A draft survey never touches the cargo itself. It weighs bulk cargo purely by measuring how much water the ship displaces before and after loading — and when done correctly, it's accurate to within half a percent. When done carelessly, it's one of the easiest survey types to get meaningfully wrong without anyone noticing until the figures are disputed.
What You'll Learn in This Lesson
- What a draft survey actually measures, and the principle it's built on
- How displacement translates into cargo weight, step by step
- How draft marks are read, and why six readings matter more than one
- The five factors most likely to compromise accuracy
- How to spot the likely cause behind an inaccurate cargo figure
Draft Survey Scope
A draft survey determines the weight of bulk cargo loaded or discharged by calculating the change in a vessel's displacement — the weight of water it displaces — rather than weighing the cargo directly. It's the standard method for bulk commodities like grain, coal, and ore, where the cargo can't practically be weighed any other way at the scale involved. Properly performed, a draft survey achieves accuracy within roughly ±0.5% of the true figure, which is precise enough to be the accepted commercial standard for settling how much cargo actually moved.
Displacement & Cargo Weight: The Calculation Logic
The method rests on Archimedes' Principle: a floating vessel displaces a volume of water exactly equal to its own weight. Measure that displaced volume accurately, convert it to weight using the water's density, and you have the vessel's total displacement at that moment.
| Step | What Happens |
|---|---|
| 1. Initial Displacement | Draft readings taken and corrected before loading/discharge begins |
| 2. Final Displacement | Draft readings taken and corrected after loading/discharge completes |
| 3. Net Displacement | Initial displacement minus final displacement (or vice versa for loading) |
| 4. Deduct Consumables | Ballast, fuel, fresh water, and stores subtracted from each displacement figure |
| 5. Cargo Weight | The remaining net difference — this is the cargo actually loaded or discharged |
Every deductible — ballast, bunkers, fresh water, stores — has to be measured with the same care as the draft readings themselves. A ballast sounding error doesn't just create a small inaccuracy; it's consistently the single largest source of error in draft survey practice, because ballast quantities are often larger than the margin of accuracy the whole calculation is trying to achieve.
Draft Marks & Readings
Draft marks are read at six points: forward, midships, and aft, on both the port and starboard sides. Averaging six readings rather than relying on one or two matters because the vessel's hull can deflect under uneven weight distribution — bending upward at midships (hogging) or downward (sagging) — which a single reading at one point can't detect or correct for. The mean of the forward and aft drafts is the Arithmetic Mean Draught (AMD); the draught at the vessel's actual centre of flotation is the True Mean Draught (TMD), which is what the hydrostatic tables are actually built around. Using AMD in place of TMD is a well-documented source of accuracy loss.
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Five Factors Affecting Accuracy
- Ballast Sounding Errors — the most common and consequential source of error, since ballast quantities are often large relative to the accuracy margin the whole survey is trying to hit
- Trim & List Beyond Tolerance — the vessel should be as close to upright as possible (list under roughly 0.5°) and at minimal trim, since larger values push corrections outside what the hydrostatic tables reliably account for
- Density Measurement — displacement must be corrected for the actual water density at the time of survey, not an assumed standard value
- Hydrostatic Table Errors — occasional typographical or arithmetic errors in a vessel's own hydrostatic tables have been documented as a real source of displacement error
- Hogging & Sagging — hull deflection that distorts the relationship between draft readings and true displacement if not properly corrected for
Draft Survey Workflow
Document review means confirming the vessel's hydrostatic and tank calibration tables before a single reading is taken — without them, draft readings can't be converted into a displacement figure at all. Where possible, ballast tanks are best either fully pressed up or completely empty before soundings, since partially filled tanks are the condition most prone to sounding error.
Common Errors & Discrepancies
Case Study: An Inaccurate Cargo Figure
Scenario: A bulk carrier's draft survey shows a cargo figure that differs from the shore scale weight by 2.1% — well outside the ±0.5% accuracy standard. The vessel was carrying significant list during the final readings, and one ballast tank was recorded as roughly half full.
Exercise: Identify the Likely Accuracy Issue
Match each finding to its most likely cause.
1. A ballast tank was recorded as roughly half full at both initial and final soundings.
2. The surveyor used the average of forward and aft drafts instead of the draught at the centre of flotation.
3. The vessel was floating with a pronounced list during both sets of draft readings.
4. Water density was assumed at a standard reference value rather than measured on site.
Best Practices Checklist
- Hydrostatic and tank calibration tables confirmed before any reading is taken
- All six draft points read and recorded, not a partial set
- True Mean Draught used for displacement lookup, not Arithmetic Mean Draught
- Ballast tanks pressed up or fully empty wherever practically possible
- Water density measured on site, not assumed from a standard table value
- Any figure outside the ±0.5% accuracy standard investigated by input, not dismissed
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Frequently Asked Questions
What does a draft survey actually measure?
It measures the change in a vessel's displacement before and after loading or discharge, and calculates cargo weight from that difference — it never weighs the cargo directly.
How accurate is a properly performed draft survey?
Within roughly ±0.5% of the true figure, which is precise enough to be the accepted commercial standard for bulk cargo.
What is the single most common source of draft survey error?
Ballast sounding errors — inaccurate measurement of ballast water quantity, which is often large relative to the accuracy margin the survey is trying to achieve.
Why are draft marks read at six points instead of one?
To detect and correct for hull deflection (hogging or sagging), which a single reading can't reveal.
What's the difference between AMD and TMD?
AMD (Arithmetic Mean Draught) is the simple average of forward and aft drafts; TMD (True Mean Draught) is the draught at the vessel's actual centre of flotation, which is what hydrostatic tables are built around. Using AMD in place of TMD is a known source of error.
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