How to use the FWA & DWA calculator
- Enter the ship's fresh water allowance from the International Load Line Certificate or stability booklet, or let the calculator work it out from the summer displacement and TPC.
- Enter the dock water density you measured alongside, in kg/m³ (a hydrometer reading of 1.010 is 1010 kg/m³).
- Read the dock water allowance: how far the relevant load line may be submerged in that water, and roughly how much extra cargo that is worth.
FWA and DWA formulas
A ship floats deeper in fresh water than in salt water because fresh water is less dense. The fresh water allowance is that difference at the summer draught, going from salt water (1025 kg/m³) to fresh water (1000 kg/m³). It is the distance between the S and F marks on the load line. In brackish dock water the ship sinks by a proportion of the FWA: the dock water allowance.
Under the Load Line Convention a ship loading in dock water may submerge her load line by the DWA, because she will rise by that amount when she reaches salt water.
Worked example: M/V YASSIN BEY
M/V YASSIN BEY, a SENA-managed multipurpose vessel, has an FWA of 237 mm and a TPC of 41.03 t/cm. She loads in a river port where the dock water density is 1010 kg/m³:
- DWA = 237 × (1025 − 1010) ÷ 25 = 237 × 15 ÷ 25 = 142.2 mm
- TPC in dock water = 41.03 × 1010 ÷ 1025 = 40.43 t/cm
- Extra cargo ≈ 14.22 cm × 40.43 t/cm = about 575 t
In fully fresh water (1000 kg/m³) the DWA equals the FWA, so M/V OSAMA BEY, with an FWA of 147 mm, could submerge her summer load line by 147 mm.
Practical points
- Sample the dock water from several depths and positions along the ship, away from outfalls. Density can change with the tide in river ports.
- Allow for fuel and fresh water burned on the way to the sea, which also lets the ship rise.
- Check which load line applies: summer, winter or tropical, depending on the zone and season of the voyage.
Shallow berths often come with brackish water. Check the clearance underway with the squat calculator.