Stability & CargoPage 10 of 12
Displacement in dock water: correct the table figure for density
The rule
- Entered with the survey mean draft, the hydrostatic table gives a displacement worked for the density its tables are computed for — 1.025 t/m³ unless the booklet says otherwise.
- What the draft really fixes is the immersed volume: table displacement ÷ table density. That volume does not change with the water she floats in.
- Sample the water alongside, read its density, and weigh the same volume of that water: corrected displacement = table displacement × observed density ÷ table density.
- Check the direction every time: water lighter than the tables' must give the smaller figure.
Formula
- Immersed volume = table displacement ÷ table density
- Corrected displacement = table displacement × observed density ÷ table density
Worked example
| Displacement read from the tables | 158420 t |
| Density the hydrostatic tables are computed for | 1.025 t/m³ |
| Density of the water sampled alongside | 1.0135 t/m³ |
- Immersed volume = 158420 ÷ 1.025 = 154556 m³
- Corrected displacement = 158420 × 1.0135 ÷ 1.025 = 156643 t
Answer156643 t
Common mistake
Turning the ratio upside down: 158420 × 1.025 ÷ 1.0135 = 160218 t.
The volume has not changed, only the weight of the water it holds, and water of 1.0135 t/m³ is lighter than the tables' 1.025: a correction that makes her heavier has gone the wrong way, and in a draft survey it turns into 3575 t of cargo that is not on board.