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Stability & CargoPage 11 of 12

Draft survey: how much cargo is on board

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Draft survey: displacement less lightship less the deductibles leaves the cargoThe draft marks are read aft, midships and forward on both sides and corrected, which gives the displacement, 158420 t. Deadweight = 158420 − lightship 22940 = 135480 t. Deductibles: ballast 1860 + fresh water 320 + fuel oil 2140 + diesel oil 180 + lubricating oil 45 + constant 380 = 4925 t. Cargo = 135480 − 4925 = 130555 t.AFTMIDSHIPSFORWARDdraft marks, read both sides → corrected → displacementdisplacement 158420 tlightship22940 tdeadweight 135480 tcargo 130555 tdeductibles 4925 t (magnified)ballast1860 tfresh water320 tfuel oil2140 tdiesel oil180 tlubricating oil45 tconstant380 tBow → · displacement bar to scale

The rule

  • The survey's drafts — read forward, midships and aft on both sides, then corrected for trim, hog or sag and the dock water density — give the displacement; this sum starts from that figure.
  • Deadweight = displacement − lightship: everything on board that is not the ship herself.
  • Deductibles = ballast + fresh water + fuel oil + diesel oil + lubricating oil + the ship's constant — the tanks sounded at the time of the survey, the constant from the ship's record.
  • Cargo = deadweight − deductibles.

Formula

  • Deadweight = displacement − lightship
  • Deductibles = ballast + fresh water + fuel oil + diesel oil + lubricating oil + constant
  • Cargo = deadweight − deductibles

Worked example

Displacement from the survey158420 t
Lightship22940 t
Ballast on board1860 t
Fresh water320 t
Fuel oil2140 t
Diesel oil180 t
Lubricating oil45 t
Ship’s constant380 t
  1. Deadweight = 158420 − 22940 = 135480 t
  2. Deductibles = 1860 + 320 + 2140 + 180 + 45 + 380 = 4925 t
  3. Cargo = 135480 − 4925 = 130555 t

Answer130555 t

Common mistake

Leaving the 1860 t of ballast off the deductibles and reporting 132415 t.

The drafts cannot tell water from cargo: ballast still in the tanks at the survey is weighed with everything else, so every tonne you leave out is reported as cargo that was never loaded.