Background
The booklet is a static document approved by the flag administration on advice of the classification society at delivery and updated as necessary thereafter. The loading computer is a dynamic tool, certified to the same data, that allows the master to evaluate any loading condition in real time as cargo, fuel, and ballast plans are developed. Together they form the bridge between naval architectural design and daily operational decision-making.
This article describes the structure of a typical trim and stability booklet, the standard set of loading conditions required by the IMO Intact Stability Code, the hydrostatic and stability data presentation, the loading computer certification framework distinguishing Class A and Class B, the inputs and outputs of a typical onboard system, and how the loadicator integrates into ship management practice including trim and list management, intact stability compliance, and damage stability verification.
Purpose of the Trim and Stability Booklet
The trim and stability booklet has two primary purposes. The first is to demonstrate to the flag administration and the classification society at the time of approval that the ship as designed and built complies with the applicable stability and strength criteria across the full envelope of expected loading conditions. The second is to provide the master with the data necessary to assess the stability and strength of any actual loading condition encountered in service.
The booklet is the regulatory record. If a ship is detained by port state control on a stability concern, the inspector will check that the booklet is on board, current, and approved, and that the loading condition presented matches the actual condition. Booklets that have been superseded by approved modifications must not remain on board as the working document.
The booklet must be prepared in a language understood by the responsible ship’s officers; for vessels carrying mixed-nationality crews this is usually English. SOLAS specifies the working language of the ship; the stability documentation and the bridge passage plan are normally produced in that language.
IMO Required Loading Conditions
The 2008 Intact Stability Code (IS Code), made mandatory under SOLAS, specifies the loading conditions which must be analysed and presented in the booklet. The standard set, applicable to most cargo vessels, comprises:
The lightship condition, demonstrating the inclining experiment results and forming the baseline for all subsequent calculations. The fully loaded departure condition with full stores and bunkers and homogeneous cargo at the design density. The fully loaded arrival condition with reduced stores and bunkers reflecting voyage consumption. The ballast departure condition with full stores and bunkers and ballast distributed for sea passage. The ballast arrival condition with reduced stores and bunkers. Intermediate loading or discharge conditions where the most adverse stability state may be encountered, particularly for bulk carriers and tankers where alternate hold loading patterns are used.
For specific ship types, additional conditions are required. Tankers must show conditions with cargo at minimum density and with cargo at maximum density. Bulk carriers must demonstrate alternate hold loading per the IACS bulk carrier requirements. Container ships must demonstrate stability with the design stack heights, including the worst combination of high tier weights and slack tanks. Passenger ships must demonstrate compliance under the probabilistic damage stability framework with all required compartment damage cases.
Each condition presents the ship in plan and elevation, the cargo distribution, the consumables loading, the ballast loading, the resulting draughts, the stability calculation, and the trim and list results.
Hydrostatic Data
Hydrostatic data describes the relationship between draught and the principal volumetric and stability parameters of the hull at varying displacement and trim. The standard tabulation, computed at even keel and a series of trim conditions, gives for each draught the displacement (in salt and sometimes fresh water), the longitudinal centre of buoyancy (LCB), the longitudinal centre of flotation (LCF), the vertical centre of buoyancy (KB), the transverse metacentric radius (BM_T), the longitudinal metacentric radius (BM_L), the transverse metacentre above keel (KM_T), the moment to change trim one centimetre (MCTC), the tonnes per centimetre immersion (TPC), and the wetted surface area.
The data is presented in tabular form for ease of interpolation and graphically as the hydrostatic curves. Modern booklets supplement the tabulation with interpolation formulae or, equivalently, with the loading computer database which holds the underlying values at high resolution.
The Bonjean curves, often presented alongside or in their own section, give the cross-sectional area of the hull below any waterline at each station along the length. The Bonjeans support manual calculation of displacement and longitudinal centres at trimmed waterlines and are essential for damage stability work where the ship floats at large heel and trim. See hydrostatics and Bonjean for the underlying theory.
KN Curves
The KN curves give the righting arm at varying angles of heel for a series of constant displacement values, computed assuming the centre of gravity is at the keel (KG = 0). The actual righting arm GZ at the ship’s actual KG is then obtained by subtracting KG sin θ from the KN value at the relevant heel angle and displacement.
This indirect formulation is used because the underlying integral, which depends only on hull form, can be precomputed once at the design stage and tabulated for all displacements, while the ship’s KG varies with each loading condition. By separating the hull form contribution (KN) from the loading-dependent contribution (KG sin θ), the calculation is reduced to a simple subtraction at each heel angle.
The KN curves are the input to the GZ curve construction, which in turn is the input to the IS Code criteria checks. See metacentric height for the connection between GM and the GZ curve at small angles.
IS Code Criteria Checks
The IS Code 2008 sets the intact stability criteria that every loading condition must meet. The general criteria, applicable to all ship types unless superseded by specific criteria, are:
The area under the GZ curve up to 30 degrees heel must not be less than 0.055 metre-radians. The area under the GZ curve up to 40 degrees or the angle of downflooding, whichever is less, must not be less than 0.090 metre-radians. The area under the GZ curve between 30 and 40 degrees, or 30 degrees and the angle of downflooding, must not be less than 0.030 metre-radians. The maximum righting arm must occur at a heel angle not less than 25 degrees, ideally not less than 30 degrees. The maximum GZ must be at least 0.20 metres. The initial metacentric height GM_0, corrected for free surface effect, must not be less than 0.15 metres.
The severe wind and rolling criterion (the weather criterion) tests the ship’s ability to withstand a steady wind heeling moment combined with a roll to windward, requiring that the residual area between the wind heeling lever and the GZ curve be positive.
Specific ship type criteria add requirements. Container ships must comply with the requirements for ships carrying timber deck cargoes where applicable, and with the second generation intact stability criteria currently being phased in by IMO. Passenger ships have stricter heel-during-turn and crowding-of-passengers criteria. Offshore supply vessels have specific criteria reflecting their service profile.
The booklet presents each of these checks for every required condition, with a clear pass/fail summary table at the front of each condition.
Loading Computer (Loadicator) Certification
The loading computer is software, running on a dedicated industrial PC or in a marine-grade tablet, that holds the ship’s hydrostatic and stability data and computes the loading condition entered by the master. Certification is required because the computer is used as the primary stability assessment tool in normal operation.
Class certification is conducted by the classification society at the time of installation and following any material change to the ship’s particulars (lightship modification, addition of new compartments, change in cargo system arrangement). The certification process verifies that the computer’s output matches the booklet within defined tolerances across a representative set of test conditions, that the user interface presents the IS Code criteria results clearly, and that the software has appropriate safeguards against data corruption.
The IMO MSC.1/Circ.1229 (later updated by MSC.1/Circ.1461 and subsequent guidance) sets out the principal performance standards. IACS UR L5 governs onboard computers for stability calculations.
Class A and Class B Loading Computers
Loading computers are categorised by capability into classes. The IACS framework distinguishes three:
Class 1 computers handle intact stability calculation only. These are the simplest tools and are typical on smaller vessels and on vessels not subject to damage stability requirements.
Class 2 computers add intact stability with longitudinal strength (shear force and bending moment) calculations. They are required on ships where longitudinal strength is a critical operational consideration, including bulk carriers, tankers, and container ships.
Class 3 computers add damage stability calculation. They are required where damage stability is part of the operational regime, including passenger ships and certain tanker categories under MARPOL.
Within each class, the loading computer must be approved with the specific ship for which it is used. The hydrostatic database, the lightship data, the tank tables, and the cargo gear capacities are all specific to the hull. A type-approved generic loading computer is not sufficient; the installation must be verified against the approved trim and stability booklet.
The terms “Class A” and “Class B” are sometimes used to distinguish the primary onboard computer (A) from a backup or secondary computer (B). Where damage stability is required, the regulations typically demand redundancy with two independent installations.
Input Data: Cargo, Fuel, and Ballast
The master enters the loading condition into the loadicator as a series of compartment loadings. For cargo holds, the entry is the weight in tonnes (or volume and density for liquid cargo) and, where required, the vertical and longitudinal centre of cargo. For fuel and lubricating oil tanks, the volume and density are entered, and the system computes the free surface effect correction from the tank tables. For ballast tanks, the same treatment applies. For stores, the operator typically maintains a standard distribution and adjusts as necessary.
For container ships, the entry is by stack and tier with a per-container weight, allowing automatic computation of the deck cargo distribution. For tankers, the entry is by ullage, temperature, and density, with the system applying API or ASTM volume correction factors to convert to mass.
Modern systems integrate with cargo gauging systems on tankers, automatic draught monitoring on bulkers, and crane weighing systems on container ships, reducing manual entry and the associated transcription errors.
Output Reports
The output of a typical loading condition calculation includes the resulting draughts forward, midships, and aft (with trim and list computed); the displacement, deadweight, and lightship breakdown; the GM_0 corrected for free surface; the GZ curve with IS Code criteria pass/fail; the longitudinal strength curves of weight, buoyancy, shear force, and bending moment with permissible envelope overlay; the torsional moment curve where applicable; and the trim correction summary.
For container ships and bulkers, the longitudinal strength is the operationally critical output. The shear force and bending moment must remain within the harbour and seagoing envelopes set by class. Exceedance leads to permanent hull deformation and potentially catastrophic structural failure.
The loading condition can be saved, recalled, and modified. Standard pre-approved conditions are typically held in the system as templates. The master can run “what-if” analyses by varying ballast distribution, cargo sequence, or bunker uplift to optimise trim, draft, and stability.
Real-Time Stability Calculation
Modern loading computers, particularly on passenger ships and large container ships, are capable of real-time stability calculation. The system reads tank levels, draught sensors, and cargo monitoring inputs continuously and computes the current GM, GZ curve, and IS Code compliance margin in real time on the bridge.
For passenger vessels, real-time damage stability monitoring under SOLAS chapter II-1 is now common. The system continuously evaluates the worst-case damage scenario from the approved damage cases against the current loading condition.
The benefits of real-time monitoring are most evident during cargo operations, where rapid changes in distribution can produce transient stability deficits that a static booklet calculation would miss.
Integration with Ship Management
The loading computer is integrated with the broader ship management workflow. The output of the load planning exercise is the statement of facts attachment showing the planned departure condition. The actual departure condition is filed alongside, with discrepancies investigated. Onboard records of stability calculations form part of the SIRE 2.0 inspection record on tankers.
For voyage planning, the loadicator interfaces with the weather routing service, the bunker calculation, and the hold preparation schedule. For ships in the alternate hold loading regime, the loadicator is the central planning tool for managing the cargo sequence.
The chief officer is normally responsible for the cargo and ballast plan and for operating the loadicator; the master signs the approved condition. On modern container ships and tankers, a dedicated cargo officer assists. The classification society surveyor at periodic surveys verifies that the loadicator is calibrated, the booklet is current, and recent loading conditions are documented.
Related Wiki Articles
- Hydrostatics and Bonjean
- Trim and List
- Metacentric Height
- Free Surface Effect
- Intact Stability
- Damage Stability
- Probabilistic Damage Stability
- SOLAS Convention
- Classification Society
- Port State Control
- SIRE Tanker Inspections
See also
Calculators
- KG Limit - Intact (from GM-required)
- IS Code - Severe-Wind & Rolling Criterion
- Trim - From Loading Centroid
- IMO DCS Annual Report Calculator (MARPOL Annex VI 22A)
Related wiki articles
- GZ Curve and Righting Arm
- Marine Anti-Heeling and Heeling Control Systems
- Marine Bow Doors and Stern Ramps
References
- IMO SOLAS Convention, Chapter II-1 (Construction - Subdivision and Stability) and Chapter VI (Carriage of Cargoes)
- IMO Resolution MSC.267(85), International Code on Intact Stability 2008 (2008 IS Code)
- IMO MSC.1/Circ.1229, Guidelines for the Approval of Stability Instruments
- IMO MSC.1/Circ.1461, Guidelines for Verification of Damage Stability for Tankers
- IMO Resolution A.749(18), Code on Intact Stability for All Types of Ships (superseded by IS Code 2008)
- IACS Unified Requirement L5, Onboard Computers for Stability Calculations
- IACS Unified Requirement S1A, Longitudinal Strength of Container Ships
- IACS Common Structural Rules for Bulk Carriers and Oil Tankers
- IMO MARPOL Annex I, Damage Stability Requirements for Oil Tankers
- IMO Resolution MSC.429(98), Revised Explanatory Notes to the SOLAS Chapter II-1 Subdivision and Damage Stability Regulations