At every port there are two calculations of the ship's condition. The planner ashore builds the loading plan in planning software, while the chief officer checks the same condition in the ship's loading computer. The two should agree. When they do not, the ship's class-approved instrument prevails. This guide explains what an approved loading instrument is, why shore-side tools must be reconciled with it and how to keep the roles clear.
What a class-approved loading instrument is
A loading instrument is an onboard program that calculates loading conditions from the ship's own data: lightship weight and centre of gravity, hydrostatic tables, cross curves (KN), tank tables and strength limits. Its output covers displacement, draughts, trim, KG, corrected GM, the GZ curve against the intact stability criteria (IS Code 2008 and SOLAS II-1), and shear force and bending moment.
Key references for these instruments include:
- IACS UR L5 on onboard computers for stability calculations.
- IACS UR S1 on loading manuals and loading instruments for longitudinal strength.
- IACS Recommendation No. 48, guidance on loading instruments.
- IMO guidelines for the approval of stability instruments, plus the mandatory carriage of stability instruments for certain ship types such as tankers.
UR L5 distinguishes several types of stability calculation:
| Type | What it calculates | Typical use |
|---|---|---|
| Type 1 | Intact stability only | Ships not required to meet a damage stability criterion |
| Type 2 | Intact stability plus a damage stability check via limit curves (for example limiting KG or GM) | Many cargo ships subject to damage stability rules |
| Type 3 | Intact stability plus damage stability by direct calculation of pre-programmed damage cases | Ships needing direct damage calculation, such as certain tankers |
Which type is required depends on the ship type and its class rules. Check the certificate and instrument manual on board your vessel.
Type approval vs ship-specific approval
Two approval steps are often confused:
- Type approval (general approval). The classification society assesses the software program with test data, independent of any one ship. This shows the calculation method is sound.
- Ship-specific approval. The program loaded with a particular ship's data is tested against the test conditions from that ship's stability booklet and loading manual. The installation on board is checked too, and the test is repeated periodically, typically at the annual survey.
A type-approved program is not automatically right for your ship if its data contain a typo. What counts on board is the instrument with ship-specific approval, together with its operating manual and test conditions.
Why shore-side planning tools must be reconciled
Shore tools help planners work faster: building alternatives, simulating rotations and exchanging BAPLIE data with terminals. But their output is only useful if it agrees with the ship's loading computer. The most common sources of difference:
| Source of difference | Example | How to align |
|---|---|---|
| Lightship and constants | The ship's constant (stores, sludge, unaccounted weight) changed after drydock | Use the latest constant from the chief officer or a draft survey |
| Tank tables and soundings | Shore tank contents are estimates | Ask for the latest sounding report before the final plan |
| Free surface | Shore tool uses maximum moments, ship uses actual moments (or the reverse) | Use the same method as the booklet |
| Container weight and VCG | Booking weight instead of VGM; VCG assumed at half height | Use VGM and the same VCG rule |
| Hydrostatics and trim | Even keel table interpolation vs trim-corrected data | Use the same tables and method as the booklet |
| SF/BM reading points | Different frame positions or limit versions | Align the reading point list and seagoing/harbour limits |
Validating against stability booklet test conditions
Before a shore tool is used for a particular ship, it should be validated with loading conditions from the approved stability booklet, such as ballast and full load departure and arrival. An illustrative comparison for one condition (KM 9.30 m and a 0.20 m free surface correction in both results):
| Quantity | Stability booklet | Shore tool | Difference |
|---|---|---|---|
| Displacement | 12,480 t | 12,472 t | −0.06% |
| Mean draught | 7.85 m | 7.84 m | −0.01 m |
| Trim (by the stern) | 0.42 m | 0.45 m | +0.03 m |
| KG | 7.92 m | 7.95 m | +0.03 m |
| Corrected GM (KM − KG − FSC) | 1.18 m | 1.15 m | −0.03 m |
| Max BM, % of seagoing limit | 82% | 84% | +2 points |
Small differences like these are normal, but their direction matters. A slightly higher KG in the shore tool makes the shore result slightly conservative. Official tolerances for the onboard instrument are set by class rules; for a shore tool, define internal tolerances and investigate every difference outside them. The GM and KG relationship is covered in metacentric height (GM) explained, and you can try the basic calculation with the GM calculator.
The workflow at every port
Reconciliation is not a one-off at initial validation. A practical routine at each call:
- The planner sends the preliminary plan and BAPLIE to the ship before arrival, with a summary of shore results (draughts, trim, GM, highest SF/BM percentage).
- The chief officer imports or enters the plan into the ship's loading computer, updates actual tank contents and compares the results with the shore summary.
- If there is a meaningful difference, both sides find the cause before cargo work starts, not when the ship is about to sail.
- After cargo work, the final condition from the ship's loading computer becomes the reference, and any differences found are logged to improve the vessel data in the shore tool.
Roles: planner, chief officer and master
| Role | Main responsibility | Tool |
|---|---|---|
| Shore planner | Builds a plan that meets POD sequence, stability, strength and segregation; prepares BAPLIE | Shore-side planning software |
| Chief officer | Checks the plan, enters the actual condition, manages ballast, accepts or asks for changes | The ship's class-approved loading computer |
| Master | Final decision on the safety and seaworthiness of the ship, including refusing cargo | Loading computer results, stability booklet, professional judgement |
This division follows SOLAS and the ship's safety management system under the ISM Code. Shore software does not change who is responsible. Planners also need a working grasp of strength limits, see shear force and bending moment.
Where STOWAGE.ID stands
STOWAGE.ID Vessel Stowage is a shore-side planning and decision-support tool. STOWAGE.ID is not currently a class-approved loading instrument and does not replace the ship's loading computer. Until such approval exists, every stability, trim and strength result from STOWAGE.ID should be treated as a planning estimate to be confirmed by the ship's approved instrument and stability booklet. Its value lies in building alternatives quickly, catching problems early and exchanging clean data with ships and terminals. You can see the workflow in the demo.