What is Ship Constant?
The ships constant is the difference between a vessels design lightship and it’s actual displacement when empty. The ships constant is determined by a draft survey. The ships constant will vary over the life of the vessel.
Unintentional Reasons for Variations
– Sedimentation in ballast or fuel oil tanks
– Corrosion
– Fouling of hull
– Coating build-up (paintwork, tank coatings, etc.)
– Garbage collection, particularly in cargo holds
– Wear and tear of mechanical devices
Other Reasons for Variations
– Modifications to ship spaces
– Accumulation of stores (i.e., in paint locker, engineering stores, spares)
– Refit, overhauls, addition/removal of equipment.
Effect of a Ships Constant
Generally, a ships lightship value will increase over time; particularly for vessels with large cyclical passenger loads, ships prone to corrosion, and ships which aren’t regularly dry-docked. One estimate of lightship increase is a rate 0.5% per annum. Additional inclining tests should be performed after any substantive additions, relocation, or removal of weights, as well as periodically over the life of a vessel. (Neely-Chaulk.com)
What is negative constant?
The condition when the empty weight of ship actual survey displacement is under the weight of the stated lightship in particulars. This is a rare case, but some of you might ever found it.
Why a ship has a negative constant?
There are several reasons why a ship has a constant negative or very small. These can be caused by the following:
1. Documents on the ship is actually a document for the identical ship (sister ships).
2. Inaccuracy of Hydrostatic Particulars.
3. Any equipment or parts of ships that have been moved / removed, for example tweendeck, crane or others – something usually done by small ship to increase the carrying capacity and of course will be decreased the actual lightship but not stated on the particulars (need to be recheck on the next vessel docking).
4. When reading, the estimates reading drafts are lower than actual of draft ships.
5. Sounding for deductibles liquid, especially water ballast could be higher than actual quantity on tanks.
6. The density does not actually represent.
What the draft surveyor should do?
If you get constant negative, check again what the reason or cause, if possible compare with the previous draft survey. And if there is no reason that can be accepted, then these circumstances must be stated in the report together with:
a. Effective result of the draft survey.
b. Results of the survey loaded – lightweight (of ship documents) – constant (information from master).
c. The master must sign a statement relating to the constant that has been informed.
d. The declared constant by master should be approved for at least 3 (three) previous shipments along with available draft survey data.
