Marine Stability Booklet and Loading Computer

The trim and stability booklet, almost always paired with an onboard loading computer (loadicator), is the master’s working instrument for verifying that the ship will float upright, within approved intact and damage stability margins, and within hull-girder strength limits in every loading condition encountered during a voyage. It is required equipment under SOLAS Chapter II-1 and is produced, approved, and updated under the rules of every recognised classification society. The GZ curve and righting arm criteria that the booklet must satisfy, the hydrostatic geometry from which they derive, and the cross-curves that feed every loading-condition check are explained in their own articles; this article covers the booklet and the loading computer as regulatory instruments, their mandatory contents, the IACS UR L5 approval framework, and the master’s legal obligations that flow from them.

For the underlying stability criteria, including the weather criterion residual-area test and the IS Code area-under-curve thresholds, see intact stability. For the probabilistic damage framework that some ships must also satisfy through their loading computer, see probabilistic damage stability.

Regulatory basis: SOLAS Chapter II-1 Regulations 5 and 5-1

SOLAS Chapter II-1 Regulation 5, titled “Information on ship’s stability,” requires every passenger ship and every cargo ship of 24 m in length and upwards to be provided with an approved stability information booklet. The requirement has been part of SOLAS since the 1974 Convention and has been progressively tightened through amendments adopted by the MSC. The phrase “provided with” is regulatory shorthand for a two-stage process: the booklet is prepared by or for the shipbuilder or owner, submitted to the flag administration (or its delegated recognised organisation, in practice the classification society), reviewed against the criteria of the applicable codes, approved if it complies, and then placed on board before the ship enters service.

Regulation 5 specifies that the booklet must contain the information necessary for the master to obtain guidance on the stability of the ship in a variety of representative service conditions. The 2008 Intact Stability Code (IS Code), adopted by Resolution MSC.267(85) and made mandatory through concurrent SOLAS and ICLL amendments with entry into force on 1 July 2010, defines “a variety of representative service conditions” as a specific set of loading conditions covering the full operational envelope.

SOLAS Chapter II-1 Regulation 5-1, inserted by amendment MSC.269(85) and entering force on 1 July 2010, addresses the loading instrument specifically for bulk carriers and oil tankers of 150 m or more. Regulation 5-1 paragraph 1 requires these ships to carry an approved loading instrument capable of providing rapidly and reliably information on the hull-girder shear forces and bending moments. Paragraph 2 extends the requirement to any ship where the master needs a loading instrument to verify compliance with the applicable strength criteria, and paragraph 3 sets out the verification standard: the instrument’s output must be verified against the approved loading manual using the test loading conditions defined in it.

The loading instrument under Regulation 5-1 is therefore not a stability computer in the narrow sense; it is a hull-strength computer with stability capability layered on top. On bulk carriers the shear force and bending moment limits, maintained within the permissible envelopes at all times, are the operationally critical outputs and the primary reason the instrument is mandatory.

The 2008 IS Code: Part A mandatory contents and required loading conditions

Part A of the 2008 IS Code, which is mandatory under SOLAS II-1/2, sets out in Chapter 2 the information that every stability booklet must contain. The minimum mandatory set includes:

The ship’s general particulars: name, IMO number, flag, length between perpendiculars, breadth moulded, depth moulded, and maximum draught.

The lightship particulars determined by an inclining experiment or, for near-sister vessels, by a lightweight survey or displacement check accepted by the flag administration: lightship displacement, KG (centre of gravity above keel), LCG (longitudinal centre of gravity forward of aft perpendicular), and the transverse KG with free-surface corrections from any permanent liquid ballast.

Hydrostatic tables and curves giving, for each draught and for the range of trims likely in service, the displacement in salt water and fresh water, the tonnes per centimetre immersion (TPC), the moment to change trim one centimetre (MCTC), the centre of buoyancy longitudinal (LCB) and vertical (KB), the metacentric radius transverse (BM_T), the transverse metacentric height above keel (KM_T), the centre of flotation (LCF), the wetted surface area, and the hull-girder section modulus at the critical sections.

Cross-curves of stability (KN curves) giving the righting lever from keel to the point of application of the buoyancy at each heel angle from 0 to at least 90 degrees, for a range of displacements covering lightship to maximum load. The KN curves are the raw input to every loading-condition GZ calculation. See cross-curves of stability and KN tables for the underlying computation method.

Tank tables or capacity plans for every tank and void space, giving volume (in cubic metres), the longitudinal, transverse, and vertical centres of volume, and the transverse free-surface moment (first moment of the waterplane area) at every sounding interval. The free-surface correction applied to each loading condition depends on these tables.

The required loading conditions, each computed in full and presented with a GZ curve, the IS Code criteria check results in tabular form, the draught and trim, the displacement, and the longitudinal-strength curves where applicable.

Required loading conditions under the IS Code

The IS Code Chapter 2.2 specifies the minimum set of conditions to be included. For a general dry-cargo vessel the set comprises: departure with full cargo, full bunkers, and full stores; arrival with full cargo, 10% bunkers, and 10% stores; departure with no cargo, full ballast, full bunkers, and full stores; arrival with no cargo, full ballast, 10% bunkers, and 10% stores; and where applicable, the worst intermediate condition identified during the loading or discharging sequence.

Bulk carriers must additionally include: alternate hold loading departure and arrival conditions (full cargo in alternate holds, empty intermediate holds); the homogeneous cargo departure and arrival; and, where the design allows, ballast hold flooding conditions. SOLAS Chapter XII, which applies to bulk carriers of 150 m or more, requires that the loading conditions demonstrated in the booklet include the specific load-distribution limits at which the hull-girder strength criteria are satisfied. SOLAS XII/8 sets the maximum and minimum loading limits per hold for each ship in a separate section of the stability booklet or in an associated loading manual.

Container ships must include: maximum homogeneous loading departure and arrival; the heaviest practical loading with maximum allowable stack weights; a lightly loaded departure condition with high-tier containers; and an empty-ship condition. The IACS UR S11 for container ships sets specific hog/sag bending moment limits that the loadicator must enforce.

Passenger ships must include: all-classes-full departure and arrival; a service condition with maximum crew and embarked persons but reduced consumables; and, in the booklet’s damage stability annex, the required subset of two-compartment damage cases that bound the intact loading envelope under the probabilistic damage stability framework (SOLAS II-1/6 as amended by MSC.216(82)).

Tankers must include: full-cargo departure with homogeneous cargo at design density and at maximum density; full-cargo arrival; ballast departure and arrival; and an intermediate passage condition where the worst combination of slack tanks and heel from free surfaces may occur. The MARPOL Annex I damage stability requirements, applicable to oil tankers under Regulation 28 read with Regulation 1, require that the loading conditions within the booklet demonstrate compliance with the residual stability criteria after the worst assumed damage to the cargo tanks, void spaces, and pump rooms lying within 0.2L from the bow.

Hydrostatic data presentation

The hydrostatic table is computed at even keel and at the range of trims that arise in service. Every major stability program computes the table at the design trims specified by the naval architect, typically in 0.5 m trim increments from maximum forward to maximum aft trim. The key quantities, and what practitioners use them for, are:

Displacement (salt water, 1.025 t/m³) at each draught gives the total mass of the loaded vessel. At other water densities the displacement is adjusted via the TPC-based dock water allowance. Most booklets also tabulate displacement in fresh water (1.000 t/m³) and brackish water (1.010 to 1.020 t/m³) for river and estuarine trades.

Tonnes per centimetre immersion (TPC) gives the change in displacement per centimetre change in mean draught. For a typical Panamax bulk carrier at summer load draught, TPC runs between 68 and 72 t/cm. The quantity is used for cargo weight verification from draught surveys.

Moment to change trim one centimetre (MCTC) is the trimming moment in tonne-metres per centimetre change in trim, computed as $\text{MCTC} = \frac{GML \times \Delta}{100 \times L_{PP}}$ where $GML$ is the longitudinal metacentric height, $\Delta$ is displacement, and $L_{PP}$ is length between perpendiculars. MCTC drives every trim-and-draught estimate in cargo planning. See trim and list and the companion trim from loading centroid calculator.

Vertical centre of buoyancy (KB) and transverse metacentric radius (BM_T) together give the transverse metacentre above keel: $KM_T = KB + BM_T$. From the loading condition’s actual KG (corrected for free surfaces) the initial transverse metacentric height is $GM_0 = KM_T - KG_{corr}$. The IS Code minimum of $GM_0 \geq 0.15$ m is checked against this value.

Centre of flotation (LCF) is the pivot point for changes in trim. A weight added at the LCF changes displacement without changing trim; a weight forward or aft of the LCF changes trim in proportion to its moment about LCF divided by MCTC.

Bonjean curves (cross-sectional area below the waterline at each station, tabulated against draught and trim) are the basis for computing displacement and centres at large heel and trim. They appear in most booklets as a separate figure or table and are indispensable for damage stability calculations where the ship floats at an arbitrary waterline. See hydrostatics and Bonjean curves.

KN curves and the GZ calculation

The KN curves are precomputed at the design stage by integrating the immersed hull volume at each heel angle for a range of assumed displacements, with the centre of gravity placed at the keel ($KG = 0$). At any given displacement $\Delta$ and heel $\theta$, the KN value is the horizontal distance from keel to the point where the vertical through the centre of buoyancy crosses the vessel’s centreline at zero heel, measured in the heeled position.

The actual righting arm for a loading condition with vertical centre of gravity $KG$ above keel is:

This separation of the hull-form integral (KN, computed once) from the loading-dependent term ($KG \sin\theta$, computed at each condition) is the practical reason cross-curves exist. The naval architect runs the hull-form integration once at design; every subsequent loading condition check is arithmetic.

The full GZ curve from 0 to 90 degrees (or to the angle of downflooding if that occurs earlier) is then constructed point by point at the condition’s displacement, using the KN table by interpolation, and the IS Code criteria are checked against the curve. The GZ curve and righting arm article explains the curve’s shape, the criteria thresholds, and what drives the position of the maximum righting arm. The cross-curves of stability and KN tables article covers the numerical integration method in detail.

IS Code general criteria: what the booklet must demonstrate

The 2008 IS Code Part A Chapter 2 sets the following general criteria, which every loading condition in the booklet must satisfy unless a more specific set of criteria applies to the ship type:

Area under the GZ curve to 30 degrees not less than 0.055 metre-radians. Area to 40 degrees (or to the angle of downflooding $\theta_f$ if $\theta_f < 40°$) not less than 0.090 metre-radians. Area between 30 and 40 degrees (or to $\theta_f$) not less than 0.030 metre-radians. Maximum GZ not less than 0.200 m, occurring at a heel angle not less than 25 degrees. Initial transverse metacentric height $GM_0$ corrected for free-surface effect not less than 0.150 m.

The severe wind and rolling criterion (weather criterion, IS Code 2.3) applies alongside the area criteria. It tests the ship under a steady wind heeling lever $l_w1$ combined with a wave-induced roll to windward. The criterion requires that the residual area $b$ between the wind heeling lever and the GZ curve (area A2, from the leeward static heel angle to the lesser of the angle of downflooding or 50 degrees) is not less than the area $a$ (area A1, from the equilibrium heel angle to the angle of maximum roll to windward). The IS Code severe wind and rolling criterion calculator implements this balance. The weather criterion is sensitive to the wind-moment lever arm, which in turn depends on the ship’s sail area, the lateral centre of the above-water profile, and the draught-dependent lever arm. Errors in the sail-area calculation were the source of several cases where vessels passed the booklet check but failed under vetting inspection after the rigging or deck structure was changed.

The booklet must explicitly state whether each condition passes or fails each criterion. A table at the front of each condition listing criterion, required value, actual value, and pass/fail status is standard practice and is required by most class rules.

The International Grain Code: stability specific to bulk grain

The International Code for the Safe Carriage of Grain in Bulk (the Grain Code), adopted by Resolution MSC.23(59) and made mandatory under SOLAS Chapter VI Part C, contains its own stability criteria for ships loading bulk grain. The Grain Code applies whenever a ship loads bulk grain, regardless of ship type; a tanker carrying a single parcel of bagged grain in a hold does not attract the Code, but a tanker loading grain in bulk does.

The Grain Code criteria differ from the general IS Code criteria in one key respect: they address the risk of grain shifting to leeward in the hold by computing an assumed heeling moment from a defined grain shift scenario. The criteria require:

1. The angle of heel due to the assumed grain shift must not exceed 12 degrees.
2. In the residual stability diagram accounting for the grain heeling moment, the residual area between the grain heeling moment curve and the GZ curve (from the angle of static heel to the lesser of 40 degrees or the angle of downflooding) must not be less than 0.075 metre-radians.
3. The initial metacentric height $GM_0$, corrected for free-surface effects of liquid in tanks, must not be less than 0.300 m (compared with the general 0.150 m minimum).

The booklet for a ship approved to carry bulk grain must include a grain stability document or grain loading booklet approved by the flag administration showing that these criteria are met at each approved grain loading condition. Where the booklet conditions do not cover the intended grain cargo density or distribution, the master must not load.

The stricter $GM_0$ floor of 0.300 m for grain is a frequent source of confusion on vessels transitioning between grain and non-grain trades. A condition that passes the general IS Code criteria with $GM_0 = 0.220$ m would fail the Grain Code. The loading computer must therefore flag which criteria set is active for any given cargo entry.

The onboard loading computer: function and certification requirement

The loading computer (loadicator, loading instrument, stability computer) is software, running on a dedicated industrial PC or marine-grade ruggedised terminal, that holds the ship’s hydrostatic database and computes the results of any loading condition entered by the master. Its core functions are:

Intact stability: GZ curve at any displacement and KG, IS Code criteria check, and $GM_0$ display with free-surface corrections from tank tables.

Longitudinal strength: weight, buoyancy, shear force, and bending moment distributions along the ship’s length, compared against the permissible envelopes for harbour and seagoing conditions and at intermediate cargo stages. This output is the primary operational constraint on bulk carriers and container ships.

Draught and trim: forward, aft, and mean draughts, trim, list, and displacement at any loading arrangement. Useful for optimising trim for fuel efficiency (see trim from loading centroid calculator) and for verifying load line compliance.

Stability instrument approval is required because the master relies on the computer’s output rather than manually working from the booklet. If the computer contains an error in its hydrostatic database or a bug in its calculation logic, the master has no practical means of detecting the discrepancy without an independent calculation from first principles. The certification requirement exists to close that verification gap before the ship enters service.

IACS Unified Requirement L5: type approval and verification categories

IACS Unified Requirement L5 (Rev.4, 2019) is the governing framework for loading computers across all IACS member societies. It defines approval categories, verification requirements, and the periodic confirmation schedule. All major flag state administrations accept IACS UR L5 as satisfying their national loading-instrument requirements.

Approval categories under UR L5

UR L5 divides loading computers into two main categories based on function:

A further distinction applies to damage-capable instruments. Ships required to carry a damage stability assessment tool (passenger ships under SOLAS II-1/6, certain tankers under MARPOL Annex I/28) may have either a pre-computed damage case database (the instrument retrieves the worst applicable scenario) or a real-time damage calculation engine. The pre-computed database approach is the more common; real-time damage computation to full Resolution A.265(VIII) / MSC.281(85) accuracy at sea remains a specialised capability.

Type approval means the class society has reviewed and approved the software logic independent of any particular ship installation. Shipboard approval (also called ship-specific approval or installation approval) means the approved software has been loaded with the specific ship’s hydrostatic database, tank tables, and lightship data, and the output has been verified against the approved booklet values.

Both type approval and shipboard approval are required. A type-approved generic loading computer supplied without verification against the ship’s own booklet data does not satisfy UR L5. This requirement is frequently misunderstood by owners who purchase a new loading computer from a vendor with IMO/class type approval, install it, and assume it is certified. The installation verification step is separate and must be conducted before the instrument is used operationally.

Verification procedure and test loading conditions

UR L5 requires that the loading computer output be verified by comparison with a set of test loading conditions computed independently from the booklet data. The test conditions are typically five to ten in number, spanning the range of draughts and trims in the approved conditions, and include at least two conditions with significant free-surface corrections. For a Category 2 instrument, the shear force and bending moment at the midship section and at the sections of maximum shear force must also be within tolerance.

The tolerance limits set by UR L5 are: 1% of displacement for mass quantities; 0.5% of ship length for longitudinal distances; 0.01 m for stability measures; 2% of the maximum permissible bending moment for bending moment values; 2% of the maximum permissible shear force for shear force values.

Verification must be repeated at each annual survey. If a change is made to the ship’s lightship data (following an inclining experiment, a significant lightship modification, or a deadweight survey), to the tank tables (following a tank cleaning, structural repair, or change to the tank configuration), or to the approved loading conditions, the loading computer must be re-verified before it is returned to operational use.

The record of verification, including the test condition inputs, the computed and booklet values, and the surveyor’s signature, must be maintained on board. Port state control inspectors have detained vessels where the verification record was absent or out of date, treating this as a deficiency in the stability information required under SOLAS II-1/5.

IACS UR S1 and S1A: the loading manual for bulk carriers and container ships

Alongside the loading computer, bulk carriers of 65 m or more and container ships of 90 m or more are required to have an approved loading manual under IACS UR S1 and S1A respectively (and their equivalent national requirements under flag administrations that have implemented these URs through class rules).

UR S1 for bulk carriers defines the loading manual as a document, separate from or integrated into the trim and stability booklet, that contains: the maximum and minimum cargo masses per hold at each cargo sequence stage; the maximum draught amidships and at each loading stage; the maximum and minimum ballast tank filling levels; and the permissible combinations of hold loading for alternate and block stowage patterns. For each departure and intermediate condition, the manual gives the shear force and bending moment at the cross-sections where the class-rule permissible values are specified.

UR S1A for container ships adds longitudinal torsional moment calculations for open-top container ships, maximum stack weights per bay, and the metacentric height limits for each departure and ballast condition.

The loading computer on a bulk carrier must enforce the UR S1 limits. If the operator enters a cargo sequence that would take the shear force above the permissible harbour value at the fore hatch coaming, the computer must flag the violation before the condition is accepted. On vessels carrying highly dense bulk cargoes (iron ore, pig iron, bauxite), the per-hold density limit and the alternate-hold weight restriction are the binding operational constraints, not the stability margin.

Longitudinal strength output: shear force and bending moment

The longitudinal strength calculation is the second major output of a Category 2 loading computer. The hull girder of a ship at sea experiences a continuous bending and shearing load driven by the relative distribution of weight and buoyancy along the length. Where the buoyancy exceeds the weight (typically amidships on a full loaded ship) the hull bends concave-downward (hogging). Where the weight exceeds buoyancy (towards the ends) the bending sense reverses. Wave action superimposes a dynamic increment of the same order as the stillwater component.

The permissible limits in the loading computer are specified separately for harbour conditions (the lighter set, since wave-induced dynamic loads are absent) and seagoing conditions (the tighter set, since the total load is stillwater plus dynamic). The harbour limits are relevant during cargo loading and discharging sequences, particularly the intermediate stages where a bulk carrier may pass through an extreme shear condition as a hold is being filled or emptied. The loading computer must check each intermediate stage explicitly against the harbour envelope, not only the planned final condition.

The shear force is most critical at the longitudinal bulkheads separating holds, because the shear jumps discontinuously at a bulkhead when cargo or ballast is distributed differently on each side. On vessels with high-density cargo in alternate holds, the shear at the odd/even hold boundaries can reach 90 to 95% of the permissible value at the optimum loading sequence, leaving little margin for the master to improvise.

Bending moment is most critical near midship. On a container ship, the maximum sagging moment typically occurs in a condition with light midships stacks and heavy forward or aft stacks. The loading computer highlights the section where the envelope is tightest and displays the ratio of actual to permissible as a percentage.

Damage stability instruments and MARPOL Annex I context

Passenger ships under SOLAS II-1/6 and certain tankers under MARPOL Annex I Regulation 28 must satisfy damage stability criteria derived from the probabilistic framework of SOLAS II-1 as amended and Resolution A.265(VIII)/MSC.281(85). The instruments for checking damage stability compliance fall into two categories:

Pre-computed damage case library: the naval architect computes all required damage cases at design and stores the results in the booklet and in a searchable database on the loading computer. At sea, the master selects the applicable damage case (or the computer identifies the worst case for the current loading condition) and the instrument confirms whether the residual stability criteria of SOLAS II-1 Regulation 8 are met. This approach is appropriate for ships with a large but defined set of damage cases (passenger ships with their fixed subdivision geometry).

Real-time damage computation: the instrument computes the damaged waterplane and righting-lever curve from first principles for any assumed damage extent, using the vessel’s current loading condition as the baseline. This capability is more complex and is found on the most capable passenger ship systems and on FPSO units and offshore vessels where the damage geometry is variable.

For oil tankers, MSC.1/Circ.1461 (2012) provides the guidelines for verification of damage stability instruments. The guidance requires that the damage stability results computed by the instrument be verified against the approved booklet calculations for each required damage case, using the same tolerances as UR L5 requires for intact stability.

MARPOL Annex I Regulation 28 sets the damage stability criteria for oil tankers (the two-compartment flooding standard at the required draught, the residual $GM$ and $GZ$ minimums). The loading computer on an oil tanker subject to Regulation 28 must either hold the pre-computed damage cases for the approved loading conditions or be capable of computing them in real time. Several tanker vetting programmes, including the OCIMF SIRE 2.0 questionnaire (see SIRE tanker inspections), ask specifically whether the loading computer includes a validated damage stability module for the ship’s worst-case damage scenario.

Master’s legal obligations and operational discipline

The master’s obligation under SOLAS Chapter II-1/5 is to verify, before sailing, that the ship’s loading condition complies with the applicable stability and strength criteria. The loading computer is the primary means of satisfying this obligation in practice. The procedural requirements that flow from this are:

Before departure, the master or chief officer must run the actual departure condition, not the planned condition, and confirm that the GZ area criteria, the $GM_0$ minimum, the weather criterion, and (on Category 2 ships) the shear force and bending moment limits are all satisfied. The computed condition must reflect the as-loaded cargo distribution (not the stowage plan), the actual bunker and ballast levels as measured by sounding or ullage at the time of departure, and the stores.

The calculation must be recorded. On tankers and bulk carriers this is typically on the loading computer printout, signed by the master. On passenger ships, the departure stability condition is often a formal document filed with the purser’s records.

If the departure condition does not satisfy the criteria, the master must not proceed to sea. The options are to redistribute cargo or ballast to improve the condition, to reduce cargo to reduce displacement, to load more ballast, or to take on additional bunkers. No regulatory framework permits departure on the grounds that “the booklet condition was close enough.”

The classification society surveyor at the annual survey and at load line surveys verifies that: the loading computer is approved and the verification record is current; the stability booklet is on board, approved, and matches the current lightship data (if an inclining experiment has been performed since the last issue, a new booklet must be on board); and the master can demonstrate the use of the loading computer by running the test loading conditions and matching the booklet output within the specified tolerances.

Port state control officers under the Paris MOU and Tokyo MOU inspection regimes have authority to detain a vessel for stability deficiencies. Detention grounds include: no approved stability booklet on board; booklet not updated after a structural modification; loading computer verification record absent or more than one annual survey interval out of date; the actual departure condition, when checked by the inspector, fails a mandatory criterion. See port state control and Tokyo MOU port state control for the broader PSC inspection framework.

Approval and updating workflow at classification society

The stability booklet is not a document that the owner or master edits. It is an approved document. Every change to the ship’s structure, tanks, or permanent equipment that affects the lightship condition or the hydrostatic data requires a booklet amendment, submitted to the class society for approval, before the amendment is placed on board.

The workflow follows these steps at delivery of a new ship: the shipbuilder’s stability software (typically a commercial naval architecture package licensed to the classification society for verification purposes) generates the booklet and the loading computer database from the same hull model. The class society reviewer checks the booklet calculations against the applicable criteria, verifies the loading computer output against the booklet for the test conditions, and issues a certificate of approval of the stability booklet and the loading computer installation. The approved booklet is placed on board and the previous document (if any) is withdrawn.

For a ship-to-ship booklet update triggered by a structural modification, the naval architect must first determine whether the modification changes the hull form (affecting hydrostatics and KN curves, requiring a full recalculation) or only the compartment arrangement (affecting tank tables and loading conditions, requiring a more limited update). A significant structural modification, defined under most class rules as a change altering the lightship displacement by more than 1% or the lightship KG by more than 10 mm, triggers an inclining experiment or a lightweight survey to re-establish the lightship data.

The loading computer database must be updated concurrently with the booklet. The vendor typically provides an updated database file which is installed on the computer and verified against the new booklet values before the surveyor endorses the updated verification record.

For electronic stability documentation (permitted under MSC.1/Circ.1553 subject to flag administration acceptance), the update procedure must include controlled distribution of the new electronic file to all access devices and confirmation that the previous version has been withdrawn or archived.

Limitations

Booklet conditions do not cover all real-world configurations. The loading conditions in the booklet are approved samples, not an exhaustive enumeration. A loading arrangement that falls outside the booklet’s sample space must be checked by the loading computer interpolating from the booklet data. If the arrangement is far outside the conditions for which the booklet was prepared (for example, a new stowage pattern for a different cargo density), the extrapolation may be unreliable and a formal stability calculation by the naval architect is the correct response.

Free-surface corrections are tank-table-dependent. The free-surface correction computed by the loading computer for a slack tank is only as accurate as the tank table. For tanks with complex geometry (wing tanks with knuckle points, tanks with non-flat bottoms), the tabulated free-surface moment changes rapidly near certain fill levels, and linear interpolation between table entries introduces error. Operators should be aware of this limitation for tanks near the critical fill levels.

Loading computer software requires regular maintenance. A type-approved loading computer that has been running without software updates for several years may not correctly implement the most recent amendments to the IS Code criteria or the latest class-rule permissible values. The annual verification confirms only that the existing software matches the booklet; it does not confirm that the software implements all current regulatory requirements. Flag administrations and class societies periodically issue circulars requiring software updates, but the responsibility for ensuring the update is applied lies with the owner.

The booklet does not address dynamic stability failure modes. The 2008 IS Code Part A criteria are static: they check the GZ curve at fixed angles for fixed loading conditions. They do not address parametric roll, surf-riding and broaching-to, or pure loss of stability in waves. The IMO Second Generation Intact Stability (2GISM) criteria, currently in interim guidelines stage (SDC 9, 2022), will eventually add dynamic failure mode screening to the required stability check. Until those criteria are mandatory, a loading condition that passes the booklet criteria may still be susceptible to dynamic instability in specific sea states.

Damage stability pre-computation assumes a fixed loading envelope. The pre-computed damage cases in the booklet and loading computer database are valid only for the range of loading conditions that were used as the base for the damage calculations. If the master loads the ship in a condition outside that envelope (for example, with a negative metacentric height in a tank partly filled with dense liquid), the pre-computed damage survival margins may not be valid.

Longitudinal strength permissible envelopes are class-rule specific. The permissible shear force and bending moment values loaded into the Category 2 instrument are derived from the class rule applicable at the time the ship was built, potentially decades ago. Class-rule permissible values can be revised as the design rules are updated. Owners should confirm at each special survey that the values in the loading computer database remain consistent with the current applicable class rule for the specific hull.

See also

Calculators

• IS Code Severe Wind and Rolling Criterion
• KG Limit: Intact Stability (GM-required method)
• Trim from Loading Centroid
• IACS UR S34 Loading Manual Check
• Damage Stability Multi-Draft A

Related wiki articles

• Intact Stability
• GZ Curve and Righting Arm
• Cross-Curves of Stability and KN Tables
• Probabilistic Damage Stability
• Hydrostatics and Bonjean Curves
• Metacentric Height
• Free Surface Effect
• Trim and List
• Damage Stability
• Classification Society
• Port State Control
• SIRE Tanker Inspections
• SOLAS Convention
• Bulk Carrier
• Tokyo MOU Port State Control