Load line is a special marking, positioned at mid ship which indicates how much of the ship is underwater (also known as, draught of the vessel) and the maximum permitted limit in waters with different densities. The density of water primarily changes based on the temperature and salinity.

 

Plimsoll Load Line (Courtesy: www.rhiw.com)

Why do we need a Load Line?

It is basically a tool that helps in detecting if the ship is carrying more weight/cargo than it should. It proves to be a simple yet an effective way to ensure that sufficient reserve buoyancy is maintained by the vessel. Reserve buoyancy as the name suggests, gives a measure of ship’s ability to sustain damage. It is water tight space above the waterline which would act as a lifejacket (unless the watertight boundary is damaged) and prevent the ship from sinking if sea water enters the ship until the added weight is lesser than the reserve buoyancy.

Load Line marks are 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.

Which vessels bear this mark?

All vessels of 24 meters and more are required to have this Load Line marking at the center of summer load waterline.

Since the immersion of ship depends on the buoyancy and buoyancy force depends on density, how can you have a standard line?

Fig.2 The Plimsoll Mark

 

Yes! You are right we cannot have a single standard line instead we have a series of extra lines painted to accommodate the effect on water density due to different seasons and area as seen above.

There are two types of Load line markings:-

  1. Standard Load Line marking – This is applicable to all types of vessels.
  2. Timber Load Line Markings – This is applicable to vessels carrying timber cargo.

The complete Load line markings consist of 3 vital parts.

  1. Deck Line – It passes through the upper surface of the freeboard & measuring 300mm by 25mm
  1. 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 Load Line’ also known as ‘Plimsol Line’.
  1. 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.

SUMMER LOAD LINE (S)

This is the primary load line from which the other load lines are derived, the International Maritime Organization under the Load Lines Convention specifies certain rules for calculation of freeboards. Freeboard is the distance between Deck Line and Summer Load Line, the load line mark is positioned based on many factors such as the type of cargo carried, the ship length, superstructures, sheer of the fore body and so on.

These have been standardised and can be obtained from Freeboard Tables which look somewhat like this.

 

Fig. 3 Freeboard Table (Courtesy: Load Lines, 1966/1988 – International Convention on Load Lines, 1966,as Amended by the Protocol of 1988)

 

By using the freeboard value from the table (sometimes we need to make certain corrections as well but we shall not get into it here) we now get the summer load line and hence the summer draught (H) as well

So how do you obtain the other lines from the Summer Load Line?

Tropical Load Line (T) = H + H(1/48)

Winter Load Line (W)= H – H (1/48)

WINTER NORTH ATLANTIC LOAD LINE (WNA)

Vessels bound to enter any part of the North Atlantic Ocean during the winters have an additional load line called the WNA load line.

WNA = W – 50mm

A separate WNA mark is present only on vessels that do not exceed length by 100 m. By default, it is same as the winter mark (W) for other ships

FRESH WATER LOAD LINE (F)

F = H + Δ / (40*t)

Where Δ is displacement in tonne & t is tonnes/cm immersion.

The distance between S and F is also referred as Fresh Water Allowance (FWA).

TROPICAL FRESHWATER MARK (TF)

TF = F + H/48

By now you must be wondering what the lines on the left-hand side (as seen in fig.2) with similar notations but with an extra L denote?

They denote the timber load line markings assigned to certain vessels which carry timber deck cargo

These are analogous to normal load lines and are calculated similarly from the Summer Timber load draught (This value is supplied in the table from the convention), the only exception is that the Winter Timber load line = LH – (LH/36)              {Where LH is the Summer Timber Load Draught}

 

 

 

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