Light Displacement Tonnage (LDT)

In ship breaking industry, LDT is light displacement tonnage which in simple terms is the weight of water displaced by the ship – the mass of the ship excluding cargo, fuel, ballast, stores, passengers, crew, but with water in boilers to steaming level. Displacement is a measurement of mass, and should not be confused with similarly named measurements of volume or capacity such as net tonnage, gross tonnage, or deadweight tonnage.

“D” is usually used to denote the depth of the ship, ie the vertical distance from the keel to the deck. The draft, “T”, is the vertical distance from the keel to the waterline. D is only indirectly related to the weight of the ship. T allows you to determine the ship’s weight from the hydrostatics.)

You have to calculate the lightship weight by doing a weight survey. If you know the draft, trim, and heel of the vessel and the density of the water it’s floating in, you can determine the overall weight of the vessel at the time of the survey from the hydrostatics. The survey involves tabulating all the weights aboard that are not considered to be part of lightship. Subtracting those weights from the displacement during the survey leaves the lightship weight as the remainder. Note that the definition of lightship must be clearly defined, ie does it include liquids in piping systems, etc.

displacement = lightweight + deadweight. You need to know the displacement and the deadweight to get the lightweight.
The displacement would be the weight of a volume of seawater = L * B * D * block coefficient of the ship.
L * B * D will give you the volume of a slab-sided barge; the ship hull will have a smaller volume.

To understand this with a simple analogy, imagine a container (not the 20 or 40 footer but simply a vessel) brimming with water inside – displacement would be the mass of water that would spill out if the ship were to be placed into it. A small glitch here would be due to variance in density. The density (mass per unit of volume) of water can vary. For example, the average density of seawater at the surface of the ocean is 1025 kg/m³ (10.25 lb/ga, 8.55 lb/US gallon), fresh water on the other hand has a density of about 1000 kg/m³ (10.00 lb/ga, 8.35 lb/US gallon). Thus the displacement in fresh water would more than it would do in salt water.

Understand that prior to passing of MS Act of 1876 in Britain, the ship-owners could load their vessels until their decks were almost awash, resulting in a dangerously unstable condition. Samuel Plimsoll, a member of Parliament, realised the problem and engaged some engineers to derive a fairly simple formula to determine the position of a line on the side of any specific ship’s hull which, when it reached the surface of the water during loading of cargo, meant the ship had reached its maximum safe loading level.

To this day, that mark, called the “Plimsoll Mark”, exists on ships’ sides, and consists of a circle with a horizontal line through the centre.

Because different types of water, (summer, fresh, tropical fresh, winter north Atlantic) have different densities, subsequent regulations required painting a group of lines forward of the Plimsoll mark to indicate the safe depth (or freeboard above the surface) to which a specific ship could load in water of various densities.

Measuring any vessel could be on the following parameters :

Length : Overall; between perpendiculars, & at waterline.

Breadth : Beam

Depth : Draft, moulded depth, freeboard

Volume : GRT, Net, Panama Canal, Thames measurement, GRT, NRT etc.,

Capacity : DWT ; TEU

Weight : Displacement, Loaded, standard, light & normal displacement.

Now having seen this, here is something on the different kinds based on tonnage. Tonnage is a measure of the size or cargo carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship’s cargo:-

Gross Register Tonnage (GRT) : represents the total internal volume of a vessel, where a register ton is equal to a volume of 100 cubic feet (5.83 m³), which volume, if filled with fresh water, would weigh around 5800 kg or 5.8 tonnes.

GRT being a measure of volume, could be complex at times. As you can understand – grain in bulk would occupy some space including its air space, baled cargo would be different.

Net Register Tonnage (NRT) : is the volume of cargo the vessel can carry; ie. the Gross Register Tonnage less the volume of spaces that will not hold cargo (e.g. engine compartment, helm station, crew spaces, etc., again with differences depending on which port or country is doing the calculations). It represents the volume of the ship available for transporting freight or passengers.

Net tonnage (NT) : this is a volumetric composition of all cargo spaces of the ship. This term has replaced NRT in recent times especially after 1994.

Gross Tonnage (GT) : as a natural corollary is the volume of all enclosed spaces (from keel to funnel) in a ship.

The reference to various measurement systems are in vogue as they would be relevant to various users – registration fee, harbour dues etc., would be based on Gross tonnage.

In ship scrapping, it is the weight of the ship that matters and hence it is LDT

Light Displacement tonnage (LDT) : Displacement is the actual total weight of the vessel. It is often expressed in long tons or in metric tons, and is calculated simply by multiplying the volume of the hull below the waterline (ie. the volume of water it is displacing) by the density of the water. (Note that the density will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.) For example, in sea water, first determine the volume of the submerged portion of the hull as follows: Multiply its length by its breadth and the draft, all in feet. Then multiply the product thereby obtained by the block coefficient of the hull to get the hull volume in cubic feet. Then multiply this figure by 64 (the weight of one cubic foot of seawater) to get the weight of the ship in pounds; or divide by 35 to calculate the weight in long tons. Using the SI or metric system : displacement (in tonnes) is volume (in m³) multiplied by the specific gravity of sea water (1.025 nominally).

The word “displacement” arises from the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the water which would otherwise occupy the “hole in the water” displaced by the ship.

Lightship or Lightweight measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.

Deadweight tonnage (DWT) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement, it is often expressed in long tons or in metric tons.