Alternative tonnage. A vessel may sometimes load a full cargo, which takes it down to the normal load line marks.
At other times it may load a full cargo of low-density, light commodities. The shipowner can request modified tonnage in addition to the normal (e.g., British) gross and registered tonnage. A special certificate is issued, showing both tonnages. When the details of the vessel are included in publications, e.g., in the “Register” published by Lloyd’s Register of Shipping, both tonnages will also be shown.
A modified tonnage may apply to a vessel as an alternative to its normal tonnage. The same criteria for modified tonnage would apply. Once again, a tonnage mark would be used but in this case it is placed at a distance below the line of the second deck from above. This distance depends on the length and depth of the vessel. It is common for the tonnage mark to be closer to the keel than the deepest load line.
In this situation the tonnage mark is not like a “badge”. If the tonnage mark is not submerged, the modified (lower) tonnage applies for various purposes, such as harbour and pilotage dues. If it is submerged, because of the loading of the vessel, the normal gross and net tonnages will determine the various charges on the vessel. Port authorities react unfavourably even to this system, their revenue depending on the submersion of a mark. The submersion was variable and unpredictable yet the vessel’s earning capacity was not affected if the cargo was light and bulky so that the normal load lines were not submerged nor was the tonnage mark.