Friday, April 15, 2011

Vessel Displacement and how it is measured

A 200 meter long  PCTC upbound Westerschelde River (photo by K.C.)


Displacement  is the weight of the vessel and all weights on board. The units most commonly used are metric tons. The concept that  weight and displacement of a floating vessel are equal is called Archimedes Principle.

Displacement = weight of the vessel + all the stuff on the vessel
Lightweight Displacement = weight of the vessel
Deadweight = all the stuff on the vessel.

Deadweight,  abbreviated dwt  is the carrying capacity of individual ships and is used to compare ships   as well as a nations entire merchant marine and loses at war.

Deadweight  includes cargo, fuel, water, lube oil, stores, the crew.  It might seem odd to refer to paying cargo as dead weight but the term deadweight implies that the ship itself is alive. A sailing ship in danger might jettison deadweight, paying or not - the origin of the concept of general average.

Displacement is measured in two ways

- By summing up all weights on board and adding them to the lightship weight. This is done by a program, basically  a spreadsheet. The lightship weight is supplied by the builder. The chief engineer supplies the fuel, lube oil. The chief mate manages the ballast. The program computes drafts, in meters, displacement in metric tons and  GM in meters.

- The vessel's drafts. The two factors that determine draft are displacement and the density of the water. Drafts can be converted to displacement using a table provided by the builder.

Using some  real numbers the PCTC pictured above:

Lightship weight is  16,108 mt.
Here are some typical weights that comprise deadweights
Cargo: 8520 mt -  5500 standard cars at 1.5 mt each
Fuel:   2000 mt (Max 3095)
Ballast:  4000 mt (max 9523 mt)
Fresh water: 400 mt  (max 420 mt)
Diesel oil:  100 mt (max 190 mt)
crew and associated gear and equipment is 135 mt

Total Deadweight is this case is 15,155 mt
 Lightship weight + deadweight=  Displacement
Displacement = 16108 mt +15,155 mt =31,263 mt

A PCTC is at one end of the deadweight/lightweight ratio  spectrum. At the other end are large tankers. For example from this site:

  A VLCC is a is a ship with a deadweight of from 200,000 to 315,000 tons. The standard VLCC is about 300,000 tons which will allow is to carry about 2 million barrels of oil. The lightweight of such a ship will be around 42,000 tons.

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