How is the weight of a Ship calculated?

Question

How is the weight of a Ship calculated?

Answer 1

As mr oil tanker has mentioned above there is a number of different ways to measure a ships tonnage... this is taken from wiki..

to refer directly to the weight of the vessel -

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

Deadweight (often abbreviated as DWT for deadweight tonnes) 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.

Gross Register Tonnage (GRT) represents the total internal volume of a vessel, with some exemptions for non-productive spaces such as crew quarters; 1 gross register ton is equal to a volume of 100 cubic feet (2.83 m³). This calculation is complex; a hold can, for instance, be assessed for grain (accounting for all the air space in the hold) or for bales (exempting the spaces between structural frames). Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, but is still a widely used term in the industry.[1][2]

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. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.

Gross Tonnage (GT) refers to the volume of all ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing. It is always larger than gross register tonnage, though by how much depends on the vessel design. It was a measurement of the enclosed spaces within a ship expressed in "tons" – a unit which was actually equivalent to 100 cubic feet.

Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.

It is calculated by using the formula :, where V = total volume in m³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by :). GT is consequently a measure of the overall size of the ship.

Net tonnage (NT) is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel’s earning space and is a function of the moulded volume of all cargo spaces of the ship.

A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc, are based on its gross tonnage, GT, or net tonnage, NT.

The Panama Canal/Universal Measurement System (PC/UMS) is based on net tonnage, modified for Panama Canal purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; a PC/UMS net ton is equivalent to 100 cubic feet of capacity.[3]

Thames measurement tonnage is another volumetric system, generally used for small vessels such as yachts; it uses a formula based on the vessel's length and beam.

Many people in many countries, including those professional people working in maritime industries for many years or even in their lifetime, often confuse "Tonnage" and "Ton". Please note that "Tonnage" refers to the unit of a ship's volume in measurement for registration and "Ton" refers to the unit of weight. They are totally different in concept.

Answer 2

Ha! This is such a mystery to everyone but the architect. Technically, a ship can be weighed based on the amount of water it displaces. For tax purposes, a ships' 'weight' is based on revenue-generating enclosed space (no kidding!) For charter purposes, the 'weight' is based on the cubic voume of cargo space, and/or based on the theoretical volume of water(in tons) that may be carried when the ship is up to it's maximim draught.

I know it doesn't help, but it's the truth!

Answer 3

There are two simple ways to calculate displacement (weight): trapezoidal and Simpson method. For both you will need area of the sections below WL.

Answer 4

By displacement. Say you put a ship in a dry dock. A given amount of water is pumped in and the ship floats, on the wall marker the water is at say 26'4". But without the ship the same water only reaches 14'8". The dry dock measures 100 feet by 350 feet. How much water did the ship displace in rising the water 11'6" in the dry dock. And this amount of water weighs? 11'6" of water over an area of 100 feet by 350 feet. One gallon of water = 8.33 pounds. 1cubic foot = 62.4 pounds. You do the math to see what this ship weighs.

Answer 5

On paper, a ship's displacement is estimated by measuring underwater volume.  On cubic foot of salt water weighs 64.2 pounds.  So if you can measure the cubic feet of the hull's shape, then you can calculate the weight displaced.

Answer 6

by the amount of water it displaces. This is the same way your body fat can be measure.