Draft survey

A shipboard cargo-mass determination that infers weight loaded or discharged from the vessel's change in displacement, using draft readings corrected for trim, water density, and non-cargo weight changes.

Version

v1 · 2026-09-28 · History

Domain-specific #

9062

Domain group

Applied Sciences & Engineering

Origin domain

Engineering & Design (beyond software)

Subdomains

Naval Architecture, Marine Cargo Surveying → Engineering & Design (beyond software)

Core Idea

A draft survey estimates cargo loaded onto or discharged from a ship by comparing vessel displacement before and after transfer. Surveyors read draft marks forward, amidships, and aft on both port and starboard, correct those observations, and use the vessel's hydrostatic data to convert the waterline condition into displaced mass. The cargo estimate is the corrected displacement change after subtracting other weight changes. The cargo estimate is the corrected displacement change after subtracting other weight changes.

How would you explain it like I'm…

How Low Does It Float

Put a toy boat in the bathtub and it floats with a little bit under the water. Add marbles and it sinks lower. Big ships work the same way: people look at how deep the ship sits before and after loading, and that tells them about how much cargo went on.

Weighing Cargo by Waterline

A draft survey is a way to figure out how much cargo was loaded onto a ship without putting the cargo on a scale. Ships have number marks painted on their sides that show how deep they sit in the water. Surveyors read these marks at the front, middle and back, on both sides, before and after loading. The deeper the ship sits, the more water it pushes aside and the heavier it is. They also have to subtract other things that changed, like fuel, fresh water and ballast water, so the answer is a careful estimate.

Displacement-Based Cargo Estimate

A draft survey estimates the mass of cargo loaded or unloaded by comparing a ship's displacement before and after. It rests on Archimedes' principle: a floating ship displaces a mass of water equal to its own mass. Surveyors read the draft marks forward, amidships and aft on both sides, correct the readings for things like the ship tilting front-to-back (trim) or side-to-side (list), and use the ship's tables to turn the waterline into displacement. The density of the water matters, since the same mass sinks deeper in fresher water. Then they subtract changes in ballast, fuel, stores and other consumables. The result is an inferred total mass for the bulk cargo, not a direct weighing, and it carries measurement uncertainty.

 

A draft survey determines cargo quantity by comparing a vessel's displacement before and after loading or discharge. Surveyors read the draft marks forward, amidships and aft on both port and starboard sides, then apply corrections for trim (fore-aft tilt), list (sideways tilt) and water density. Using the vessel's hydrostatic data, the corrected mean draft is converted into displacement, the mass of water displaced, which by Archimedes' principle equals the ship's total mass. The cargo figure is the change in corrected displacement minus all other weight changes, such as ballast water, fuel, stores and consumables. Archimedes' principle supplies the physics, but readings and inventory remain uncertain, so the result is an inferred bulk mass rather than a direct weighing of individual cargo units.

Scope of Application

Use draft survey for documented paired-state ship measurements with hydrostatic tables, density observations, weight reconciliation, and uncertainty. Use draft survey for documented paired-state ship measurements with hydrostatic tables, density observations, weight reconciliation, and uncertainty.

• Bulk shipping. Measures loaded commodities.
• Ports. Verifies transfer quantities.
• Marine surveying. Documents vessel condition.
• Trade settlement. Supports mass reconciliation.
• Coal cargoes. Uses standardized procedure.

Clarity

A change in vessel immersion combines cargo and every other weight or buoyancy change; attribution requires a balance, not just draft subtraction. The closest near miss sets the boundary: A single-state displacement calculation is closest: it estimates total vessel displacement, whereas a cargo draft survey needs before-and-after reconciliation.

Manages Complexity

Precision depends on readable marks, calm conditions, hull data, density sampling, trim correction, and accurate ballast and consumables records. Reporting should distinguish calculation resolution from credible uncertainty. The central practical scale–indirect inference tradeoff is this: The method handles huge bulk loads without direct scales but compounds corrections. A second standard procedure–field conditions tension matters because Waves, inaccessible marks, and changing ballast challenge standardization.

Abstract Reasoning

Use three linked moves: record six draft readings before transfer; measure density and correct trim, list, and hydrostatic displacement; inventory ballast, fuel, stores, and constants. As a collapse test, the case exits when either state is missing, hydrostatic conversion is absent, or untracked ballast and density changes prevent cargo attribution. A fourth check is to repeat observations after transfer. A final check is to reconcile corrected displacement difference and report uncertainty.

Knowledge Transfer

Before-and-after mass balance transfers to other inventory measurements, but vessel buoyancy, draft marks, and hydrostatic corrections delimit a draft survey. The nearest stopping boundary is explicit: A single-state displacement calculation is closest: it estimates total vessel displacement, whereas a cargo draft survey needs before-and-after reconciliation. The inclusion test remains: A measurement is a draft survey when paired corrected draft observations are converted to displacement and reconciled for non-cargo changes to estimate transferred cargo mass. The structure no longer applies when the case exits when either state is missing, hydrostatic conversion is absent, or untracked ballast and density changes prevent cargo attribution. No canonical parent prime is currently asserted; broader structural comparisons remain related-prime analogies until separately adjudicated in the DAG. Observations and corrections estimate mass. Cargo is isolated from other weight changes.

Neighborhood in Abstraction Space

Draft survey sits in a moderately populated region (47^th percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.

Family — Domain-Specific Measurement Parameters (36 abstractions)

Nearest neighbors

• Stowage plan for container ships — 0.90
• Fuel Fraction — 0.87
• Turner angle — 0.87
• Transport problem — 0.86
• ARGUS distribution — 0.85

Computed from structural-signature embeddings · 2026-10-08