The key difference between a draft survey and a shore scale weight is that a draft survey calculates cargo quantity indirectly from the vessel, whereas a scale weighs the cargo directly ashore. A draft survey assesses the whole parcel in a single calculation, while a weighbridge or belt scale weighs it truck by truck or continuously. Each method has its own sources of uncertainty.
How do the two methods work?
A draft survey calculates the vessel's displacement before and after the operation and, after deducting variable weights, arrives at the cargo quantity. The measurement takes place on board and the cargo itself is never weighed.
Shore weighing uses road weighbridges, rail scales or conveyor belt weighers. With a weighbridge each truck is weighed loaded and empty to give a net weight; a belt scale continuously measures material passing over the belt and totalises the result.
What are the strengths and limits of a draft survey?
A draft survey can be used where there is no shore scale or where scale figures need independent verification. It produces a single figure for the whole parcel, and all parties can attend the readings.
Its main limitation is sensitivity to survey conditions. Swell, current, varying water density, slack tanks or gaps in the ship's documentation can all affect the outcome. On small parcels, the measurement uncertainty represents a larger share of the cargo quantity.
What are the strengths and limits of shore scales?
A calibrated and regularly verified scale weighs the cargo directly and gives a result that is easy to understand. Truck-by-truck records also make it possible to track a parcel in parts.
The reliability of scale figures depends on calibration status, tare practice, whether every vehicle was actually weighed and on handling losses between the scale and the ship. With belt weighers, belt tension, flow rate and zero setting can all influence the totals.
Why do ship and shore figures differ?
Because the two methods measure at different points and on different principles, an exact match should not be expected. The size of the difference indicates which source deserves closer scrutiny.
- Cargo left on the quay, spilled or lost to wind between the scale and the ship
- Changes in moisture content while cargo sits in the stockpile
- Scale calibration or tare deviations
- Draft survey uncertainty from density, readings or tank soundings
- An unnoticed change in the constant during the operation
Which figure goes on the bill of lading?
The basis for the B/L figure is normally set by the sales contract and the contract of carriage. Some contracts specify the load port draft survey, others the shore scale weight, and in some cases both figures are recorded side by side.
Where there is a significant gap between the shore figure and the ship's figure, the master may wish to record it. Knowing the contractual measurement clause before operations start helps avoid disputes later.
How is a difference at the discharge port assessed?
When the discharge port draft survey differs from the B/L figure, the first question is whether the surveys at the two ports are comparable. Use of the same tables, a consistent constant and the same treatment of deductibles at both ends form the basis of any assessment.
Moisture content, cargo remaining in the holds after discharge, handling losses and the bunker consumption records for the voyage are then reviewed. Together, these help distinguish a genuine shortage from differences that simply arise from the measurement methods, and they give cargo interests and P&I correspondents a factual basis for discussion.
Which method should be preferred, and when?
Where a scale has current calibration and complete records, its figure may serve as the primary source, with a draft survey providing an independent cross-check. In ports without a scale, or where scale figures are disputed, the draft survey becomes the primary method.
Many operations use both, so that large discrepancies are spotted early and investigated while the cargo is still accessible. Karan Gözetim provides draft survey services from its base in Gemlik; whether a comparison with shore figures is needed is agreed at quotation stage.
Frequently asked questions
Can a belt scale replace a draft survey?
A properly calibrated belt weigher is useful for monitoring loading rate and total quantity. Where independent verification is required, a draft survey is often used alongside it as a complementary check.
Does cargo moisture affect the draft survey result?
A draft survey measures the total weight of cargo at that moment, including its moisture. If moisture content changes between loading and discharge, results at the two ports will differ accordingly.
When should a difference between ship and shore figures be investigated?
An acceptable difference is often defined in the contract or by trade custom. Where that limit is exceeded, both the scale records and the draft survey calculation should be reviewed in detail.
Topics
- draft survey vs shore scale
- draft survey or weighbridge
- belt scale vs draft survey
- shore scale weight bulk cargo
- bill of lading quantity basis
- ship figure vs shore figure