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Stability & CargoPage 3 of 12

Loading to the marks in dock water (FWA and DWA)

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Load line mark in dock water: she may load deeper than S by the dock water allowanceLoad line mark on the starboard side, bow to the right. S is the Summer line at 16.1 m; F, the fresh water line, is the FWA of 426.4 mm above it; T and W sit 1/48 of the Summer draft above and below S, and TF is the FWA above T. In dock water of 1.012 t/m³ she may load until the water is the DWA of 221.7 mm above S — 0.52 of the way from S (salt water, 1.025) to F (fresh water, 1.000): 16.1 + 0.222 = 16.32 m. In salt water she rises back to S.STWFTFFWA 426.4 mmDWA221.7 mmWATER DENSITY · t/m³1.000 fresh1.012 dock1.025 saltSummer draft 16.1 m + DWA 0.222 mmax draft alongside 16.32 mat sea (1.025) she rises 221.7 mm, back to SStarboard side, bow → · levels read at the upper edge of each line

The rule

  • FWA (in mm) = displacement ÷ (4 × TPC), both read at the Summer draft.
  • DWA = FWA × (1.025 − dock water density) ÷ 0.025: the allowance is pro rata between salt water (1.025) and fresh water (1.000).
  • Lighter water gives less buoyancy, so she floats deeper: ADD the DWA to the Summer draft.
  • When she sails into salt water of 1.025 she rises by the same DWA and sits on her Summer mark.

Formula

  • FWA (mm) = displacement ÷ (4 × TPC)
  • DWA = FWA × (ρ salt water − ρ dock water) ÷ (ρ salt water − ρ fresh water)
  • Maximum draft alongside = Summer draft + DWA

Worked example

Summer load line draft16.1 m
Displacement at the Summer draft168000 t
TPC at the Summer draft98.5 t/cm
Measured dock water density1.012 t/m³
  1. FWA = 168000 ÷ (4 × 98.5) = 426.4 mm
  2. DWA = 426.4 × (1.025 − 1.012) ÷ 0.025 = 426.4 × 0.52 = 221.7 mm = 0.222 m
  3. Dock water is lighter, so she floats deeper: 16.1 + 0.222 = 16.32 m
  4. Maximum mean draft alongside = 16.32 m

Answer16.32 m

Common mistake

Adding the whole FWA, as though the dock water were fresh, and loading to 16.53 m.

Water of 1.012 is only 0.52 of the way from salt to fresh, so it earns 0.222 m, not the full 0.426 m; load to the full figure and her Summer mark is still 0.205 m under water when she reaches the sea.