicable to all types of vessels.
- Timber Load Line Markings – This is applicable to vessels carrying timber cargo.
The complete Load line markings consist of 3 vital parts:
- Deck Line – It is a horizontal line measuring 300mm by 25mm. It passes through the upper surface of the freeboard.
- Load Line Disc-It is 300mm diameter and 25mm thick round shaped disc. It is intersected by a horizontal line. The upper edge of the horizontal line marks the ‘Summer saltwater line’ also known as ‘Plimsol Line’.
- Load Lines – Load lines are horizontal lines extending forward and aft from a vertical line placed at a distance of 540mm from the centre of the disc. They measure 230mm by 23mm. The upper surfaces of the load lines indicate the maximum depths to which the ships maybe submerged in different seasons and circumstances.
S – Summer:- It is the basic freeboard line at the same level as the Plimsol Line. Other load lines are marked based on this Summer freeboard line.
T – Tropical:- It is 1/48th of summer draft marked above the Summer load line.
W – Winter:- It is 1/48th of 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 winter months.
F – Fresh Water:- It is the summer fresh water load line. The distance between S and F is the Fresh Water Allowance (FWA).
TF – Tropical Fresh Water:- It is the fresh water load line in Tropical. It is marked above the T at an amount equal to FWA.
Every ship that has been surveyed and marked in accordance with the present Load line convention are issued by the authorized administrator, an International Load Line Certificate. The certificate will have a validity of not more than 5 years and will contain all vital information that includes the assigned freeboard and fresh water 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 that have been covered under the survey. If such changes need to be made, the authorized authority conducting the survey shall be contacted.
The fundamental purpose of a Load Line is to allow a maximum legal limit upto which a ship can be loaded. By prescribing such limits, the risk of having the vessel sailing with inadequate freeboard and buoyancy can be limited. A vessel should be having sufficient freeboard at all times, any exceptions made will result in insufficient stability and excessive stress on the ship’s hull. This is where load lines play an important role, as it makes the task of detecting whether the vessel is overloaded and it’s freeboard tremendously easy and effortless.
However, since the buoyancy and immersion of the vessel largely depend on the type of water and it’s density, it is not practical to define a standard freeboard limit for the ship at all times. For this reason, the convention has put regulations which divides the world into different geographical zones each having different 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.
Technically, no ship is able to absolutely float above, as it travels across the waters. A certain portion of the ship is always immersed in the sea water and the level of submerging decided by the gross weight of the respective ship. Even the type of water in which the ship is floating influences the application of the load line. Sometimes even the oceanic regions and changing seasons affect the Plimsoll mark, as well. Researchers have found out that the temperature and salinity of the water also plays a key role in deciding the height until which the ship can be loaded.
Importance of Load Line Ship:
They indicate the density of seawater, which determines how much lift force the water can apply to the ship. So, basically, the lines show how much cargo a ship can hold in different water bodies.