Ship tonnage: Its use and an aproximate calculation method.
DEFINITIONS
The concept of ship tonnage is related to her size or volume, but it is not a physical particular than can be expressed in any measurement unit, but it is the result of some calculations performed with the ship's dimensions by applying a set of agreed rules.
Until recent times the words ship tonnage were suggesting that they referred to a weight when actually it is a volumetric concept. Ship tonnage nowadays is adimensional.
There are three tonnage classes: Gross, Net and Compensated gross
1) Gross tonnage (GT) means the whole size of a ship according to regulations in the International Convention on Tonnage Measurement of Ships, 1969 (Ref.1). It is used to define the size of a ship, so to apply fees depending on it, when using port services and also when defining many technical requirements applicable to a ship as a function of her size.
2) Net tonnage (NT) means the useful capacity of a ship according to the same convention, that is, her spaces devoted to carrying cargo or passengers. It is used to suggest the economic earning capacity of a ship, that depends on the size of her cargo spaces and passenger accommodations.
3) Compensated gross tonnage (CGT) is related to the amount of work required to build a ship and it depends on her size, as measured by the GT, and her sophistication, as defined by a coefficient increasing with the ship type complexity. Its definition and calculation procedure are set down by the OECD (see Ref.2). CGT is being used to measure and compare the capacity or production of a shipyard, a group, a country etc., for the purpose of statistics and comparisons.
CALCULATIONS
In the book of Ref.3, published by the Spanish Association of Naval Architects (Ref.4), some approximate methods are found to estimate gross and net tonnages, as to get exact results requires tedious and lengthy calculations, that, on the other hand, are needed when any ship is built. But very often, a simple approximate estimation of tonnages can be enough and that is why approximate methods were included. Nevertheless, even those methods are some times too elaborated, as shown in the mentioned book (that also incorporates a computer program to make naval architecture preliminary calculations). By this reason I show below simpler methods, that can be used with a simple desk calculator.
a) Gross tonnage (GT)
GT can be estimated by the formula
GT = 0,35 L.B.D.Cb
The factors in the formula are the following ship dimensions:
L: length bet. perpendiculars
B: breadth
D: depth to upper deck
Cb: block coefficient
In case that Cb is not known it can be estimated as follows:
*fast ships, as reefers, container ships: bet. 0,65 and 0,70
*conventional cargo ships: bet. 0,70 and 0,80
*slow ships, as tankers, bulkcarriers: bet. 0,80 and 0,85
b) Net tonnage (NT)
NT is calculated by a formula a little too complex and because its practical interest is lesser than that of GT, I simply consider its value, as a first approximation, equal to 0,30 GT.
c) Compensated gross tonnage (CGT)
CGT is calculated by following formula, based on GT and two coefficients
CGT = a * GT^b (simbol ^ express potentiation)
Coefficients a and b are defined by OECD in Ref.2, wherefrom I have copied those applicable to most common ship types
Ship type ( a, b)
Bulkcarrier (29, 0.61)
Crude-oil Tanker (48, 0.57)
Chemical carrier (84, 0.55)
Product tanker (48, 0.57)
General cargo (27, 0.64)
Coaster (27, 0.64)
Reefer (27, 0.68)
LPG carrier (62, 0.57)
Container ship (19, 0.68)
REFERENCESRef.2 Compensated Gross Ton (CGT) System
Ref.3 El Proyecto Básico del Buque Mercante,(Basic design of merchant ships) by R. Alvariño, J.J. Azpíroz, M. Meizoso. Published by Editorial Fund from AINE
Ref.4 Spanish Association of Naval Architects. Editorial Fund
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