DRAFT SURVEY IN SPAIN

Independent cargo quantity determination by vessel displacement at Spanish commercial ports.

VALENCIA Marine Services provides professional Draft Survey Spain services for bulk carriers, general cargo vessels and other commercial ships requiring an independent determination of cargo quantity loaded or discharged.

A draft survey calculates cargo weight from the change in vessel displacement, using observed vessel drafts, hydrostatic data, water density, ballast quantities, bunkers and other relevant weights and corrections.

The service may be carried out as an Initial Draft Survey, Final Draft Survey, or both as part of the same cargo operation.

REQUEST A DRAFT SURVEY

WHAT IS A DRAFT SURVEY?

A draft survey is a method used to determine the weight of cargo loaded onto or discharged from a vessel by comparing the vessel's displacement before and after cargo operations.

The survey normally involves:

reading vessel drafts;
determining vessel displacement;
measuring water density;
reviewing hydrostatic data;
determining ballast quantities;
considering bunker quantities;
considering freshwater and other consumables;
applying relevant corrections;
calculating the final cargo quantity.

The accuracy of the result depends on the quality of the observations, vessel data, environmental conditions and information provided.

HOW DOES A DRAFT SURVEY WORK?

The basic principle is:

Corrected Final Displacement − Corrected Initial Displacement = Cargo Quantity

In practice, the calculation requires several technical stages.

1. DRAFT READINGS

Relevant tanks may include, depending on vessel configuration:

fuel oil tanks;
marine diesel oil tanks;
marine gas oil tanks;
settling tanks;
service tanks;
storage tanks;
other fuel tanks included within the appointment.

The surveyor establishes which tanks form part of the required calculation.

2. DRAFT MARK OBSERVATION

Draft marks should be read as accurately as practicable.

Factors that may affect observation include:

vessel movement;
waves;
swell;
poor lighting;
fouling;
inaccessible draft marks;
berth configuration;
visibility;
water turbulence.

Where readings are uncertain, record this as a survey limitation.

3. TRIM AND LIST

The vessel's attitude must be considered because draft readings can be affected by:

trim by bow or stern;
port or starboard list;
vessel geometry;
position of draft marks.

Appropriate trim and list corrections are applied according to the vessel's hydrostatic information.

4. WATER DENSITY

Water density directly affects displacement calculations.

Where practicable, the surveyor should obtain representative water samples from around the vessel and determine the observed density.

Relevant factors can include:

seawater;
brackish water;
freshwater;
local salinity;
temperature;
river influence;
rainfall;
berth location.

Incorrect water density can materially affect the calculated cargo quantity.

5. HYDROSTATIC DATA

The vessel's hydrostatic information is required to determine displacement from the observed drafts.

Relevant data may include:

hydrostatic tables;
displacement tables;
tonnes per centimetre immersion;
longitudinal centre of flotation;
mean draft data;
trim correction information.

The calculation depends on the accuracy and applicability of the vessel documentation provided.

6. BALLAST QUANTITIES

Ballast is one of the principal non-cargo weights affecting vessel displacement.

Depending on the vessel and available arrangements, ballast quantities may be determined from:

soundings;
ullages;
tank calibration tables;
remote gauging systems;
vessel records.

Where necessary, ballast density may also need to be considered.

7. BUNKER QUANTITIES

Fuel quantities on board form part of the vessel's deadweight and must be considered in the draft survey calculation where relevant.

Bunker quantities may be derived from:

tank measurements;
vessel records;
bunker survey data;
other agreed information.

8. FRESHWATER AND OTHER CONSUMABLES

Other weights that may require consideration include:

freshwater;
lubricating oil;
slops;
stores;
provisions;
crew effects;
operational consumables;
other liquids or weights that change during cargo operations.

The significance of each item depends on the vessel and operation.

VESSEL CONSTANT

The vessel constant represents the difference between the vessel's calculated condition and the known or expected lightship-related condition after accounting for measurable onboard weights.

Where relevant, the constant may be derived or considered according to:

vessel records;
previous draft surveys;
agreed operational data;
current calculation conditions.

An unreliable constant can affect cargo quantity results.

DRAFT SURVEYS

INITIAL DRAFT SURVEY

The Initial Draft Survey is normally performed before cargo loading or discharge begins.

It establishes the vessel's initial displacement and onboard weight condition.

The survey may include:

six draft readings;
water density;
ballast quantities;
bunker quantities;
freshwater;
other relevant weights;
vessel constant;
initial displacement calculation.

This provides the baseline for the final cargo calculation.

FINAL DRAFT SURVEY

The Final Draft Survey is carried out after cargo loading or discharge has been completed.

The same general procedure is repeated to determine the final corrected displacement.

The difference between corrected initial and final vessel condition is then used to calculate the cargo quantity.

BUNKER SURVEY PROCEDURE

1. APPOINTMENT

The client provides:

vessel name;
IMO number;
vessel type;
port;
ETA;
requested survey date;
survey purpose.

2. VESSEL ATTENDANCE

The marine surveyor attends the vessel at the agreed location.

3. DOCUMENT REVIEW

The marine surveyor reviews relevant bunker and tank documentation, where available.

4. TANK IDENTIFICATION

The surveyor identifies the tanks included in the survey.

5. SOUNDING OR ULLAGE MEASUREMENTS

Actual observed tank levels are recorded.

6. TEMPERATURE RECORDING

Obtain relevant bunker temperatures where required.

7. VESSEL CONDITION

Consider trim and list where applicable.

8. QUANTITY CALCULATION

Convert measured levels to volumes using the applicable tank calibration data and corrections.

9. ROB DETERMINATION

The calculated quantities are consolidated into the final Remaining On Board result.

10. REPORTING

A bunker survey report is prepared where included in the appointment.

CARGO LOADED/DISCHARGED BY DRAFT SURVEY

CARGO LOADED BY DRAFT SURVEY

For a loading operation:

Final Corrected Displacement − Initial Corrected Displacement = Cargo Loaded

The calculation must account for any changes in:

ballast;
bunkers;
freshwater;
consumables;
other vessel weights.

CARGO DISCHARGED BY DRAFT SURVEY

For a discharge operation:

Initial Corrected Displacement − Final Corrected Displacement = Cargo Discharged

Again, consider changes in non-cargo weights.

DRAFT SURVEY CALCULATION PROCESS

A professional calculation may include:

STEP 1 — OBSERVE DRAFTS

Record six draft readings.

STEP 2 — DETERMINE MEAN DRAFT

Calculate corrected mean draft according to vessel trim and geometry.

STEP 3 — OBTAIN HYDROSTATIC DISPLACEMENT

Use vessel hydrostatic data.

STEP 4 — APPLY TRIM CORRECTIONS

Apply first and second trim corrections where applicable.

STEP 5 — APPLY WATER DENSITY CORRECTION

Correct displacement from standard density to observed dock-water density.

STEP 6 — DETERMINE DEDUCTIBLES

Calculate ballast, bunkers, freshwater and other relevant onboard weights.

STEP 7 — DETERMINE NET DISPLACEMENT

Establish the corrected vessel condition.

STEP 8 — COMPARE INITIAL AND FINAL RESULTS

Calculate the final cargo quantity.

DRAFT SURVEY DATA REQUIRED

Before attendance, it is useful to have access to:

vessel hydrostatic tables;
General Arrangement;
tank calibration tables;
ballast tank plans;
bunker tank information;
lightship data;
vessel constant information;
previous draft survey data where relevant;
cargo operation details.

The survey result depends partly on the quality and reliability of the vessel data provided.

DRAFT SURVEY REPORT

A professional report may include:

vessel name;
IMO number;
vessel type;
port;
berth;
survey date;
appointing party;
cargo operation;
cargo description where provided;
initial draft readings;
final draft readings;
mean drafts;
trim;
list;
water density;
hydrostatic data used;
ballast quantities;
bunker quantities;
freshwater;
vessel constant;
corrections applied;
initial displacement;
final displacement;
calculated cargo quantity;
observations;
survey limitations.

The objective is to provide a transparent and technically traceable calculation.

SOURCES OF DRAFT SURVEY ERROR

A draft survey is an indirect cargo quantity measurement and several factors can affect accuracy.

Potential sources of error include:

inaccurate draft readings;
heavy swell;
vessel movement;
poor draft-mark visibility;
inaccurate water density;
incorrect ballast quantities;
inaccurate tank tables;
unaccounted liquid transfers;
inaccurate bunker quantities;
unreliable vessel constant;
incorrect hydrostatic data;
trim correction errors;
list;
timing differences between measurements.

A professional survey should identify significant conditions that may affect calculation reliability.

DRAFT SURVEY LIMITATIONS

Differences in calculated bunker quantities can arise from several technical factors, including:

inaccurate tank calibration data;
vessel trim;
vessel list;
temperature variation;
density information;
sounding measurement;
inaccessible tanks;
tank geometry;
measurement timing;
fuel movement between tanks.

Where relevant discrepancies are observed, document them rather than conceal them by adjusting results without technical justification.

DRAFT SURVEY VS SHORE SCALE

A shore scale and a draft survey use different measurement principles.

DRAFT SURVEY

Calculates cargo quantity from vessel displacement.

SHORE SCALE

Measures cargo through shore-based weighing equipment.

Differences between the two methods may occur because they rely on different measurement systems and operational data.

A draft survey should therefore be treated as an independent vessel-based cargo quantity calculation.

DRAFT SURVEY VS BUNKER SURVEY

The two services use tank and vessel measurements but have different objectives.

DRAFT SURVEY

Determines cargo quantity.

BUNKER SURVEY

Determines fuel quantity.

DRAFT SURVEY VS CARGO HOLD INSPECTION

These services may both be required during the same cargo operation but are technically distinct.

DRAFT SURVEY

Determines the weight of cargo loaded or discharged.

CARGO HOLD INSPECTION

Assesses the visible condition and readiness of the vessel's cargo spaces.

WHEN IS A DRAFT SURVEY REQUIRED?

Draft surveys are commonly requested for:

bulk cargo loading;
bulk cargo discharge;
commercial quantity verification;
shipper interests;
receiver interests;
charterer requirements;
shipowner requirements;
cargo disputes;
independent cargo quantity determination.

They are particularly common for commodities transported in bulk where vessel displacement provides a practical quantity-control method.

COMMERCIAL VESSELS

Draft surveys may be carried out on suitable commercial vessels including:

bulk carriers;
general cargo vessels;
multipurpose vessels;
other ships carrying bulk cargo where reliable draft and hydrostatic data are available.

DRAFT SURVEY AT SPANISH PORTS

VALENCIA Marine Services provides draft survey attendance at major Spanish commercial ports.

Regular coverage includes:

Valencia • Sagunto • Castellón • Alicante • Cartagena • Tarragona • Barcelona • Algeciras

Attendance at other Spanish ports can also be considered subject to vessel location, cargo operation and surveyor availability.

WHY APPOINT VALENCIA MARINE SERVICES

INDEPENDENT CARGO QUANTITY DETERMINATION

Measurements and calculations are carried out independently according to the agreed scope.

ENGINEERING APPROACH

Draft survey calculations are approached from a Naval Architecture and Marine Engineering perspective.

TRACEABLE CALCULATIONS

The survey report records the principal inputs, corrections and calculated results.

COMMERCIAL VESSEL EXPERIENCE

The service is designed for merchant ships and bulk cargo operations.

CLEAR TECHNICAL REPORTING

Relevant measurements, data, calculations and limitations are presented in a structured format.

EXPERIENCED MARINE SURVEYOR

Carlos Montamarta is a Naval Architect & Marine Engineer with more than 20 years of professional experience in marine surveying and ship inspection.

His marine survey assignments combine practical vessel inspection with technical expertise in naval architecture, hydrostatics, vessel displacement, marine engineering and technical reporting.

Professional marine surveying services are provided through VALENCIA Marine Services SLU.

INDEPENDENT • DISCREET • PROFESSIONAL

FREQUENTLY ASKED QUESTIONS

WHAT IS A DRAFT SURVEY?

A draft survey determines the quantity of cargo loaded or discharged by calculating changes in the vessel's displacement before and after cargo operations.

HOW ACCURATE IS A DRAFT SURVEY?

Accuracy depends on factors such as draft readings, water density, vessel hydrostatic data, ballast quantities, trim, list and other onboard weights.

HOW MANY DRAFT READINGS ARE TAKEN?

Where practicable, six readings are taken: forward, midship and aft on both port and starboard sides.

ARE BUNKERS INCLUDED IN THE CALCULATION?

Relevant bunker quantities are considered as part of the vessel's non-cargo weights.

WHY IS WATER DENSITY IMPORTANT?

Vessel displacement changes with water density, so the observed dock-water density must be considered in the calculation.

ARE BALLAST TANKS MEASURED?

Ballast quantities must normally be determined because changes in ballast directly affect vessel displacemen

WHAT IS A VESSEL CONSTANT?

The vessel constant represents an onboard weight difference used in the calculation and may be derived from vessel records or survey data.

DO YOU PERFORM BOTH INITIAL AND FINAL DRAFT SURVEYS?

Yes. For a complete cargo quantity determination, an Initial Draft Survey and Final Draft Survey are normally required.

CAN A DRAFT SURVEY BE USED FOR DISCHARGE OPERATIONS?

Yes. Draft surveying can determine cargo discharged as well as cargo loaded.

DO YOU PROVIDE A WRITTEN REPORT?

Yes, where included in the appointment, a professional report can document the survey data, calculations, final cargo quantity and relevant limitations.