⚓ FWA & DWA Calculator

Calculate Fresh Water Allowance (FWA) and Dock Water Allowance (DWA) for any vessel. Determine load line submersion limits for dock water loading operations.

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Fresh Water Allowance (FWA)

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Dock Water Allowance (DWA)

Valid range: 1.000 – 1.025 t/m³

FWA Result will appear here

DWA Result will appear here

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Worked Example

Given:

  • • Displacement: 28,500 tonnes
  • • TPC: 33.75 t/cm
  • • Dock water density: 1.015 t/m³
1

Calculate FWA

FWA = 28,500 / (4 × 33.75) = 28,500 / 135 = 211 mm
2

Calculate DWA

DWA = [211 × (1.025 − 1.015)] / 0.025 DWA = [211 × 0.010] / 0.025 = 84.4 mm

Interpretation:

  • • In fresh water, the vessel will sink 211 mm deeper than in salt water.
  • • In this dock water (1.015 t/m³), the load line may be submerged 84.4 mm below the summer mark.

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Results are for reference only. Please double-check all values.SOLAS Load Line Convention

⚙️ How to Use

Step 1: FWA Inputs

  • Enter the vessel's Summer Displacement in tonnes from the stability booklet.
  • Select TPC (metric) or TPI (imperial) and enter the value from hydrostatic tables.
  • Choose preferred output unit: mm or cm.

Step 2: DWA Inputs

  • FWA auto-populates from the calculation above, or enter it manually.
  • Select a dock water density preset or enter a custom value.
  • Density must be between 1.000 and 1.025 t/m³.

Step 3: Calculate

  • Press Calculate to see FWA and DWA results.
  • The IMO compliance note indicates the permitted load line submersion.

Note: These calculations are based on SOLAS Load Line Convention rules. Always verify results against your vessel's official hydrostatic documentation.

📘 Formula Reference

FWA (TPC method):

FWA (mm) = Displacement / (4 × TPC)FWA (cm) = Displacement / (40 × TPC)

FWA (TPI method):

FWA (mm) = Displacement / (40 × TPI)

DWA formula:

DWA (mm) = [FWA × (1.025 − ρ)] / 0.025

TPC = Tonnes Per Centimeter in seawater (1.025 t/m³)

TPI = Tons Per Inch in seawater

ρ = Density of dock water (t/m³)

🌍 Common Port Water Densities

Port TypeDensityExamples
Fresh Water1.000 t/m³Great Lakes, River ports
Low Brackish1.005 t/m³River estuaries
Med. Brackish1.010 t/m³Enclosed bays
High Brackish1.015–1.020Semi-enclosed seas
Sea Water1.025 t/m³Open ocean

📖 Terminology & Definitions

FWA (Fresh Water Allowance)

The number of millimeters by which the mean draft changes when a ship moves between salt water (1.025) and fresh water (1.000) while at her summer load displacement.

DWA (Dock Water Allowance)

An allowance made for the change in a ship's draft when she is loading in water of a density between fresh water and salt water.

Displacement

The actual weight of the ship and everything on board. It is equal to the weight of the volume of water displaced by the hull.

TPC / TPI

TPC: Tonnes Per Centimeter immersion. The weight required to increase the mean draft by 1 cm.
TPI: Tons Per Inch immersion. The imperial equivalent for 1 inch of draft change.

Relative Density

Standard Sea Water is taken as 1.025 t/m³. Pure Fresh Water is 1.000 t/m³. Anything in between is considered Brackish or Dock Water.