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. Water density, trim, list, ballast, fuel, stores, and consumables therefore matter; Archimedes' principle supplies the physical basis but does not eliminate inventory and observation uncertainty. The result is an inferred bulk mass, not a direct weighing of individual cargo units.

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.

Structural Signature¶

Sig role-phrases:

paired vessel states. Provide before and after conditions surrounding loading or discharge. Constitutive comparison. If altered: One isolated draft cannot determine transferred cargo mass.
six-point draft readings. Sample forward, midships, and aft on both sides. Constitutive observation set. If altered: A single mark cannot characterize trim and list adequately.
hydrostatic displacement relation. Maps corrected draft to displaced water mass under Archimedes' principle. Identity-bearing model. If altered: Draft difference is not itself cargo weight.
environmental and trim corrections. Adjust for water density, trim, list, and vessel hydrostatics. Constitutive accuracy step. If altered: Ignoring density biases displacement.
non-cargo inventory correction. Accounts for ballast, fuel, stores, and other weight changes. Necessary attribution boundary. If altered: Untracked ballast can be mistaken for cargo.

What It Is Not¶

Draft reading. Were paired states and reconciliation performed?
Displacement. Is cargo change rather than total vessel mass sought?
Cargo tally. Is count replacing buoyancy inference?
Ullage survey. Is tank volume rather than vessel displacement measured?

Scope of Application¶

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.

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.

Abstract Reasoning¶

Record six draft readings before transfer.
Measure density and correct trim, list, and hydrostatic displacement.
Inventory ballast, fuel, stores, and constants.
Repeat observations after transfer.
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.

Examples¶

Canonical¶

A bulk carrier is surveyed before and after coal loading. Six draft readings, dock-water density, trim, ballast, fuel, and hydrostatic tables yield the corrected cargo mass.

Mapped back: paired vessel states → before and after loading; six-point draft readings → port/starboard at three stations; hydrostatic displacement relation → ship tables; environmental and trim corrections → density and trim; non-cargo inventory correction → ballast and fuel.

Applied / In Practice¶

The stern draft increases after transfer, but ballast was also taken aboard and no forward or midship readings exist; the observation cannot support a complete cargo draft survey.

Mapped back: paired vessel states → partial; six-point draft readings → missing; hydrostatic displacement relation → not completed; environmental and trim corrections → absent; non-cargo inventory correction → ballast unresolved.

Structural Tensions¶

T1: practical scale vs. indirect inference. The method handles huge bulk loads without direct scales but compounds corrections. Diagnostic: Which uncertainty dominates?

T2: standard procedure vs. field conditions. Waves, inaccessible marks, and changing ballast challenge standardization. Diagnostic: Were deviations documented?

Structural–Framed Character¶

Description turns on paired vessel states, six-point draft readings, hydrostatic displacement relation, environmental and trim corrections, non-cargo inventory correction. Skeletal core. An unweighed transfer is recovered from a before-and-after system balance with nuisance changes removed. Domain-bound accent. Draft marks, trim, water density, hydrostatic tables, ballast, and ship displacement define the method. Transfer remains bounded because Why not prime. Difference-based mass balance is portable; this is a marine cargo survey. The negative boundary is concrete: Any visual draft reading, displacement table lookup, scale weighing, cargo tally, ullage survey, stability calculation, or uncorrected change in waterline is not automatically a draft survey. Draft survey is measurement-operational: buoyancy theory structures a field reconciliation of two vessel states. Its character: cargo mass inferred from corrected displacement change.

Structural Core vs. Domain Accent¶

Skeletal core. An unweighed transfer is recovered from a before-and-after system balance with nuisance changes removed.

Domain-bound accent. Draft marks, trim, water density, hydrostatic tables, ballast, and ship displacement define the method.

Why not prime. Difference-based mass balance is portable; this is a marine cargo survey.

Measurement. Observations and corrections estimate mass.
Mass balance. Cargo is isolated from other weight changes.
No strict parent is asserted.

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

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

Not to Be Confused With¶

Draft reading. Tell: Were paired states and reconciliation performed?
Displacement. Tell: Is cargo change rather than total vessel mass sought?
Cargo tally. Tell: Is count replacing buoyancy inference?
Ullage survey. Tell: Is tank volume rather than vessel displacement measured?

References¶

Frozen Wikipedia discovery revision: https://en.wikipedia.org/wiki/Draft_survey (revision 1185775075).
Preserved source candidate: http://www.unece.org/energy/se/coal/code.html
Preserved source candidate: http://draft.avocetdata.net/
Preserved source candidate: https://web.archive.org/web/20151115233512/http://draft.avocetdata.net/
Preserved source candidate: http://www.northpublications.com/lp_guides/Draft_Surveys/index.html#/23/zoomed
Preserved source candidate: https://web.archive.org/web/20140303191502/http://www.northpublications.com/lp_guides/Draft_Surveys/index.html#/23/zoomed

The frozen Wikipedia revision is discovery provenance. The retained source set was reviewed for identity, formal or operational relation, and scope. The encyclopedia's structural synthesis is bounded to those claims; a thin authority surface is recorded as a nonblocking source-strengthening repair rather than concealed.