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Stability & CargoPage 12 of 12

The ship's constant on a survey: what she displaces but cannot name

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The ship's constant: what the survey displacement holds beyond the lightship and everything declaredSurvey displacement 138423 t. Less the lightship, 23180 t, leaves 115243 t on board. Declared: cargo 112600 + ballast 150 + fresh water 275 + bunkers 1900 = 114925 t. What is left is the constant: 115243 − 114925 = 318 t. The bunker figure from arrival, 1460 t, is not used: she bunkered in this port.displacement 138423 tcargo 112600 ton board 115243 tlightship 23180 tdeclared items and the constant, magnifiedballast150 tfresh water275 tbunkers1900 tconstant318 tbunkers on arrival1460 tnot used — she bunkered hereconstant = 115243 − 114925 = 318 tDisplacement bar to scale · small items magnified on one scale

The rule

  • Start from the survey displacement and take off the lightship from the ship's certificate. The displacement should already be corrected for the water alongside; here no density is given, so the table figure is used as it stands.
  • Then take off every weight declared on board at the time of the survey: cargo, ballast, fresh water, and bunkers and lubricating oil as measured on completion.
  • What is left is the constant: sediment in the tanks, stores and spares nobody weighs, and the error of the tables themselves — weight the ship carries but cannot name.
  • Compare it with her last surveys: a constant that jumps means a deductible was missed or misread, not that the ship has changed.

Formula

  • On board = displacement − lightship
  • Constant = on board − (cargo + ballast + fresh water + bunkers on completion)

Worked example

Displacement from the hydrostatic tables at the survey mean draught138423 t
Lightship from the ship's certificate23180 t
Cargo measured by the ullage report112600 t
Ballast water remaining on completion150 t
Fresh water on completion275 t
Bunkers and lubricating oil on completion1900 t
Bunkers and lubricating oil on arrival, before this port1460 t
  1. On board = 138423 − 23180 = 115243 t
  2. Declared = 112600 + 150 + 275 + 1900 = 114925 t
  3. Constant = 115243 − 114925 = 318 t

Answer318 t

Common mistake

Using the bunker figure from arrival, 1460 t, instead of the 1900 t measured on completion: the constant comes out 758 t.

She bunkered in this port, so the arrival figure is 440 t short of what is in her tanks now, and every one of those tonnes lands in the constant.