The History
In the 17th and 18th centuries the Industrial Revolution swept Britain. The British Isles became increasingly dependant on merchant shipping for imports of the raw materials needed to support her. These transport links became the veins and arteries of this fledgling trading empire, meaning more and more ships to futher and further flung ports, bringing back.
Such a demand for transport led to corners being cut by many merchants in their attempts to undercut the competition. In this era prior to the undersea telegraph lines, satellite communications and the Internet, very little was heard from a merchant vessel for weeks, or sometimes months, until it arrived at its destination, if it arrived at all. The number of ships sunk with the loss of all hand was becoming ever higher, to the point that the public began to become concerned, and demanded that something should be done.
Up stepped Samuel Plimsoll
Plimsoll, son of Thomas Plimsoll and Priscilla Willing, was a Bristol boy, born and raised within sight of one of the most important ports in the Empire. He became involved in the coal trade, mainly shipping to London, and soon became one of Britain's leading experts on the trade. During the course of researching the two books he wrote, The Export Coal Trade of England and The Inland Coal Trade he became aware of the horrific dangers sailors of the time were exposed to.
After being elected to the House of Commons as MP for Derby in 1868, he campaigned for legislation to prevent sailors being sent to their deaths in unsafe and overloaded ships. He was particularly disgusted with the provisions laid out in the Merchant Shipping Act which allowed ship owners to imprison sailors who wouldn't sail on a ship after they'd signed a contract to work on it, despite often having never seen the vessel so being unaware whether it was unseaworthy. In 1873 he went as far as publishing the pamphlet entitled 'Our Seamen' which highlighted the problem of unscrupulous ship owners. This report so shocked many in Parliament that they understandably attempted to amend the Merchant Shipping Act to cover this area.
Even more understandably, many more MP's, specifically those with business interests in the shipping area, opposed any legislation, leading to a stalemate. In 1875, Benjamin Disraeli, the then Conservative prime minister, gave his support to a change to the act, and proposed the 'Unseaworthy Vessels Bill' which would provide for an 'International Load Line'.
This mark provided for the marking of a line on a ship's sides which would disappear below the water line if the ship was overloaded. It took a little more time for the for the line to be taken seriously as, unfortunately, the Act allowed the shipowners to paint the line where they saw fit, so some chose to paint it on the funnel of the ship. In 1890 government officials were employed to check the 'Plimsoll Line' was maintained and accurate, and today the Maritime and Coastguard Agency employs inspectors still called marine surveyors to perform this job. The International Community finally adopted the Plimsoll line in 1930 with the snappily titled 'International Convention on Load Lines'.
In Practice
Dependent upon a number of factors including salinity and local temperature, the density of water will vary, causing your common-or-garden supertanker to float at different levels. Water is most dense at 4 degrees Celsius and less dense at higher or lower temperatures, where as, as far as buoyancy is concerned, the more salt the better.
The mark is painted onto the side of cargo vessels to indicate the limit to which they can be legally loaded.
A modern Plimsoll line has the following sections marked:
There are often another set of line adjecent to the mark, which contain the same abbreviations prefixed with an 'L'. These marks are to be applied when the ship is carrying lumber. Modern load line postioning in currently worked out using a number of factors, including the hatch types the ship uses. Further information can be found at www.admiraltylawguide.com/conven/protoloadlines1988.html.
Inspired by wandering around Bristol docks too many times to mention ... no not like that, you dirty dirty pervert ...
Sources include
http://www.pacifier.com/~rboggs/PLIMSOLL.HTML
http://www.derbyuk.net/samuel_plimsoll.html
http://www.spartacus.schoolnet.co.uk/TUplimsoll.htm