Dock Water Allowance (DWA & FWA) Calculator
Fresh, Brackish & Sea Water Density • Plimsoll Load Line Assessment • Draft Survey Control
Interactive Plimsoll Mark & Load Lines
Marine Dock Water Allowance (DWA) and Fresh Water Allowance (FWA) Calculation
Accurate hydrometer readings and water density assessments are critical for cargo planning, draft surveys, and maximum payload optimization prior to vessel departure. Under Archimedes’ principle, when a vessel moves from low-density brackish or river water into high-density open ocean water (1025 kg/m³), the required displacement volume decreases, causing the ship to rise and its draft to decrease. Conversely, navigating into fresh water causes bodily sinkage.
The NavLib Professional DWA & FWA Calculator provides Chief Officers, navigation officers, and marine cargo surveyors with an exact mathematical tool to verify draft adjustments and enforce international load line compliance.
1. Fresh Water Allowance (FWA) Fundamentals
Fresh Water Allowance (FWA) is the physical difference in millimeters between a vessel’s Summer Load Line draft in standard salt water (density = 1025 kg/m³) and her draft in pure fresh water (density = 1000 kg/m³) at constant displacement.
FWA is specified in the vessel’s approved Trim and Stability Booklet or derived using hydrostatic parameters:
FWA (mm) = Displacement / (4 * TPC)
Where:
Displacement is the vessel’s total displacement in metric tons;
TPC is the immersion rate (tons per centimeter immersion in salt water).
2. Dock Water Allowance (DWA) Formula
Harbor basins, river berths, and enclosed docks rarely contain purely fresh or purely oceanic water. Due to tidal influence and river runoff, dock water density generally ranges between 1005 and 1020 kg/m³.
Dock Water Allowance (DWA) determines the exact number of millimeters a ship may safely submerge beyond her statutory Summer Load Line mark in dock water of observed density:
DWA = FWA * (Sea Water Density – Actual Density) / 25
Or, using the standard salt-water constant (1025 kg/m³):
DWA = FWA * (1025 – Actual Density) / 25
3. Operational Applications on the Cargo Post:
Maximizing Permissible Loading Draft: When loading in dock water with density below 1025 kg/m³, the ship may legally submerge her Summer Mark (S) by the distance of DWA:
Max Permissible Draft = Summer Draft + DWA
Upon reaching open sea, buoyant lift returns the waterline precisely to the Summer mark, avoiding overloading detentions during Port State Control (PSC) inspections.
Deriving Sea Water Arrival Draft: When measuring departure draft in port (Observed Draft), the open ocean draft (Sea Water Draft) is determined as:
Sea Water Draft = Observed Draft – DWA
The interactive scaled Plimsoll Mark canvas visually maps waterline elevation against international statutory marks (TF, F, T, S, W, WNA) in real time. All survey figures can be exported to Microsoft Excel (.xlsx) or printed as an official Vessel Draft Survey Density Report.