Maritime Compliance: Load Lines
A
ship’s waterline is the line where its hull meets the surface of the water. A
load line, also called Plimsoll mark, is a marking indicating the extent to
which the weight of a load may safely submerge a ship, by way of a waterline
limit. It is positioned amidships on both sides of a vessel’s hull and
indicates the draft of the ship and the legal limit to which a ship may be
loaded for specific geographical areas and seasons of the year. The purpose of
the load line is to ensure that a ship has sufficient freeboard (the height
from the waterline to the main deck) and thus sufficient reserve buoyancy
(volume of ship above the waterline). It should also ensure adequate stability
and avoid excessive stress on the ship’s hull as a result of overloading. Ships
intended for the carriage of timber deck cargo are assigned a smaller freeboard
as the deck cargo provides protection against the impact of waves.
The exact location of the load line markings is calculated and/or verified by a classification society, which then issues a load line certificate.
The first international convention on load lines was
adopted in 1930. It was based on the principle of reserve buoyancy. In 1966 the
IMO adopted a new convention, determining the freeboard of ships by subdivision
and damage stability calculations. The International Convention on Load Lines
1966 applies to all vessels engaged in international trade and determines the
permitted draft/freeboard for a vessel in different climate zones and seasons,
which are defined in a special international load line zones and areas map.
Load line markings are as follows:
TF
Tropic Freshwater – the deepest permitted draft considering the relatively
benign weather conditions in tropical waters and the fact that the vessel
floats deeper in freshwater due to lower density than in saltwater.
F
Freshwater – the deepest permitted draft in freshwater considering that the
vessel floats deeper in freshwater than in saltwater.
T
Tropics – deepest permitted draft in tropical waters considering the benign
weather conditions in tropical waters.
S
Summer – deepest permitted draft during summer considering benign weather
conditions.
W
Winter – deepest permitted draft during winter considering rougher weather
conditions.
WNA Winter North Atlantic –
deepest draft permitted considering the rough weather conditions likely to be
encountered during winter in the North Atlantic.
Draft
surveys
In
addition to the association with reserve buoyancy and stability, load
lines make it possible to determine, with reasonable accuracy by draft survey,
the amount of cargo on board a ship.
A draft survey is used to determine the weight of any
materials being loaded into or discharged from a vessel. It measures the
displacement of the water both before and after the loading or unloading, with
the resulting difference between the two displacements representing the weight
of the cargo. A draft survey is based on solid mathematics but usually
involves one of the ship’s officers gauging the draft of the vessel from visual
reference to the vessel’s forward, amidships and aft draft marks along with the ascertained list the vessel may
have. The combination of pure science and
practical experience allows the officer to calculate the amount of cargo on
board to within more or less 0.5 per cent of the actual quantity.
In many ports, a draft survey is the only way of independently confirming the amount of cargo the shipper declares as having been shipped and thus the only way of checking the accuracy of the quantity stated in the bill of lading (which is normally based on the shipper’s figures) and how much cargo was on board the vessel prior to the start of discharge.
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