Enter the dock water allowance (draft correction), fresh water allowance (FWA), density of sea water, density of dock water, and density of fresh water into the calculator to determine the missing value.

Dock Water Allowance Calculator

Enter any 4 values to calculate the missing variable

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Dock Water Allowance Formula

The following formula is used to calculate the dock water allowance (a draft correction) for a given set of values.

DWA = FWA \cdot \frac{(\rho_s - \rho_d)}{(\rho_s - \rho_{fw})}

Variables:

  • DWA is the dock water allowance (draft correction; same units as FWA, e.g., mm or inches)
  • FWA is the fresh water allowance (draft correction between sea water and fresh water at the relevant load line)
  • ρs is the density of sea water
  • ρd is the density of dock water
  • ρfw is the density of fresh water (often taken as 1000 kg/m³)

To calculate the dock water allowance, multiply the fresh water allowance (FWA) by the ratio of the density difference between sea water and dock water to the density difference between sea water and fresh water. DWA and FWA must be in the same length units, and all densities must be in consistent units.

What is Dock Water Allowance?

Dock water allowance (DWA) is a draft correction used in ship loading practice when a vessel is in dock (or estuarial) water whose density is typically less than standard sea water. Because buoyancy depends on water density, a ship will generally sink deeper (increase draft) in less-dense dock water than it would in sea water at the same displacement. DWA is commonly calculated from the vessel’s fresh water allowance (FWA) and the measured dock-water density so that loading can be adjusted appropriately before the ship proceeds to sea.

How to Calculate Dock Water Allowance?

The following steps outline how to calculate the Dock Water Allowance.

  1. First, determine the fresh water allowance (FWA) for the vessel at the relevant load line.
  2. Next, determine the density of sea water (ρs) you are using as the reference (often 1025 kg/m³).
  3. Next, determine the density of fresh water (ρfw) (often 1000 kg/m³).
  4. Next, measure or determine the density of dock water (ρd).
  5. Finally, calculate the Dock Water Allowance using the formula DWA = FWA · (ρs − ρd) / (ρs − ρfw).
  6. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Fresh water allowance (FWA) = 150 mm

Density of sea water (ρs) = 1025 kg/m³

Density of fresh water (ρfw) = 1000 kg/m³

Density of dock water (ρd) = 1010 kg/m³

Then DWA = 150 mm × (1025 − 1010) / (1025 − 1000) = 150 × 15 / 25 = 90 mm.