The mass required to change a vessel's mean draft by one centimetre, used to calculate draft changes during loading or discharging.
In practice
For example, if a vessel has a TPC of 50 t/cm and the chief mate wishes to reduce the mean draught by 10 cm, approximately 500 tonnes of cargo or ballast must be removed. Similarly, if 200 tonnes of fuel are burned during a passage, the draught will decrease by 200 divided by the TPC at that draught. TPC also facilitates cargo quantity estimation during loading by draught survey: a surveyor measures the draught before and after loading, applies the TPC to the change in mean draught, and corrects for trim and density to determine the cargo received. Draft surveys under this method are a commercially recognised basis for quantity determination when weighbridge or flow-meter measurement is not available.
Regulatory detail & full definition
Tonnes Per Centimetre (TPC) is the mass in metric tonnes required to change a vessel's mean draught by one centimetre when the ship is floating upright at a particular displacement. It depends on the area of the waterplane at that draught and the density of the water in which the vessel is floating. The TPC is tabulated in the hydrostatic tables of the ship's stability booklet and is used by the chief mate to calculate draft changes resulting from loading or discharging cargo, transferring ballast, or consuming fuel and fresh water.
For example, if a vessel has a TPC of 50 t/cm and the chief mate wishes to reduce the mean draught by 10 cm, approximately 500 tonnes of cargo or ballast must be removed. Similarly, if 200 tonnes of fuel are burned during a passage, the draught will decrease by 200 divided by the TPC at that draught. TPC also facilitates cargo quantity estimation during loading by draught survey: a surveyor measures the draught before and after loading, applies the TPC to the change in mean draught, and corrects for trim and density to determine the cargo received. Draft surveys under this method are a commercially recognised basis for quantity determination when weighbridge or flow-meter measurement is not available.
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