the dimensions of a ship can be expressed by using termsm which descrbe the charateristics of the ship. each terme has a specific abbreviation. the type of ship determines the term to be used. for instance, the size of a container vessel is expressed in the number of containers it can transport﹔ a roll﹣on roll﹣off carrier's size is given by the total deck﹣area in squre metres and a passenger ship in the number of people it can carry. at the imo﹣conference in 1969 the new units "gross tonnage" and "nett tonnage" were introduced, to establish a world﹣wide standard in calculating the size of a ship. in many countries the gross tonnage is used to determine port dues and pilotage, or to determine the number of people in the crew.
register ton
to determine the volume of a space the register ton is used. one register ton equals 100 cft. or 2.83 m3.
gross tonnage
the gross tonnage is calculated using a formula that takes into account the ship's volume in cubic metre below the main deck and the enclosed spaces above the main deck.
this volume is then multiplied by a constant, which results in a dimensionless number (this means no values of t or m3 should be placed after the number). all distances used in the calculation are moulded dimensions.
in order to minimier the daily expenses of a ship, the ship owner will keep the gt as low as possible.
nett tonnage
the nett tonnage is also a dimensionless number that describes the volume of the cargo space. the nt ca be calculated from the gt by subtraction the volume of space occupied by:
﹣ crew
﹣ navigation equipment
﹣ propulsion equipment
﹣ workshops
the nt may not be less than 30% of the gt.
displacement (in m3)
the displacement equals the volume of the part of the ship below the water line including the shell plating, propeller and rudder.
displacement δ (in ton)
the displacement is the weight of the volume of water displaced of the ship. one could also say: the displacement equals the total mass of the ship.
displacement δ (t) = waterdisplacment (m3) * density of water (t/m3)
density of fresh water is 1 t/m3﹔
density of salt water is 1.025 t/m3.
biblio: ship knowldge ﹣ a modern encyclopedia
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