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Load Lines Explained

International Load Line Zones and Areas
During the 19th century, when British trade with the rest of world was growing rapidly, the high loss of ships being experienced annually as a result of poor maintenance and overloading created a serious cause for concern. The condition of ships in some instances gave rise to them being referred to as ‘coffin ships.'
In 1835, Lloyd’s Register introduced rules regarding the loading of ships in an attempt to reduce the losses, but this only applied to those ships registered with them. 
Since there was no law or authority to control the quantity of cargo a certain type of ship could safely carry, owners of ships not registered with Lloyd’s could do as they pleased, many having no regard for safety.
This disregard for safety at sea created a problem for seamen. They were refusing to go to sea, and in 1855, a group of seafarers wrote to Queen Victoria, protesting that Courts were finding them guilty of desertion when they complained or refused to go to sea in these so called ‘coffin ships’.
 
Various attempts over the years were made, similar to that of Lloyd’s Register, to ensure that only safe amounts of cargo were loaded, but unfortunately there was still no compulsory system to force ship owners to act on protecting their ships and lives at sea. 
In 1870, Samuel Plimsoll MP and a coal merchant, commenced research into this situation, and in 1876 the Merchant Shipping Act made load lines compulsory. However, the position of the line was not finalised by law until 1894. In 1906, foreign ships were also required to carry a load line if they visited British ports. 
The original load line symbol, the "Plimsoll Line", 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. The International Convention on Load Lines (ICLL) was adopted on 5 April 1966 and entered into force on 21 July 1968, with subsequent amendments since then.
 
The International Convention on Load Lines (ICLL) is an IMO Convention that is applicable for all vessels engaged in international trade, except for ships of war, ships less than LOA 24 m., pleasure yachts not engaged in trade, and fishing vessels. The ICLL defines the maximum allowed draft of the vessel, and how this is to be marked on the side of the vessel.
 
The purpose of a load line is to ensure that a ship has sufficient freeboard (the height from the waterline to the maindeck) and thus sufficient reserve buoyancy and bow height in compliance with the requirements of the ICLL; the requirement for reserve buoyancy is to ensure that the vessel is not broken down by heavy seas, while the requirement for bow height is to limit the amount of green seas on the deck.
 
All applicable vessels have a load line symbol painted, and permanently marked,   amidships on each side of the ship. This makes it easy for anyone to determine if a ship has been overloaded. The exact location of the load line is calculated and/or verified by a Classification Society, and that society issues the relevant certificates. 
 
The letters on the load line marks have the following meanings, and refer to the seasonal zones, areas and periods which determine the appropriate load line in a particular sea area at a given time. These are set out in the ICLL.
 
TF - Tropical Fresh Water
F – Fresh Water
T – Tropical Seawater
S – Summer Temperate Seawater
W – Winter Temperate Seawater
WNA – Winter North Atlantic.
 
Fresh water is considered to have a density of 1,000 kg./cu.m. and sea water 1,025 kg./cu.m., i.e. more dense. Fresh water marks make allowance for the fact that the ship will float deeper in fresh water than in salt water. A ship loaded to her fresh water (F) mark in fresh water will float at her summer (S) mark once she has passed into sea water. Similarly if loaded to her tropical fresh (TF) water mark she will float at her tropical (T) mark once she passes into sea water.
Letters may also appear to the sides of the mark indicating the Classification Society that has surveyed the vessel's load line, for example AB for the American Bureau of Shipping, LR for Lloyd's Register.