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The tonnage measurement system


The tonnage of ships above 24 metres in length *) which are engaged on international voyages is determined according to the International Convention on Tonnage Measurement of Ships, 1969 (London-Rules). Under the Convention system, the volume of all ship’s enclosed spaces (from keel to funnel) is measured to the inside of the hull framing, whereas special open spaces are excluded, so they are not considered in the total volume.

measurement.jpg Tonnage measurement according to the London 69 Rules


The dimensionless gross tonnage (GT) is computed from the total volume V (in m3) multiplied by the factor K1 (from 0.22 up to 0.32), using the formula GT = K1 * V. The gross tonnage indicates a vessel’s total volume, not her weight or deadweight, which is a common misconception.

The net tonnage (NT) indicates a vessel’s earning space. NT is also dimensionless and depends on the total volume of all cargo spaces, the draught, moulded depth and number of passengers. Net tonnage is determined using a special formula and is not allowed to be less than 0.3 GT.

According to IMO-regulations a reduced gross tonnage is certified for 'open top' containerships and tanker with a double hull (ships equipped with segregated ballast tanks SBT).

*) The definition of length, as given in the International Convention on Load Lines, 1966, means ”96 per cent of the total length on a waterline at 85 per cent of the least moulded depth measured from the top of the keel, or the length from the fore side of the stem to the axis of the rudder stock on that waterline, if that be greater”.



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If you have any additional questions, please e-mail Rainer Koch or phone +49 40 3190 - 7210


More information on the subject:
Small craft tonnage measurement


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 © 2018 Bundesamt für Seeschifffahrt und Hydrographie Last Update: 19.04.2013 18:01:28  
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