Ship constant and draught survey

Constellation Marine Services has a number of marine surveyors experienced to perform draught surveys on behalf of agents, ship owners, charterers, P&I clubs.

The ship constant is the difference between a vessel’s design lightship and its displacement when i.e light ship displacement = light ship weight + light ship constant.

The lightship weight includes the ship and its full equipment, engine room spares, water in the boiler and the lubricating oil in the engines.

And what is not included are personnel, cargo, fuel, stores and water.

Once the shipbuilder declares the lightship weight – this becomes permanent for the particular ship.

The lightship plays an important role in all stability calculations and draught survey.

Ship constant and draught survey-Reasons for variation

  • Sedimentation in ballast or fuel oil tanks.
  • Fouling of hull.
  • Coating build-up (paint work, tank coatings)
  • Garbage collection (particularly in cargo holds)
  • Modification to ship spaces.
  • Accumulation of stores (i.e. in paint locker, engineering stores, spares)
  • Refit, overhauls, addition/removal of equipment.

Generally a  ship’s  lightweight will increase over time particularly ships which are prone to corrosion and ships which aren’t regularly dry docked.

One estimate of lightship increase is at the rate of 0.5% per annum. However design light ship is taken into account for all draught survey calculations. Therefore the constant changes due to increased light ship weight.

A draught survey is a method of cargo weight determination by ship’s displacement calculations, empty and loaded, taking into account any change in weight of the bunkers, ballast and consumables on board.

An important point to be taken into account is the density of water.

If the density of water is not included in the calculations – there will be a shortage of cargo which will be reflected in the calculated figures.

Reason for variation of density of water may be because the water is not sea water but brackish water.

Density of the water also depends on the season – summer, winter or the tidal effect.

Hydrometers are commonly used to determine the density of water.

Two types of hydrometers are in use – draft survey hydrometer which is calibrated in vacuum and the load line hydrometer which is calibrated in air.

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