Plimsoll Lines- A Detailed Synopsis

Fig.1: Plimsoll Line and 
various loadlines
(Courtesy: www.rhiw.com )

Surely you know about the different kinds of ships in existence today and how the cargo or the method of loading or unloading determines the class of ship concerned.

 Among merchant ships today, the largest are the Containers, the ones with the bulk cargo (Bulk Carriers) and the tankers. Speaking in terms of deadweight, these ships range from somewhere around 50,000 DWT in Handymax Carriers to a whopping 5,50,000 DWT ULCC’s (Probably among the largest man-made objects around). 

Having said about the magnitude of things here, let us concentrate on a part of the ship, or rather a certain mark on the ship hull having a centre-dashed circle and vertical lines with branches on both sides and a horizontal line on top.

A SLICE OF HISTORY

Fig.2: Samuel Plimsoll
(Courtesy: Getty Images )

In the early 19^th Century, there used to be ships which were loaded to such great draughts and hence such less freeboard that they were at great risks of being drowned in rough seas. Such ships carried crew who risked their lives every time they went to the sea. Actually these ships were worth more to their owners if they drowned than they would have been had they actually completed their journey! This was because these ships were ‘over insured’ and the owners who would gain from the insurance money, took advantage of the situation during that time.

 It was not until this Gentleman named Samuel Plimsoll, a coal merchant who had written a popular book on disasters of ship overloading, started campaigning for safety at seas. Ultimately, his efforts paid off and the load lines became compulsory in British ships and spread worldwide over. Today they have been standardized and are visible on the ship hull as given in this photo. Let us try to understand these markings.

First of all, think about it, when large ships travel very long distances, they pass through oceans, the largest of seas, and sometimes, through freshwater passages in between. Now, if you remember, this means that they will be passing through different climates and different densities of water, also the salinity in sea water is also a factor to be considered. The ship’s weight, for all normal purposes would not change as much, but the different densities mean that the pressure would vary from seawater to freshwater, and so the immersed volume and draughts would change accordingly. The vessel would float to deeper depth in fresher water than in seawater. Now from certain calculations, we can find out the depths for each climatic or geographic condition directly from one primary load line. This primary load line as in almost all cases is the Summer load line. Together these are a shorthand representation of the freeboard of the same ship in different seas on Earth.

OF LINES AND DISCS

Fig.4: Screenshot from our Prezi on Plimsoll Lines.

Take a look at this screenshot from our Prezi on Plimsoll Lines, we will take each of them at a time. First, starting from the top, we have the deck line, which by convention is of 300 mm length and 25 mm breadth, for that reason all such lines are of same thickness. But the other horizontal lines are of 230 mm length. These marks are inverted on the other side of the ship hull.

DECK LINE

The deck line is placed at exact intersection of the freeboard deck with the outer shell of hull plating. In case you want to mark the deck line, which serves as a reference to be place somewhere else, you will have to correct the freeboard calculation accordingly.

LOADLINE MARK AND PLIMSOLL DISC

Immediately below the Deck line, we have what is called the load line mark which passes through the disc of outer diameter 300 mm, called the Plimsoll Disc. The upper edge of the line passes through the centre of the disc. The vertical line is placed a distance of 540 mm from the center of the plimsoll disc. From this, the load lines stretch on both sides to a length of 230 mm each.

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 and their implementation under supervision of classification societies and flag states. This load line mark’s position depends on many factors such as length of ship, superstructures, terms linked with overall raking of the fore body and so on. These have been standardized and can be obtained from Freeboard Tables which look somewhat like this.

Fig.5: Freeboard Table for ships of 'A'-type

(Courtesy:Load Lines, 1966/1988 - International Convention on Load Lines,1966,as Amended by the Protocol of 1988)

Certain formulae are used to correct this freeboard in case values of draught slightly deviates from assumptions (T>L/15) or similar corrections accounting for block co-efficient, height of superstructures, etc.  For the purpose of such calculations, ships have been classified as type A & B. There are certain factors which decide this like the type of cargo, watertight spaces, permeability of cargo compartments, etc.

Now here's the interesting part. Refer to Fig.4 or for that matter, to any Plimsoll Lines on a ship. How are each of those loadline marks obtained? Are the vertical distances between each of the different draughts different for different ships? Or are they same? Well, read on.

TROPICAL LOAD LINE (T)

The tropical load line is obtained by an addition from the summer draught (considered T hereinafter) measured from keel to the centre of Plimsoll Disc by amount 1/48^th of T. That is, it is T/48 above the Summer Load Waterline (S).

FRESH WATER LOAD LINE (F)

This is marked above the Summer Load Waterline (S) by the following amount:

Δ is the mass displacement in salt water (in tonnes) at the summer load line.

T is the tonnes per centimeter immersion in salt water at the summer load waterline.( The TPC for any draught is the mass which must be loaded or discharged to change a ship’s mean draught in salt water by one centimeter)

When it becomes difficult to find out whether freshwater and tropical freshwater are the same things, the position of the latter line relative to former is found in same manner as that of summer load line and tropical summer load line.

TROPICAL FRESHWATER MARK (TF)

The tropical freshwater mark (TF) is always marked at (T + F) above the Summer Load Waterline (S).

WINTER LOAD LINE (W)

The winter load line is obtained this time by a deduction from the Summer Load Waterline (S), an amount of T/48. That is, it lies T/48 below the Summer Load Waterline.

WINTER NORTH ATLANTIC LOAD LINE (WNA)

When a vessel is bound to enter any part of the North Atlantic Ocean during its winter period an additional load line called the WNA load line is assigned 50 millimetres below the winter mark. By default, it is same as the winter mark (W) for other ships. A separate WNA mark is present only on vessels that donot exceed length of 100 m.

ADDITIONAL LOAD LINES (Used on Ships with TImber Freeboards)

Fig.6: The Timber Load Lines for vessels 
with deck timber cargo

(Courtesy: 1873 issue of Vanity Fair(edited))

Now, take a look at the left side of the vertical line, there another set of load lines with an additional ‘L’ prefixed to them, these are called the Timber Load Line Marks or ‘L’ for Lumber Load Line Marks. These are additional load lines assigned to certain vessels which carry timber deck cargo and are granted additional freeboard as this ship will have greater buoyancy and protection against the sea and waves. 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 that Winter Timber load line is 1/36^th of the Summer Timber Load Draught below the Summer Timber load line. The displacement used in the formula is that of the vessel at her Summer Timber Load Draught.

Some vessels like Ro-Ro ships and Passenger Vessels have sub divisional load lines which are nothing but load lines for different loading conditions based on passengers and cargo, in any case, the these should not be above the deepest load line in salt water.

There is one more thing which you must have seen on the load line mark passing through the disk (450 mm in length) which seems to bear the initials ‘NK’, this is called the ‘Mark of assigning Authority’. They tell you which Classification Society has surveyed the load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyd's Register and IR for the Indian Register of Shipping and so on.

Such Load Line Convention rules do not apply to certain kinds of vessels like the warships, new ships of 24 length or less or those existing ones of less than 150 GT, even the yachts not engaged in trade and for that reason fishing vessels. Certain Geographical regions are free from the observations of the Convention. Definitely, these lines have made our life safer at seas and international trade fairer.

PREZI ON PLIMSOLL LINES (A Clearer View)

It often becomes tough for us to memorize the Plimsoll Lines unless we have a good on-ship experience or good experience in working on a ship design project. For Naval Architects, it is highly important to have a clear view of the Plimsoll Lines, and the following Prezi is an attempt to make that easier for you:

Wait till the Prezi loads. Once loaded, you need to click on the arrow or use your arrow keys to watch the Prezi. It is recommended you watch it in Full Screen Mode for the best view. LSD

    Article By: Sudripto Khasnabis