For shipowners, charterers, and cargo receivers operating within the Bay of Bengal, the draft survey remains the ultimate commercial arbiter of cargo quantity. At Chattogram Port—where heavy siltation, shifting currents on the Karnaphuli River, and variable water densities are everyday factors—conducting an accurate draft survey is both a technical science and a financial safeguard. A slight miscalculation can trigger massive commercial disputes over short-landing or over-delivery of dry bulk commodities.
Understanding the standard procedures, acceptable tolerances, and common pitfalls is essential for anyone protecting their stake in a Chattogram port call.
1. The Core Procedure: Step-by-Step at Chattogram
A standard draft survey calculates the weight of cargo loaded or discharged by measuring the displacement of the vessel before and after operations. At Chattogram, the procedure follows strict international standards adapted to local hydrographic conditions:
- Initial Draft Readings: Before cargo operations commence, independent surveyors read the vessel’s drafts across six standard locations (forward, midship, and aft on both port and starboard sides). Environmental factors like wave action or swell near the outer anchorage must be accounted for.
- Density Sampling: Water density on the Karnaphuli River fluctuates dramatically depending on the tide, seasonal monsoons, and freshwater discharge. Surveyors take dedicated water density samples right alongside the vessel’s draft marks to calculate buoyancy accurately.
- Deductibles Measurement: All non-cargo weights on board—including Ballast Water (FW), Fresh Water (FW), Fuel Oil (IFO/MGO), Diesel Oil, and lubricating oils—must be precisely sounded and calculated using the vessel’s approved sounding tables.
- Final Calculation: Applying the hydrostatic particulars, trim corrections, and list corrections yields the net weight of the cargo handled.
2. Tolerances and the Reality of Local Variables
In commercial shipping, a standard draft survey tolerance of 0.5% between the ship’s figures and shore/scale figures is generally accepted. However, achieving this at Chattogram requires rigorous attention to detail:
- The Mud Factor: Chattogram port berths often accumulate soft silt and mud layers. Surveyors must determine true water depth versus soft bottom interference to prevent false buoyancy readings.
- Trim and List Limits: Extreme trim or list can distort hydrostatic table readings. Surveyors must ensure the vessel is as upright and even-keeled as possible, or apply rigorous mathematical corrections.
3. What Can Go Wrong? Common Pitfalls at Chattogram
Even with experienced crew and surveyors, things can easily go wrong if local conditions are overlooked:
- Unrecorded Ballast Sloshing or Transfer: Pumping ballast during or immediately before a draft reading without informing the surveyor throws off the entire calculation.
- Slack Tanks and Unsounded Spaces: Failing to account for unpumpable residues or neglecting to check all double-bottom and peak tanks introduces massive errors into the deductible weight.
- Incorrect Water Density Assumptions: Assuming a standard seawater density (1.025) instead of measuring the actual brackish river water density of the Karnaphuli can result in hundreds of tons of calculated discrepancy.
- Draft Mark Obsolescence or Poor Visibility: Marine growth, rust, or paint scaling around the draft marks can obscure exact readings, leading to human error during visual inspection.
The Takeaway
Draft surveys at Chattogram Port are high-stakes operations where minor oversights lead to major financial liabilities. By deploying independent, experienced surveyors who account for local Karnaphuli water densities, mud interference, and strict deductible soundings, owners and receivers can ensure their figures remain unassailable.