SOLAS Chapter II-1: Construction and Stability

Background

Scope and structure of Chapter II-1

Chapter II-1 is the longest chapter of SOLAS and the most heavily amended. It governs almost every aspect of how a ship is built and powered, from the spacing of watertight bulkheads to the redundancy of the steering gear and the runtime of the emergency generator. The chapter is split into seven parts:

• Part A General (definitions, application, exemptions).
• Part A-1 Structure of ships (the Goal-Based Standards regime).
• Part B Subdivision and stability (probabilistic damage stability).
• Part C Machinery installations.
• Part D Electrical installations.
• Part E Additional requirements for periodically unattended machinery spaces.
• Part F Alternative design and arrangements.
• Part G Ships using fuels of flashpoint below 60 degrees Celsius (the IGF Code).

Each part contains numbered Regulations. Cross-references between Chapter II-1 and other chapters are extensive: subdivision interacts with Chapter II-2 fire zones, machinery installations interact with Chapter III life-saving arrangements, and the IGF Code in Part G operates alongside the IGC Code for ships carrying low-flashpoint cargoes in bulk.

Relationship to the rest of SOLAS

Chapter II-1 sets the structural and engineering envelope inside which the rest of SOLAS operates. A ship that fails Chapter II-1 cannot be certificated regardless of its compliance with Chapter II-2 (fire), Chapter III (life-saving) or any other chapter, because the survey-and-certification regime under Chapter I requires evidence of compliance with all applicable chapters before issue of the Cargo Ship Safety Construction Certificate or the Passenger Ship Safety Certificate.

The principal certificates whose issue depends on Chapter II-1 compliance are:

• Passenger Ship Safety Certificate (passenger ships), issued under Regulation I/12, valid for a maximum of 12 months and renewed at annual surveys.
• Cargo Ship Safety Construction Certificate (cargo ships of 500 GT and above), issued under Regulation I/12, valid for a maximum of 5 years and renewed at periodical surveys.
• Cargo Ship Safety Certificate (combined construction, equipment and radio for harmonised survey under the Harmonised System of Survey and Certification, HSSC).
• International Load Line Certificate, although issued under the Load Line Convention, depends on the freeboard derived in part from Chapter II-1 subdivision and stability.

The Chapter II-1 compliance evidence chain is documented in the statutory survey package maintained by the flag state administration (or by the recognised organisation acting under flag delegation, typically a major classification society). The package includes the inclining experiment record, the as-built stability information booklet, the damage stability calculation, the steel structure plans, bilge and ballast pumping diagrams, fire main diagram, electrical single line diagram, machinery space arrangement, and the operating and maintenance manuals for the major machinery items.

Relationship to subsidiary codes

Chapter II-1 imports several subsidiary mandatory codes through SOLAS reference:

• IGF Code (International Code of Safety for Ships using Gases or other Low-flashpoint Fuels), Resolution MSC.391(95), mandatory in Part G since 1 January 2017.
• IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk), Resolution MSC.5(48) as updated, mandatory for gas carriers under Chapter VII Part C.
• IBC Code (International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk), Resolution MSC.4(48) as updated, mandatory for chemical tankers under Chapter VII Part B.
• IACS Common Structural Rules (CSR), mandatory through GBS verification for bulk carriers and oil tankers built with contracts placed on or after 1 July 2016.
• Polar Code (International Code for Ships Operating in Polar Waters), Resolution MSC.385(94), with structural and machinery requirements interacting with Part C and Part D.

The interaction between SOLAS Chapter II-1 and these codes means that a ship’s compliance package is rarely confined to SOLAS alone. The compliance burden is distributed across SOLAS, the relevant subsidiary code, the classification society rules, and where applicable IACS unified requirements (URs) and unified interpretations (UIs) that translate the SOLAS requirements into engineering practice.

Major amendment history

Chapter II-1 has received structural rewrites at seven distinct moments since 1974, each addressing a specific cluster of casualties or technological developments:

• 1981 amendments introduced the first SOLAS provisions on emergency steering and protected escape routes from machinery spaces.
• 1990 amendments introduced probabilistic damage stability for dry cargo ships (Regulations 25-1 to 25-10 of the time), departing for the first time from the deterministic worst-case approach inherited from earlier conventions. The amendments also strengthened watertight integrity requirements after several cargo ship losses involving progressive flooding through unsealed openings.
• 1996 Stockholm Agreement (an informal regional agreement among Northwest European states implemented as a SOLAS amendment) raised damage stability requirements for ro-ro passenger ships following the loss of MS Estonia in September 1994. The agreement required vessels operating in Northwest European waters to survive damage with up to 50 cm of water on the vehicle deck. The provisions were later largely absorbed into the 2009 amendments.
• 2006 MSC.216(82) amendments introduced two landmark changes: the harmonised probabilistic damage stability methodology for passenger and cargo ships under a common framework, and the Safe Return to Port (SRtP) requirement at Regulation II-1/8-1. The SRtP requirement entered into force 1 July 2010 and applies to passenger ships of 120 metres or more (or with three or more main vertical fire zones) with keels laid on or after 1 July 2010. It requires such ships to remain capable of proceeding to a safe port under their own power after flooding of any single watertight compartment or after a fire casualty within the defined casualty threshold, with all essential systems operational. The 2006 MSC.216(82) amendments were also the vehicle for the Goal-Based Standards mandate in Part A-1.
• 2009 harmonisation amendments brought the probabilistic methodology fully into effect, harmonising the treatment of passenger and cargo ships and bringing watertight integrity requirements into closer alignment with the probabilistic methodology. The explanatory notes at MSC.281(85) provide the detailed working method.
• SOLAS 2020 amendments (MSC.421(98), adopted 15 June 2017, in force 1 January 2020) raised the required subdivision index R for passenger ships, applying to ships with building contracts placed on or after 1 January 2020 or keels laid on or after 1 July 2020. The amendments also mandated damage control drills on all passenger ships at intervals not exceeding three months. MSC.436(99), also in force 1 January 2020, extended computerized stability support requirements to existing passenger ships constructed before 1 January 2014. The accompanying explanatory notes revision is MSC.429(98), later revised to MSC.429(98)/Rev.2.
• 2024 watertight integrity amendments (MSC.474(102) adopted 11 November 2020, MSC.482(103) adopted 13 May 2021, MSC.496(105) adopted 28 April 2022, all in force 1 January 2024) harmonized Parts B-2 to B-4 with the probabilistic damage stability approach. MSC.474(102) updated Regulations 12 (peak and machinery space bulkheads), 13 (openings in watertight bulkheads below the bulkhead deck in passenger ships), 15 (shell openings below the bulkhead deck), 16 (watertight closure construction and tests) and 17 (internal watertight integrity above the bulkhead deck), revising assumptions on progressive flooding, valves in the collision bulkhead, and the treatment of watertight doors in the probabilistic calculation. MSC.482(103) introduced water-level detectors for certain cargo vessels. MSC.496(105) updated emergency electrical power system requirements. These 2024 amendments apply to ships constructed on or after 1 January 2024; existing ships are affected only on modification.
• 2017 IGF Code (Part G) entered into force 1 January 2017 to govern ships using low-flashpoint fuels (LNG, methanol, ethanol, hydrogen, ammonia under development), responding to the rapid uptake of alternative fuels driven by MARPOL Annex VI emissions limits.

A further round of amendments under development at IMO MSC addresses ammonia as marine fuel, hydrogen fuel cell installations, large lithium-ion battery installations on roll-on roll-off vehicle decks, and onboard carbon capture systems.

Part A: General

Application thresholds

Part A defines the application and scope of the chapter. The general principle is that Chapter II-1 applies to ships on international voyages, with specific thresholds set in individual Regulations:

• All passenger ships are covered, regardless of size (passenger ships carry more than 12 passengers; even very small passenger boats fall within Chapter II-1 if on international voyages, though many provisions are waived or scaled for small passenger ships).
• Cargo ships of 500 gross tonnage and above are covered for most provisions; cargo ships below 500 GT are partially exempt.
• Bulk carriers and oil tankers of 150 metres in length and above with building contracts placed on or after 1 July 2016 are subject to the GBS regime in Part A-1.
• Ships using low-flashpoint fuels are subject to Part G (the IGF Code) regardless of size.
• Tankers (chemical tankers, gas carriers, oil tankers) are subject to specific reinforcement of certain provisions, for example the steering gear redundancy in Regulation 29 applies in expanded form to tankers of 10,000 GT and above.

Geometric definitions

The geometric definitions in Part A are load-bearing because the probabilistic damage stability calculations in Part B depend on them. The principal geometric quantities are:

• Length L (subdivision length): the greatest projected moulded length of the ship at or below the deck or decks limiting the vertical extent of flooding with the ship at the deepest subdivision draught. For most cargo ships this is approximately 96 percent of the waterline length.
• Breadth B (moulded breadth): the maximum moulded breadth amidships.
• Depth D: the moulded depth measured at the centreline from the top of the keel to the top of the bulkhead deck.
• Deepest subdivision draught (d_s): the waterline corresponding to the summer load line draught.
• Partial subdivision draught (d_p): the waterline corresponding to the lightest seagoing service draught plus 60 percent of the difference between d_s and the lightship draught.
• Light service draught (d_l): the waterline corresponding to the lightest seagoing service draught.

The three loading conditions (d_s, d_p, d_l) are used together in the probabilistic damage stability calculation: the attained subdivision index A is computed as a weighted average over the three conditions with weights 0.4 / 0.4 / 0.2 reflecting the assumed time-distribution of operational conditions.

Operational definitions

Part A also defines:

• Passenger ship: a ship that carries more than twelve passengers, where a passenger is defined as a person other than the master, the crew or other persons employed in any capacity on board.
• Cargo ship: any ship that is not a passenger ship.
• Ro-ro passenger ship: a passenger ship with cargo or vehicle decks normally accessible by drive-through ramp from open or enclosed embarkation deck.
• High-speed craft: a craft capable of a maximum speed equal to or exceeding 3.7 times the volumetric displacement to the power of one-sixth.
• Length L for steering gear (Regulation 29) is defined separately from the subdivision length: the steering gear L is taken as 96 percent of the total length on a waterline at 85 percent of the moulded depth.

Exemptions and equivalents

The IMO permits flag administrations to grant exemptions from specific Chapter II-1 provisions, subject to demonstration that the exemption does not reduce the level of safety, notification to IMO and affected port states, and recording of the exemption on the ship’s certificate.

Equivalents (Regulation I/5) permit alternative arrangements achieving the same objective; they are commonly used during the introduction of new technology before specific prescriptive rules are developed.

Part A-1: Structure of ships and Goal-Based Standards

GBS for bulk carriers and oil tankers

Goal-Based Standards apply to bulk carriers and oil tankers of 150 metres or more, the building contracts for which were placed on or after 1 July 2016. The GBS framework operates at five conceptual levels:

• Tier I: Goals. High-level safety statements. The GBS Tier I goal reads: ships are to be designed and constructed for a specified design life to be safe and environmentally friendly when properly operated and maintained under the specified operating and environmental conditions, in intact and specified damage conditions, throughout their life.
• Tier II: Functional requirements. Translation of goals into engineering objectives. The 14 functional requirements include design life of 25 years for the principal hull structure, structural strength under static and dynamic loading, fatigue life consistent with the design life, residual strength after damage, protective coatings, structural redundancy, watertight and weathertight integrity, and accessibility for inspection.
• Tier III: Verification of compliance. The methodology by which classification societies demonstrate that a design satisfies the functional requirements. The principal vehicle is the IACS Common Structural Rules.
• Tier IV: Rules and regulations. The classification society rules and IMO instruments. For bulk carriers and oil tankers above 150 metres, the IACS CSR provides the structural rules meeting Tier III verification.
• Tier V: Industry practices and standards. Workmanship, quality systems, and industry standards (welding qualification, material certification, in-service inspection regimes).

The SOLAS regulation introducing GBS is II-1/3-10, established by MSC.290(87) (adopted May 2010, in force 1 January 2012). The GBS Standards themselves are in Resolution MSC.287(87) (also adopted May 2010). Both resolutions were adopted at the same (87th) session of the Maritime Safety Committee.

IACS Common Structural Rules

The IACS Common Structural Rules (CSR) for bulk carriers and oil tankers were developed jointly by the major classification societies and provide the unified structural design rules satisfying GBS Tier III verification. The rules cover:

• Hull girder strength under still-water and wave bending moments and shear forces.
• Local strength of plating and stiffeners under hydrostatic, hydrodynamic and cargo loads.
• Buckling strength of plating and stiffeners under combined in-plane and out-of-plane loading.
• Fatigue life of structural details under spectrum loading derived from the design wave environment.
• Residual strength after damage, including post-grounding and post-collision scenarios.
• Coating performance and corrosion margins.

The CSR incorporates direct calculation methods (finite element analysis, spectral fatigue analysis) for the verification of complex structural details.

GBS Verification Audit Scheme

The verification of compliance under Tier III is performed by the IMO under the GBS Verification Audit Scheme, governed by Resolution MSC.454(100) (Revised Guidelines for Verification of Conformity with Goal-based Ship Construction Standards). International GBS Audit Teams established by the IMO Secretary-General audit classification society rules against the functional requirements. The outcome was submitted to MSC in May 2016 for approval; rules that pass verification are then applied to ships built on or after 1 July 2016. Where a rule fails to verify, the classification society must amend and re-submit.

Functional requirements and Ship Construction File

Each ship built under GBS must have a Ship Construction File (SCF) prepared at delivery and maintained throughout the ship’s life. The SCF documents construction details and supports inspection, maintenance, repair and recycling decisions. SCF content includes:

• General arrangement, midship section, lines plan and longitudinal section.
• Materials specifications including grade, heat-treatment condition, and manufacturer.
• Welding procedure specifications and welder qualifications.
• Non-destructive examination records (radiographic, ultrasonic, dye penetrant).
• Hull steel survey plan and as-built thickness measurements.
• Loading manual including cargo distribution patterns and loading sequence constraints.
• Inspection and maintenance plan covering the survey interval and equipment required.
• Coating specification and as-applied coating record.

The SCF must be available to the master and to the flag-state surveyor on demand throughout the ship’s life.

Part B: Subdivision and stability

Historical evolution of subdivision rules

Subdivision rules in maritime safety go back to the 19th century. The first systematic compartment standards appeared in the 1854 British Merchant Shipping Act, requiring iron passenger ships to have at least one watertight bulkhead. The first SOLAS Convention 1914, drafted after the loss of RMS Titanic in 1912, introduced the concept of “permissible length” of compartments and the criterion that the ship should survive flooding of any one compartment.

The 1929, 1948 and 1960 SOLAS conventions progressively tightened the criterion to two-compartment damage for passenger ships, retaining a deterministic worst-case approach. The 1990 amendments first introduced probabilistic methodology for cargo ships. The 1996 Stockholm Agreement applied a stricter standard to ro-ro passenger ships in Northwest Europe. The 2006 MSC.216(82) amendments introduced the harmonised probabilistic framework and the Safe Return to Port requirement. The 2009 amendments consolidated the probabilistic methodology. The SOLAS 2020 amendments (MSC.421(98)) raised the required index for passenger ships. The 2024 amendments (MSC.474(102)) aligned watertight integrity prescriptive rules with the probabilistic framework.

The progressive development reflects two related insights from casualty experience: that worst-case deterministic scenarios under-represent the actual probability distribution of damage, and that ship designers respond to deterministic rules by optimising against the specified worst case rather than across the full operational risk profile.

The probabilistic damage stability methodology

The probabilistic methodology adopted in the 2006/2009 amendments represents a fundamental departure from the deterministic worst-case standard known as SOLAS 90. The probabilistic approach treats damage as a random event with a probability distribution over location, length and penetration, computes the conditional survival probability for each damage case, and combines them into an overall survival index.

The mathematical structure is:

• A (attained subdivision index) is the weighted sum over all relevant damage cases and loading conditions of the product of (a) the probability p that the damage occurs at the considered location and (b) the probability s that the ship survives the damage:

  $$A = 0.4 \cdot A_s + 0.4 \cdot A_p + 0.2 \cdot A_l$$

  where $A_s$, $A_p$, $A_l$ are the attained indices at the deepest subdivision draught $d_s$, the partial subdivision draught $d_p$, and the light service draught $d_l$ respectively. Each draught-specific index is the sum:

  $$A_x = \sum_{i=1}^{n} p_i \cdot s_i$$

  over the n damage cases considered.

• R (required subdivision index) is calculated from formulas that depend on ship length and, for passenger ships, on persons on board:

  • For cargo ships of length $L \geq 100\,\text{m}$: $R = 1 - \dfrac{128}{L + 152}$
  • For cargo ships $80 \leq L < 100\,\text{m}$: a transition formula links the cargo ship R to a lower bound.
  • For passenger ships (SOLAS 2009 base formula): $R = 1 - \dfrac{5000}{L_s + 2.5N + 15225}$, where $N = N_1 + 2N_2$, $N_1$ is the number of persons for whom lifeboats are provided and $N_2$ is the number of persons (including crew) in excess of $N_1$; R typically ranges from 0.7 to 0.85.

• The compliance criterion is $A \geq R$.

The subdivision and attained index R calculator implements the cargo ship formula. The SOLAS probabilistic R for passenger ships calculator implements the passenger ship formula.

For a 200-metre cargo ship, the cargo ship formula gives $R = 1 - 128 / 352 \approx 0.636$. For a 300-metre passenger ship certified for 6,000 persons with lifeboat capacity for 2,500, $N = 2{,}500 + 2 \times 3{,}500 = 9{,}500$, giving $R = 1 - 5000 / (300 + 23{,}750 + 15{,}225) \approx 0.87$.

SOLAS 2020 changes to the required index R

Resolution MSC.421(98) (in force 1 January 2020) raised the required subdivision index R for passenger ships. The revision was driven by the investigation findings from the Costa Concordia casualty (2012) and from the EU-funded HARDER research project, which had produced the statistical damage distributions underlying the probabilistic method. Ships with building contracts placed on or after 1 January 2020 (or keels laid on or after 1 July 2020) must satisfy the revised, higher R values. Ships contracted before 1 January 2020 remain subject to the 2009 R formulas.

The same resolution also mandated damage control drills on all passenger ships at intervals not exceeding three months, requiring use of the onboard damage stability computer to conduct stability assessments for simulated damage conditions during each drill. This operational requirement applies to existing as well as new passenger ships.

MSC.436(99), also in force 1 January 2020, extended computerized damage stability support requirements to passenger ships constructed before 1 January 2014, closing a gap between new and existing tonnage for stability-management tools available to the master.

Damage probability p

The probability p that a given damage case occurs is computed from the joint probability distribution of damage location, length and penetration. The IMO casualty database analysis has produced parametric forms for these distributions:

• Longitudinal location is approximately uniform over the subdivision length, with slight skew toward the bow (collision damage) and the stern (grounding damage).
• Damage length has a roughly exponential distribution with median around 6 percent of L.
• Damage penetration from the side shell is approximately uniform conditional on length, with maximum penetration approximately B/5.
• Vertical extent depends on damage type: collision damage typically extends from the waterline up to the bulkhead deck; grounding damage extends from the keel up to the inner bottom or higher.

For each ship, the probabilistic methodology constructs a finite set of damage cases spanning the joint distribution. Modern probabilistic damage stability software (NAPA, GHS, MAESTRO and similar) computes the probability of each case automatically from the ship’s compartmentation geometry.

Survival probability s

The survival probability for a given damage case depends on the residual stability characteristics of the damaged ship. The criteria are:

• Range of positive stability after damage: at least 16 degrees beyond the equilibrium heel angle.
• GZmax (maximum righting arm) after damage: the survival factor s falls to 0 when GZmax is at or below 0.05 m, and reaches its maximum contribution when GZmax is at or above the 0.12 m reference value.
• Area under the GZ curve: at least 0.0175 m-radians.
• Equilibrium heel angle: not exceeding 7 degrees (single compartment damage) or 15 degrees (two-compartment damage).
• Final waterline: must not immerse non-watertight openings (escape openings, ventilation intakes, etc.).
• Wind heeling moment: the ship must survive a defined wind heeling moment without breaching the heel-angle criterion.

For passenger ships with ro-ro spaces, additional criteria address water on the vehicle deck (the ro-ro requirement that originated in the Stockholm Agreement).

If the damaged condition fails any of these criteria, s = 0; if it passes, s is computed from a formula scaling with the strength of the margin:

where K is a factor depending on heel angle, capped at 1.0. The exact form is set out in SOLAS Regulation 7-2, with detail in MSC.281(85).

Required index R values

R values were calibrated so that the average attained survival probability across the existing fleet under the new probabilistic regime would be at least as high as under the old deterministic regime, while permitting ship designers to optimise within a probabilistic budget. The calibration was based on a sample of approximately 100 ships of various types and sizes, with R values selected to reproduce the historical compliance pattern under SOLAS 90.

2024 watertight integrity amendments

The 2024 amendments (MSC.474(102), in force 1 January 2024) addressed a known inconsistency between the prescriptive watertight integrity rules in Parts B-2 to B-4 and the probabilistic damage stability approach in Parts B and B-1. The amendments revised:

• Regulation 12 (peak and machinery space bulkheads): updated structural and positional requirements for the collision bulkhead, including revised rules on valves penetrating the collision bulkhead.
• Regulation 13 (openings in watertight bulkheads below the bulkhead deck in passenger ships): revised treatment of watertight doors in the probabilistic calculation, particularly for doors that may remain open during navigation.
• Regulation 15 (shell openings below the bulkhead deck): updated flood-path and progressive flooding assumptions.
• Regulation 16 (construction and initial tests of watertight closures): aligned testing criteria.
• Regulation 17 (internal watertight integrity above the bulkhead deck in passenger ships): harmonized the internal integrity standard with the probabilistic framework.

These changes apply to ships constructed on or after 1 January 2024; existing ships are affected only on modification.

Comparison with deterministic SOLAS 90

The pre-2009 deterministic standard required cargo ships to survive a single specified worst-case damage scenario. This was simple to apply but inflexible: a ship that survived a worst case by a small margin received the same compliance status as a ship that survived it by a large margin, and ships at risk in damage scenarios outside the specified case received no credit for surviving them or no penalty for failing them.

The probabilistic methodology penalises the design across the full damage probability distribution, rewarding redundancy and watertight integrity wherever they help across the population of cases. It rewards longer subdivision (more bulkheads), better watertight door management and smaller damage propagation paths.

The trade-off is computational complexity. A modern ship requires a probabilistic damage stability calculation involving hundreds or thousands of damage cases, performed using class-society or third-party software, with results submitted as part of the design package for class approval. The damage stability multi-draft calculator supports the multi-condition index computation.

The probabilistic damage stability article covers the mathematical structure in greater depth.

Software tools

Modern probabilistic damage stability calculation requires software. The principal commercial tools are:

• NAPA: developed by NAPA Ltd (Finland), used by the majority of European yards and many class societies.
• GHS: General HydroStatics, developed by Creative Systems Inc., widely used in North America.
• MAESTRO: Modelling, Analysis and Evaluation of Ship Damage and Survivability, a research-grade tool with class society certifications.
• DELFTship and FREE!ship: simpler tools for preliminary design.

The class societies maintain their own tools (PoseidonNG by DNV, Eagle by ABS, Mars2000 by BV, etc.) for verification of submitted calculations.

Watertight integrity requirements

The probabilistic methodology assumes a defined level of watertight integrity. Specific Regulations within Part B prescribe the structural and operational requirements:

• Watertight bulkheads: location, extent and structural standard. The forward collision bulkhead is positioned a minimum distance abaft the forward perpendicular calculated from a formula in Regulation 12 (as amended by MSC.474(102) for ships from 2024). Aft peak and machinery space watertight bulkheads have similar prescriptive requirements.
• Watertight doors: doors in watertight bulkheads must be of approved type, with operation by hand from above the bulkhead deck and by power from the bridge. The treatment of doors that may remain open during navigation was specifically revised by MSC.474(102) to improve consistency with the probabilistic survival calculation.
• Side scuttles: portholes below the bulkhead deck must have hinged storm covers and must be kept closed at sea.
• Cargo hatches: hatches on weather decks must be of weathertight design with hose-tested cleating; hatches on the bulkhead deck and below must be watertight.
• Air pipes and ventilators: must be carried to a height above the bulkhead deck sufficient to prevent flooding of the protected space in the worst-case heel and trim.

Safe Return to Port

Safe Return to Port (SRtP) is the design requirement introduced by SOLAS Regulation II-1/8-1 (via MSC.216(82), in force 1 July 2010) for passenger ships of 120 metres or more in length, or with three or more main vertical fire zones, with keels laid on or after 1 July 2010.

The requirement is that the ship shall be designed so that after flooding of any single watertight compartment, or after a fire casualty that does not exceed the defined casualty threshold, the ship is capable of proceeding to a safe port under its own power with all essential systems operational. “Essential systems” in this context include propulsion (at a safe speed), steering, fire detection and firefighting, internal communication, watertight door operation, and bilge pumping.

The SRtP requirement in Part B is complemented by the parallel SRtP fire requirements in Chapter II-2 Regulations 21 and 22. Full compliance typically requires:

• Duplication of propulsion and essential machinery arranged in separate fire zones.
• Cross-connections for ballast, fuel and electrical supply between zones.
• Documented Damage Control Plans showing system status after each specified casualty scenario.
• Operational manuals for flooding and fire casualty cases, detailing manual actions to maintain essential system operation.
• Drills using the onboard damage stability computer to simulate casualty scenarios.

For ships designed to high A index values (A substantially above R), the SRtP capability also reduces the risk of rapid or total capsize after damage, providing a margin beyond the regulatory minimum.

Part C: Machinery installations

Main and auxiliary machinery

Part C requires machinery installations to be capable of providing propulsion, electrical power, steering, cooling and other essential services in normal and emergency conditions. The general requirements include:

• Capability to operate the ship safely under all foreseeable conditions including astern operation, with reversal of propulsion direction available within a defined time.
• Means of starting and stopping main propulsion machinery from the navigation bridge as well as from the machinery space.
• Protection against fire, flooding and other foreseeable damage to essential machinery.
• Adequate redundancy in critical systems (steering, electrical, propulsion where required by ship type).

Specific Regulations cover:

• Engine starting arrangements: starting air system with at least two starting air receivers, capacity for at least 12 starting cycles, with starting from local control panel and from the bridge.
• Fuel oil arrangements (Regulations 26 and 36): fuel tank arrangements, fuel pipe routing (arranged so that any leak does not reach a hot surface, with screened drip trays under flanges and pumps), fuel quick-closing valves operable from outside the machinery space, fuel temperature monitoring, fuel oil purifier installations.
• Lubrication oil arrangements: similar to fuel oil arrangements but with the additional consideration that lube oil leaks contribute to engine room fire and to bilge contamination.
• Cooling systems: sea-water cooling intakes (typically dual main intake and emergency intake), fresh-water cooling circuits, cooling tower or radiator arrangements where applicable.
• Compressed air systems: starting air at typically 30 bar, working air for control and instrumentation typically 7 bar, with separate receivers and dryers.
• Bilge pumping arrangements (Regulation 35-1): main bilge pumps with sufficient capacity to dewater the largest watertight compartment from the deepest waterline within a defined time, with strums in each compartment, valve manifolds for selective pumping, and an emergency bilge pump available outside the main machinery space.

The bilge pumping arrangements are tied to subdivision in Part B because the ability to dewater a damaged compartment can affect the survival probability calculation. The SOLAS calculation does not credit dewatering in the survival assessment (the assumption is that damaged-compartment flooding is irreversible), but adequate pumping capacity remains a key operational safeguard.

Steering gear (Regulation 29)

Regulation 29 imposes one of the most specific performance standards in SOLAS. Every ship must have:

• A main steering gear of adequate strength, capable of putting the rudder over from 35 degrees on one side to 35 degrees on the other side at maximum ahead service speed, and from 35 degrees on either side to 30 degrees on the other side in not more than 28 seconds at the same speed.
• An auxiliary steering gear of adequate strength, capable of putting the rudder over from 15 degrees on one side to 15 degrees on the other side in not more than 60 seconds at half maximum service speed (or 7 knots, whichever is greater).
• Two independent power units for the main steering gear on tankers, chemical tankers and gas carriers of 10,000 GT and above, with the second power unit able to take over within 45 seconds in the event of failure of the first.
• Two independent control systems on tankers and gas carriers of 10,000 GT and above, with each control system serving its associated power unit.
• A means of bringing into operation, from the navigation bridge, the steering gear power units within 45 seconds of failure of one or more units.

The 28-second rule reflects the manoeuvrability margin needed to avoid collision and grounding in close-quarters situations. For a 300-metre VLCC with a rudder area of 60 square metres at 16 knots service speed, the rudder forces are very large and the steering gear hydraulic system must deliver flow rates of several hundred litres per second. The single-fault tolerance principle in Regulation 29 means that no single failure (loss of one power unit, one control system, or one rudder actuator) results in loss of steering.

Boilers, pressure vessels and piping

Part C also covers steam boilers, oil-fired auxiliary boilers, pressure vessels (including bottle storage for compressed gases), and piping systems for fuel, lubricant, bilge, ballast and cargo. The requirements are largely by reference to recognised classification society rules.

Specific Chapter II-1 requirements include:

• Boiler protective devices: low-water cut-off, high-water cut-off, low-fuel-pressure cut-off, high-pressure cut-off.
• Pressure vessel certification: design, manufacture and periodic survey to class society standard with the certification recorded on the Cargo Ship Safety Construction Certificate.
• Fuel oil pipe penetration of bulkheads: fuel piping passing through watertight bulkheads must have stop valves on the upstream side, operable from outside the machinery space.
• High-pressure fuel injection lines: must be jacketed or fitted with leak-detection so that any leak from the high-pressure pipe is contained or signalled.

Communications between bridge and engine room

Reliable communication between the navigation bridge and the engine control room is required by Part C. The arrangement typically includes:

• Engine telegraph (mechanical or electronic) with bridge-mounted indicator and engine-room-mounted indicator, with both indicators locked into agreement before the order is considered acknowledged.
• Bridge-to-engine-control-room dedicated phone line, separate from the general ship phone system.
• Public address system audible in machinery spaces.
• Engineer’s alarm system that alerts the duty engineer to malfunctions detected by the automation.

Part D: Electrical installations

Main source of power

Every ship must have a main source of electrical power of sufficient capacity to supply all services necessary for maintaining the ship in normal operational and habitable condition without recourse to the emergency source. The main source typically consists of:

• Two or more main generator sets configured so that any one generator can supply the essential services with the largest single generator out of operation (the n - 1 redundancy rule). Typical cargo ship arrangement: three diesel-generator sets each rated approximately 50 percent of total electrical demand, with two running and one on standby.
• Main switchboard distributing power to ship services, with sectionalising breakers permitting isolation of sub-distribution panels for fault clearing.
• Synchronisation panels for parallel operation of generators.
• Load-sharing controllers allocating load between parallel generators.
• Excitation system (typically permanent-magnet pilot exciter, brushless rotating exciter, automatic voltage regulator) for each generator.

The generator capacity must accommodate the largest concurrent electrical demand including motor starting transients.

Emergency source of power

Every ship must have a self-contained emergency source of electrical power located outside the main machinery space and above the bulkhead deck, capable of supplying for a specified period the services essential for safety in an emergency:

• For passenger ships and ro-ro passenger ships: 36 hours of operation.
• For cargo ships of 500 GT and above on international voyages: 18 hours of operation.

These durations were updated by MSC.496(105) (in force 1 January 2024) to reflect operational experience with the existing requirements and to align with the Safe Return to Port design life of essential systems.

The emergency services typically include:

• Emergency lighting along escape routes and in survival craft embarkation stations.
• Navigation lights and signalling lights.
• Internal communication equipment (engine telegraph, intership phone, public address).
• Communication equipment (GMDSS for the period required by Chapter IV).
• Fire detection and alarm systems.
• The steering gear (or a fraction of its capacity) if powered electrically.
• The watertight door indication and remote-operation system.
• The bilge alarm system.
• Lighting in the navigation bridge.
• Fire pump (if main fire pumps are electric, the emergency source must support at least one).

The emergency generator must start automatically on failure of the main supply and reach full load within a specified time (typically 45 seconds), and must be located in a self-contained compartment with its own fuel supply, ventilation and starting arrangements.

Transitional source of power

For the interval between failure of main supply and start of the emergency generator, a transitional source of power (typically a battery bank) must be available to maintain emergency lighting, navigation lights and essential alarms. The battery must supply these loads for at least 30 minutes without recharging.

The architecture is three-tiered: main supply for normal operations, transitional supply (battery) for the seconds between main failure and emergency generator start, and emergency supply (generator) for hours of independent operation after a main-system loss.

Switchboards and electrical distribution

The main switchboard requirements include:

• Construction with non-conducting deck and rear screens, IP-rated enclosures, and accessible from the front for switching and from the rear for maintenance.
• Bus tie breakers for sectionalisation, permitting isolation of fault zones without losing power to unfaulted sections.
• Protective relays for over-current, earth fault, reverse power, under-voltage, over-voltage, over-frequency, under-frequency.
• Synchroscope and synchronising lights for parallel generator operation.
• Generator load and bus voltage indication.

Lighting and emergency lighting

Lighting requirements include:

• Normal lighting throughout accommodation, machinery and working spaces.
• Emergency lighting at all assembly stations, escape routes, embarkation stations, working areas of machinery spaces, and navigation positions.
• Low-location lighting (LLL) along passenger ship escape routes, photoluminescent or electrically powered.
• Hazardous-area lighting in fuel transfer zones, paint stores and other hazardous areas, with luminaires of certified explosion-protected type.

Hazardous area electrical

Electrical installations in hazardous areas (defined zones around fuel and cargo systems where flammable atmosphere may be present) must be of certified explosion-protected type. The IEC 60079 series (Equipment for explosive atmospheres) defines the protection methods (Ex d flameproof, Ex e increased safety, Ex i intrinsically safe, Ex p pressurised, Ex n non-incendive, etc.). The classification of a given space (Zone 0, Zone 1, Zone 2 for gas atmospheres) follows from the ship’s hazardous-area drawings.

Part E: Periodically unattended machinery spaces

UMS notation requirements

Part E sets additional requirements for ships whose machinery spaces are not continuously manned. The UMS notation (granted by classification society and recognised by SOLAS) requires:

• Bridge control of main machinery with full ship control from the bridge including starting, stopping, reversing and adjusting power. The bridge controls must be designed to fail-safe.
• Engine control room with monitoring for all essential parameters (oil pressure, temperatures, fuel rack position, exhaust temperatures, vibration on rotating machinery).
• Automation of essential functions including auto-start of standby pumps (lubricating oil, fuel oil transfer, cooling water circulation, sea water cooling), automatic load sharing on generators, automatic ballast adjustment, automatic boiler control, and automatic refrigeration plant control.
• Fire detection in the machinery space with audible and visible alarm to the bridge and the duty engineer’s accommodation.
• Bilge alarm with high-bilge-level detection in machinery spaces and the engine room.
• Engineer alarm system that alerts the duty engineer to any UMS alarm; the engineer must respond within a defined time (typically 30 minutes) or the system escalates to the bridge.

Bridge control and engine control room

The bridge control system provides the master with direct manoeuvring control of the main propulsion machinery from the bridge. The system requirements include:

• Bridge-mounted control lever with detents at “stop”, “dead slow”, “slow”, “half” and “full” ahead, and corresponding astern positions.
• Synchronised display in the engine control room.
• Override capability from the engine control room (engineer can take local control from the engine).
• Time-delay limits between maximum-ahead to maximum-astern reversal to protect the main engine from thermal shock.
• Critical RPM lock-out in the speed range that would excite hull resonance or torsional resonance of the shafting.

Automation of essential functions

The UMS-approved ship has automation that handles routine engine room tasks without continuous engineer attention:

• Standby pump auto-start: when a running pump fails (low pressure detected), the standby pump starts automatically and the failure is alarmed.
• Generator auto-synchronisation: when load increases above the running generator capacity, the next standby generator starts, synchronises and parallels automatically.
• Auto-refrigeration: cargo refrigeration on container ships and reefer carriers operates without continuous attention, with temperatures alarmed if outside set range.
• Fuel oil purifier auto-operation: purifier operation cycles between purification and discharge automatically based on oil throughput.
• Auto-stop: dangerous conditions (low lube oil pressure, high cooling water temperature, low fuel pressure, high exhaust gas temperature beyond limit) trigger automatic engine slowdown or shutdown.

Alarm management systems

Modern ships have hundreds to thousands of alarm points. Alarm management requires:

• Prioritisation: critical safety alarms (fire, collision, grounding, machinery shutdown) above operational alarms (low fuel level) above advisory alarms (planned maintenance due).
• Suppression of cascading alarms: when one alarm causes secondary alarms, the secondary alarms are suppressed during the response period.
• Alarm indication on bridge and engine control room with visible and audible alerting.
• Alarm acknowledgement requiring active operator action.
• Alarm history with retention sufficient to support post-incident analysis.

UMS classification has become almost universal on modern cargo ships, with continuously manned engine rooms the exception (typically only on older ships or specialised vessels with high-redundancy machinery rooms).

Part F: Alternative design and arrangements

Part F was added to allow flag states to approve designs that depart from the prescriptive requirements of Chapter II-1 provided that an engineering analysis demonstrates that an equivalent level of safety is achieved. The procedure requires:

• Identification of the prescriptive Regulations from which alternative design is sought.
• Definition of the alternative design and arrangement.
• Engineering analysis demonstrating that the alternative provides at least the same level of safety as the prescriptive requirement, typically using quantitative risk analysis (QRA) with hazard identification, probability quantification and consequence assessment.
• Approval by the flag administration in consultation with the IMO.

Examples of approved alternatives include:

• Diesel-electric propulsion without a direct mechanical link from prime mover to propeller, where the prescriptive rules envisage direct-drive arrangements.
• Pod propulsion (azimuth drives) with rudder/propeller integration where the prescriptive Regulation 29 envisages a rudder and propeller as separate elements.
• Novel hull forms with unconventional structural arrangements (catamaran, trimaran, SWATH).
• Unconventional emergency power architectures with battery banks or fuel cells in lieu of diesel emergency generators.
• Arctic and polar service modifications under Polar Code interaction.

The alternative design framework was reused in Chapter II-2 for fire safety equivalence and in Chapter III for life-saving equivalence. It enables the introduction of hydrogen as marine fuel and other emerging technologies under the IGF Code Part G in advance of fully developed prescriptive rules.

Part G: Ships using low-flashpoint fuels (IGF Code)

Scope of the IGF Code

The International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code) was adopted in 2015 by Resolution MSC.391(95) and entered into force 1 January 2017 as a mandatory instrument under Chapter II-1 Part G. It applies to:

• Ships subject to SOLAS using fuels with a flashpoint lower than 60 degrees Celsius.
• Initially focused on natural gas (LNG and CNG); extended through amendments to address methanol (2024 amendments to the IGF Code MSC.526(106)), and with provisions for ammonia and hydrogen at various stages of development.
• Excludes gas carriers (governed by the IGC Code) and ships using oil fuels with flashpoint above 60 degrees Celsius.

Functional requirements

The IGF Code is structured on a goal-based pattern similar to GBS:

• Goal: ships using low-flashpoint fuels shall be designed and constructed for safe operation without compromising other safety requirements.
• Functional requirements translate this into engineering objectives covering fuel storage, distribution, machinery space, hazardous area zoning, gas detection, fire and explosion protection, ventilation, electrical installations in hazardous areas, and emergency shutdown.
• Prescriptive provisions cover specific equipment requirements (for example bunkering arrangements, double-walled fuel piping, gas-tight bulkheads).
• Alternative design is permitted under Part F where prescriptive provisions are not yet developed (used heavily for methanol, ammonia and hydrogen).

LNG bunkering

The IGF Code specifies bunkering operation requirements for LNG-fuelled ships:

• Bunkering manifold isolated from accommodation and sources of ignition.
• Emergency Shutdown (ESD) system with three independent shutdown levels (manual, automatic via gas detection, automatic via excessive movement of bunkering vessel).
• Gas detection at the manifold and along the fuel transfer route.
• Crew training in bunkering procedures with documented procedures and drills.
• Pre-bunkering checklist including weather conditions, mooring arrangements, vapour return arrangements, jetty equipment compatibility.

LNG bunkering takes longer than oil bunkering at equivalent energy quantity, with typical transfer rates of 500 to 2,000 cubic metres per hour. The IMO bunkering guidelines (MSC.1/Circ.1546) provide operational details.

Methanol-specific provisions

Methanol as marine fuel was first adopted on dual-fuel methanol ships in the 2010s. The 2024 IGF Code amendments (MSC.526(106)) added methanol-specific prescriptive provisions:

• Lower flashpoint (around 12 degrees Celsius) requires extended hazardous-area zoning compared to LNG.
• Cofferdams around methanol fuel tanks to provide secondary containment.
• Toxicity hazard management (methanol is toxic by ingestion and skin absorption, unlike LNG).
• Inerting requirements for tanks during commissioning and during cargo operations.
• Water-spray fire suppression above methanol manifold areas.

Ammonia and hydrogen development

Ammonia as marine fuel is in the development stage at IMO MSC. Provisional design requirements address:

• Ammonia toxicity (acute exposure threshold around 25 ppm, lethal around 300 ppm) requiring extensive gas detection, air-tight accommodation, decontamination provisions.
• Cold service requirements for liquefied ammonia at minus 33 degrees Celsius.
• Material compatibility (ammonia attacks copper and zinc alloys; only stainless steel and certain coatings are suitable for piping).

Hydrogen as marine fuel is in the demonstration stage with several pilot vessels in service. Provisional design requirements address:

• Hydrogen flammability (wide flammable range of 4 to 75 percent in air, very low ignition energy).
• Cryogenic storage at minus 253 degrees Celsius (boiling point) for liquid hydrogen, or compressed gas storage at 350 to 700 bar.
• Material compatibility (hydrogen embrittlement of steels and certain non-ferrous alloys).
• Hazardous-area zoning with much larger zones than for LNG due to the flammability range.

IGF Code training requirements

The IGF Code requires specific training for crew on IGF-compliant ships:

• Basic training for all crew with familiarisation with the fuel system.
• Advanced training for designated officers and crew operating fuel system and bunkering operations.
• STCW IGF certification for officers in command of fuel system operations.

The training is documented in seafarer endorsements under the STCW Convention Section A-V/3.

Notable amendments and casualties

Herald of Free Enterprise, 1987

The British ro-ro passenger ferry Herald of Free Enterprise capsized outside Zeebrugge harbour on 6 March 1987 after the bow doors had been left open. The vessel took water as it left port at speed; the open bow door allowed free water onto the vehicle deck. Free-surface effect on the vehicle deck rapidly degraded transverse stability, the vessel rolled to port, sustained progressive flooding, and capsized in approximately 90 seconds. 193 lives were lost.

The casualty exposed:

• Vulnerability of ro-ro passenger ships to free-surface effect on the vehicle deck (a flat, full-breadth deck without sub-divisions amplifies the free-surface moment).
• Inadequate bridge monitoring of door status (the bridge had no indication that the bow doors were open).
• Inadequate operational procedures for door closure.
• Organisational failure that allowed the operational gap to persist.

The amendments responded with:

• Indication of bow door status and inner door status on the navigation bridge.
• Requirement to close bow doors before leaving harbour, with the master responsible for verification.
• Watertight integrity of the bow visor and inner door arrangement to higher standards.
• Operational procedures requiring positive verification of door closure, signed off by the master before sailing.

The casualty was also a foundational driver of the ISM Code (Chapter IX), which addresses the organisational and procedural dimensions that Chapter II-1 prescriptive rules cannot.

MS Estonia, 1994

The ro-ro passenger ferry MS Estonia sank in the Baltic Sea on 28 September 1994 with 852 lives lost. The ship was on a routine overnight crossing from Tallinn to Stockholm in heavy weather (significant wave height approximately 5 metres, wind speed approximately 25 m/s). The investigation concluded that the bow visor failed in heavy weather, with locking devices and hinges yielding under repeated wave impact. After visor failure, the inner ramp also failed, water flooded the vehicle deck and rapidly destabilised the ship through free-surface effect. The vessel sank in approximately 30 minutes.

The casualty drove the Stockholm Agreement of 1996, which raised damage stability requirements for ro-ro passenger ships in Northwest European waters by requiring vessels to survive damage with up to 50 cm of water on the vehicle deck. The agreement was binding on Northwest European states (Sweden, Norway, Denmark, Finland, Estonia, Latvia, Lithuania, Poland, Germany, Netherlands, Belgium, France, UK, Ireland) and the principles were incorporated into general SOLAS amendments through the 2002 and 2009 cycles.

The casualty also contributed to:

• Strengthened bow visor design requirements in IACS unified requirements.
• Routine inspection of bow visor locking and hinge arrangements during periodical surveys.
• Operational restrictions on ro-ro passenger ships in heavy weather.

Costa Concordia, 2012

The Italian-flagged passenger ship Costa Concordia struck a rock near Isola del Giglio on 13 January 2012. 32 lives were lost. The ship was performing an unauthorised salute manoeuvre at 15 knots; the vessel struck a sub-surface rock that opened a 53-metre breach in the port side, flooded multiple compartments, and ran aground.

The casualty exposed weaknesses in passenger evacuation procedures. It drove amendments tightening passenger evacuation requirements in Chapter III and emergency drill requirements, and directly contributed to the SOLAS 2020 amendments (MSC.421(98)) raising the required subdivision index R for new passenger ships. The detailed lessons are captured in MSC.1/Circ.1446 (Lessons learned from incidents and casualties).

The casualty also reinforced the IMO Safe Return to Port philosophy: a ship with a high attained index A and a functional Damage Control Plan was more likely to remain stable long enough to execute a controlled abandonment.

Bulk carrier losses and Chapter XII

A series of bulk carrier losses in the 1990s (including Derbyshire, lost in Typhoon Orchid in 1980 with 44 dead, Marika 7 in 1990, Leros Strength in 1997, and others) led to dedicated amendments in the form of Chapter XII (Additional Safety Measures for Bulk Carriers, 1997 and amended 2002) addressing damage stability, hold strength, fore-end watertight integrity and freeboard. While Chapter XII is technically separate from II-1, it operates on the same engineering base.

The 26-year Derbyshire investigation completed in 2000 attributed the loss to failure of the No. 1 hatch cover, allowing flooding of No. 1 hold and progressive structural failure. The investigation drove strengthened hatch cover design (for ships above 100,000 tonnes deadweight), strengthened forward fore-end deck plate scantlings, and independent forecastle reserve buoyancy requirements.

MV Sewol, 2014

The South Korean ferry MV Sewol capsized on 16 April 2014 with approximately 304 dead. The casualty involved improper modification of the ship to add accommodation (raising the centre of gravity), insufficient ballast for the modified loading condition, improperly secured cargo on the vehicle deck, and an excessive helm input triggering rapid heel that exceeded the modified ship’s reduced stability margins. The casualty was a textbook case of cumulative deficiencies in stability management, post-modification verification, and crew competence.

Modern container ship structural concerns

The MOL Comfort (2013) split in two and sank in the Indian Ocean carrying about 4,500 TEU; the MV ONE Apus (2020) lost approximately 1,800 containers overboard in the Pacific in heavy weather. These and other recent large containership casualties have raised concerns about hull-girder strength under torsional loading at sizes above 20,000 TEU. IMO MSC and IACS continue to address these through unified requirements and SOLAS amendments under development.

Documentation and certification

Stability information booklet

Every ship covered by Part B must carry on board an approved Stability Information Booklet containing the data needed by the master to assess stability in service:

• Lightweight, displacement and centre-of-gravity data from the inclining experiment.
• Hydrostatic data and Bonjean curves: displacement, KM (height of metacentre above keel), KB (height of centre of buoyancy above keel), LCB, LCF, TPC (tonnes per centimetre immersion), MCT 1 cm (moment to change trim 1 cm), waterline area, all as functions of draught.
• Cross curves of stability (KN tables) covering the range of operating drafts and heel angles up to 60 degrees or beyond, used to construct the GZ curve at any displacement and KG.
• Damage stability data showing compliance with Part B for the loading conditions documented, including the attained subdivision index A and the underlying calculation by damage case.
• Critical KG curve, GM minimum curve, or equivalent guidance for the master, showing the maximum allowable centre of gravity height as a function of displacement.
• Loading instructions covering acceptable loading patterns, including any operational limitations imposed by damage stability.
• Tank tables giving capacity and centre of gravity at each fill level for all fuel, lube oil, fresh water, ballast water, slop, and other liquid tanks.
• Free surface moment data for each liquid tank.

The booklet is approved by the flag state at delivery and updated whenever modifications affect lightweight or stability characteristics.

Inclining experiment and lightweight survey

Every ship is required at delivery to undergo an inclining experiment to determine the lightweight displacement and the height of the lightweight centre of gravity (KG). The experiment is conducted in calm conditions with the ship floating freely, weights are shifted across the deck, and the resulting heel is measured.

The KG is computed from the moment created by the shift divided by the displacement times the tangent of the heel angle:

where KG is the height of the centre of gravity above keel, KM is the height of the metacentre above keel (from hydrostatic data), GM is the metacentric height, w is the inclining weight (typically 0.5 to 2 percent of displacement), d is the transverse distance through which the weight is moved, $\Delta$ is the displacement, and $\theta$ is the resulting heel angle.

The inclining experiment must be conducted with:

• Wind speed less than approximately 4 m/s (otherwise wind heel masks weight-induced heel).
• All slack tanks identified and their free-surface moment included in the calculation.
• Persons on board kept constant throughout the experiment.
• Heel measurement using pendulums (typically 4 metres long) suspended in oil dampers, with measurements averaged over multiple weight shifts.

A lightweight survey is repeated at intervals (typically every five years for passenger ships, every ten or fifteen years for cargo ships, with shorter intervals after modifications) to detect changes in lightweight from accumulated marine growth, modifications, repairs and equipment additions or removals.

Loading computer

Modern ships of significant complexity (typically tankers, bulk carriers, container ships, passenger ships) are required to carry an approved loading computer that calculates intact and damage stability for proposed loading conditions in real time. The computer is type-approved by the classification society and validated against the Stability Information Booklet.

Loading computer requirements include:

• Type approval by the classification society or by the flag state, demonstrating accuracy against the booklet across the full range of loading conditions.
• Real-time calculation of displacement, draughts at each station, trim, list, GM (metacentric height) corrected for free surface effect, GZ (righting arm) and area under the GZ curve at the proposed loading, hull-girder bending moment and shear force at each frame, and damage stability (A index) at the proposed loading for each Part B damage case.
• Pre-loading what-if simulation to verify the loading sequence does not exceed strength or stability limits at any intermediate stage.
• Records output for the Cargo Securing Manual and for the bulk loading sequence (BLU Code).

For bulk carriers and oil tankers under Chapter XII, the loading computer is mandatory and includes the bulk loading rate-arm calculation (the bulk loading rate arm calculator implements the corresponding methodology).

Damage stability documentation

Documentation required for the damage stability calculation under Part B includes:

• The damage stability calculation submission for class approval, with compartmentation diagram, permeability assignments per compartment, damage case enumeration with p and s values, attained index A breakdown by damage case and loading condition, and required index R calculation.
• Damage stability instructions for the master, including the loading conditions verified for compliance and the operational restrictions if any.
• The watertight door operational manual and watertight door indication test record.
• For passenger ships constructed on or after 1 January 2020: the damage control plan and drill records required by MSC.421(98).

Approved deviation from prescriptive rules

Where Part F alternative design has been used, the approval documentation includes:

• The engineering analysis demonstrating equivalent safety.
• The flag state approval and any port state notifications.
• The operational restrictions if any apply.
• The maintenance and inspection requirements specific to the alternative arrangement.

See also

• SOLAS Convention parent article
• SOLAS Chapter II-2: Fire Protection, Detection and Extinction
• SOLAS Chapter III: Life-Saving Appliances and Arrangements
• SOLAS Chapter V: Safety of Navigation
• SOLAS Chapter VI: Carriage of Cargoes and Oil Fuels
• SOLAS Chapter XII: Additional Safety Measures for Bulk Carriers
• Damage stability
• Probabilistic damage stability
• Intact stability
• Stockholm Agreement
• GZ curve and righting arm
• Free surface effect
• Subdivision and floodable length
• Hydrostatics and Bonjean
• Cross curves of stability and KN tables
• Metacentric height
• Freeboard and reserve buoyancy
• Load Line
• LNG as marine fuel (IGF Code Part G)
• Methanol as marine fuel
• Ammonia as marine fuel
• Hydrogen as marine fuel
• ISM Code
• Polar Code
• Classification Society
• Hull strength and longitudinal bending
• IGF Code
• IGC Code
• IBC Code

Calculators:

• Bulk, Shore Loading Rate Check Calculator
• Subdivision - Required Index R (cargo ships)
• SOLAS - Probabilistic R for Passenger Ships
• Damage Stability - Multi-Draft A
• Subdivision p-factor
• Subdivision s-factor