The Dock Water Allowance (DWA) of a ship is the number of millimetres by which the mean draught changes when a ship passes from salt water to dock water, or vice-versa, when the ship is loaded to the Summer displacement.
Consider the load line marks shown. The top of the Summer mark and the top of the Fresh mark both act as the limits of a scale of density, indicating the position of the salt water and fresh water waterlines respectively for a ship loaded to the Summer displacement. If such a ship was to be floating in water of an intermediate density, termed Dock Water; the change in draught when going from salt water to dock water can be easily determined.
Consider the scale marked on the section of load line shown.
If the ship were to go from SW to dock water of RD 1.010, the draught would change by the DWA. The amount of the DWA is simply a fraction of the FWA as shown, in this case 3/5ths or 15/25ths of the FWA value.
The DWA, as a fraction of the FWA, is found by the formula: