Fresh Water Allowance (FWA) is the amount by which a ship's mean draft changes when she moves from the sea standard salt water (SW @ 1.025 t/m3) to fresh water (FW @ 1.000 t/m3).
FWA (in mm) = Displacement / (4 x TPC)
FWA (in cm) = Displacement / (40 x TPC)
FWA (in cm) = Displacement / (40 x TPC)
For box-shaped vessel FWA remains same at all drafts (as TPC remains constant), otherwise FWA increases with the draft (as TPC increases with the draft).
Dock Water Allowance (DWA) is the amount by which a ship's mean draft changes when she moves from the sea standard salt water (SW @ 1.025 t/m3) to dock water (DW @ various t/m3).
The formula that may be used is:
DWA = FWA * (1.025 – density of DW) / 0.025
Recently I found a meticulous video explanation on the topic. Tribute to the author of the video Capt. A.N. Rizvi.
