The International Load Line or Plimsoll Line
(
waterline), positioned amidships, indicates the
legal limit to which a ship may be loaded for specific water types
and temperatures. To an observer on the ship the water appears to
rise or fall against the
hull.
Temperature affects the level because warm water provides less
buoyancy, being less dense than cold water.
The
salinity of the water also affects the
level, fresh water being less dense than salty
seawater.
For vessels with
displacement
hulls, the
hull speed is determined
by, amongst other things, the waterline length. In a
sailing boat, the length of the waterline can change
significantly as the boat
heels, and
can dynamically affect the speed of the boat.
In aircraft design, the term waterline refer to the vertical
location of items on the aircraft. This is the (normally) "Z" axis
of an XYZ coordinate system, the other two axes being the Fuselage
Station (X) and Buttock Line (Y).
The purpose of a 'load line' is to ensure that a ship has
sufficient
freeboard and thus sufficient
reserve
buoyancy. The freeboard of
commercial vessels is measured between the lowest point of the
uppermost continuous deck at side and the waterline and this must
not be less than the freeboard marked on the Load Line Certificate
issued to that ship. All commercial ships, other than in
exceptional circumstances, have a load line symbol painted
amidships on each side of the ship. This symbol must also be
permanently marked, so that if the paint wears off it remains
visible. The load line 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.
This symbol, also called an
international load
line or
Plimsoll line, indicates the
maximum safe
draft, and therefore the
minimum freeboard for the vessel in various operating
conditions.
History
The first official loading regulations are thought to date back to
maritime legislation originating with the kingdom of Crete in 2,500
BC when vessels were required to pass loading and maintenance
inspections. Roman sea regulations also contained similar
regulations.
In the Middle Ages the Venetian Republic, the city of Genoa and the
Hanseatic league required ships to load to a load line. In the case
of Venice this was a cross marked on the side of the ship and of
Genoa three horizontal lines.
The first 19th century loading recommendations were introduced by
Lloyd's Register of British and Foreign Shipping in 1835, following
discussions between shipowners, shippers and underwriters. Lloyds
recommended freeboards as a function of the depth of the hold
(three inches per foot of depth). These recommendations, used
extensively until 1880, became known as "Lloyd's Rule".
In the 1860s, after increased loss of ships due to overloading, a
British MP,
Samuel Plimsoll, took up
the load line cause. A Royal commission on unseaworthy ships was
established in 1872, and in 1876 the United Kingdom Merchant
Shipping Act made the load line mark compulsory, although the
positioning of the mark was not fixed by law until 1894. In 1906,
laws were passed requiring foreign ships visiting British ports to
be marked with a load line. It was not until 1930 (The 1930 Load
Line Convention) that there was international agreement for
universal application of load line regulations.
In 1966 a Load Lines Convention was held in London which
re-examined and amended the 1930 rules. The 1966 Convention has
since seen amendments in 1971, 1975, 1979, 1983, 1995 and
2003.
Standard load line marks
Load Line Mark and Lines and Timber
Load Line Mark and Lines for power driven merchant vessels
Load Line Mark and Lines for
commercial sailing vessels
The original "Plimsoll Mark" 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.
Letters may also appear to the sides of the mark indicating the
classification society that
has surveyed the vessel's load line.
The initials used
include AB for the American
Bureau of Shipping, LR for Lloyd's
Register, GL for Germanischer
Lloyd, BV for Bureau Veritas, IR
for the Indian Register of
Shipping, RI for the Registro Italiano Navale and NV for
Det Norske
Veritas
. These letters should be approximately 115
millimetres in height and 75 millimetres in width. The
Scantling length is usually referred to
during and following load line calculations.
The letters on the Load line marks have the following meanings:
- 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 1000 kg/m³ and sea
water 1025 kg/m³. Fresh water marks make allowance for the fact
that the ship will float deeper in fresh water than salt water. A
ship loaded to her Fresh Water mark in fresh water will float at
her Summer Mark once she has passed into sea water. Similarly if
loaded to her Tropical Fresh water mark she will float at her
Tropical Mark once she passes in to sea water.
The Summer load line is the primary load line and
it is from this mark that all other marks are derived. The position
of the summer load line is calculated from the Load Line Rules and
depends on many factors such as length of ship, type of ship, type
and number of superstructures, amount of sheer, bow height and so
on. The horizontal line through the circle of the Plimsoll mark is
at the same level as the summer load line.
The Winter load line is one forty-eighth of the
summer load draft below the summer load line.
The Tropical load line is one forty-eighth of the
summer load draft above the summer load line.
The Fresh Water load line is an amount equal to
\tfrac{\triangle}{40T} centimetres above the summer load line where
\triangle is the displacement in metric tonnes at the summer load
draft and
T is the metric tonnes per
centimetre immersion at that draft.
In any case where \triangle cannot be ascertained the fresh water
load line is at the same level as the tropical load line.
The position of the
Tropical Fresh load line
relative to the tropical load line is found in the same way as the
fresh water load line is to the summer load line.
The Winter North Atlantic load
line is used by vessels not exceeding 100 metres in length
when in certain areas of the North Atlantic Ocean
during the winter period. When assigned it
is 50 millimetres below the winter mark.
Timber load line marks
Certain vessels are assigned Timber Freeboards but before these can
be assigned certain additional conditions have to be met. One of
these conditions is that the vessel must have a
forecastle of at least 0.07 the length of the
vessel and of not less than standard height, which is 1.8 metres
for a vessel 75 metres or less in length and 2.3 metres for a
vessel 125 metres or more in length with intermediate heights for
intermediate lengths. A poop or raised quarter deck is also
required if the length is less than 100 metres. The letter L
prefixes the load line marks to indicate a timber load line. Except
for the Timber Winter North Atlantic freeboard the other freeboards
are less than the standard freeboards. This allows these ships to
carry additional timber as deck cargo, but with the facility to
jettison this cargo.
The letters on the Timber Load line marks have the following
meanings:
- LTF – Timber Tropical Fresh Water
- LF – Timber Fresh Water
- LT – Timber Tropical Seawater
- LS – Timber Summer Seawater
- LW – Timber Winter Seawater
- LWNA –Timber Winter North Atlantic
The Summer Timber load line is arrived at from the
appropriate tables in the Load Line Rules.
The Winter Timber load line is one thirty-sixth of
the Summer Timber load draft below the Summer Timber load
line.
The Tropical Timber load line is one forty-eighth
of the Summer Timber load draft above the Summer timber load
line.
The Timber Fresh and the
Tropical Timber
Fresh load lines are calculated in a similar way to the
Fresh Water and Tropical Fresh water load lines except that the
displacement used in the formula is that of the vessel at her
Summer Timber load draft. If this cannot be ascertained then these
marks will be one forty-eighth of the Timber Summer draft above the
Timber Summer and Timber Tropical marks respectively.
The Timber Winter North Atlantic load line is at
the same level as the Winter North Atlantic load line
Subdivision load line marks
Passenger ships having spaces which are adapted for the
accommodation of passengers and the carriage of cargo alternatively
may have one or more additional load line marks corresponding to
the subdivision drafts approved for the alternative conditions.
These marks show C1 for the principal passenger condition, and C2,
C3, etc., for the alternative conditions, however in no case shall
any subdivision load line mark be placed above the deepest load
line in salt water.
Subdivision Load Line Marks
 Passenger vessel with no allowed
Subdivision Load line
|
 Passenger vessel with one allowed
Subdivision Load line
|
 Passenger vessel with two allowed
Subdivision Load lines
|
See also
References
- Statutory Instruments 1998 No. 2241 The Merchant Shipping (Load
Line) Regulations 1998 Sections 5(1) and 5(3)
- Notes on Cargo Work: Kemp and Young: ISBN 0853090408
- Précis of IMO web site. History of the Load Line.[1]
- Statutory Instruments 1998 No. 2241 The Merchant Shipping (Load
Line) Regulations 1998
- Notes on Cargo Work: Kemp and Young: ISBN 0853090408
- Notes on Cargo Work: Kemp and Young: ISBN 0853090408
- The Merchant Shipping (Load Line) Regulations 1998, as amended
by the Merchant Shipping (Load Line) (Amendment) Regulations
2000
- Notes on Cargo Work: Kemp and Young: ISBN 0853090408
- Merchant Shipping (Instructions to Surveyors) (Passenger Ships)
Regulations (Hong Kong)
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