The Tonnes Per Centimetre Immersion(TPC) for any draft is the mass which must be loaded or discharged to change a ship’s mean draft in salt water by one centimetre.

The TPC varies with the draught and with the water density. Changes in draught cause a change in displacement and the TPC assists in calculating the change. 

The area between the TPC curve and the draught axis to any given draught represents the displacement of the ship at that draught, while its centroid represents the vertical position of the centre of buoyancy.

TPC in Dock Water

 When a ship is floating in dock water of a relative density other than 1.025 the weight to be loaded or discharged to change the mean draft by 1 centimetre (TPCdw).

TPCdw/TPCsw=Relative density of dock water/Relative density of  sea water

                        TPCdw= (RDdw/1.025)*TPCsw