Dock water allowance (DWA)

Dock water allowance (DWA). When a vessel moves from salt water to water of lower density it will increase its draught and vice versa.

The amount of change is known as the “dock water allowance”. The dock water allowance depends on the “Fresh water allowance” (“FWA”) which represents the change in draught when moving between salt water (density 1.025 tonnes per cubic metre) and fresh water (1 tonne per cubic metre). The DWA can be estimated from a formula:

DWA = FWA (1.025 – d)/(1.025 – 1.000)

or it may be expressed as:

DWA = FWA (1025 – d)/25

where d = the density of the water in which the ship floats.

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Written by Ship Inspection

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  1. If water of density = 1.025 T/m3 is SW where DWA = 0 applies, then water of density = 0.9954 T/m3 in Panama Canal is dock water where DWA = FWA * (1.025-0.9954)/(1.025-1) = 1.184 * FWA applies.
    Where “fresh water of unit density has SG = 1, there dock water above has SG = (0.9954 T/m3)/(1 T/m3) = 0.9954.
    For special purposes the same dock water has SGT1/T1 = SG4C/4C = 1. Consequently FW of SG = 1 is water of:
    a) SGT1/T1 = 1.00462 = reading of standard T1 hydrometer;
    b) SG4/4C = 1.0027 = reading of standard 4C hydrometers.
    Other way hydrometers of readings = 1 in pure water have readings false for stability purposes.

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