Can a passenger vessel face a more demanding heeling condition below its maximum passenger capacity? João Henrique Volpini Mattos examines this question through the movement of passengers to one side, drawing on operating experience and comparative stability calculations for six classes of passenger vessel.
The central finding is important for intact-stability assessment: the full-capacity loading condition is not automatically the most severe. Deck arrangement, the initial seated/standing distribution, available free area, displacement and the resulting transverse and vertical centres of gravity can make an intermediate passenger load more critical.
Why passenger movement matters
Passengers may gather on one side for external reasons, such as observing something alongside the vessel or seeking shelter from sun, rain or spray. They may also move because of an onboard emergency, including smoke, fire or a perceived loss of stability. The resulting transverse shift creates a heeling moment that must be considered in the intact-stability assessment.
The IMO 2008 Intact Stability Code requires the angle of heel caused by crowding passengers to one side not to exceed 10 degrees. It also specifies a minimum assumed passenger mass of 75 kg, a vertical centre of gravity 1.0 m above deck for standing passengers and 0.3 m above the seat for seated passengers. Brazilian requirements are applied through NORMAM-201/DPC for open-sea navigation and NORMAM-202/DPC for inland navigation, with criteria that depend on the operating area.
The intermediate-loading problem
A simple arrangement can demonstrate the issue. When a departure condition already includes standing passengers, the free deck area is partly occupied. At a lower passenger count, all passengers may initially be seated, leaving more clear area into which people can move. Depending on the seating plan and deck geometry, that movement can produce a larger transverse moment than the nominal full-capacity case.
For longer voyages on which only seated passengers are permitted, the relationship can be different. This is why the assessment should not rely on passenger count alone: the starting distribution and the practical destination area for a crowd movement are both part of the loading condition.
Rules and calculation basis
The paper uses the Brazilian NORMAM-202/DPC rules for inland navigation, particularly Area 1, as its main calculation basis. The assessment considers:
- the equilibrium angle under the isolated effect of passenger accumulation on one side;
- deck-edge immersion and downflooding limits;
- the relationship between the static-stability curve and the passenger heeling-arm curve;
- the number and distribution of standing and seated passengers;
- the area available at the assumed passenger density; and
- the most unfavourable practical combination of transverse and vertical passenger centres of gravity.
For each deck, the free passenger areas were divided into longitudinal strips. Their areas and centroids were used with the assumed passenger density and mass to develop curves of grouped passenger number against transverse centre of gravity and heeling moment.
Six vessel classes compared
The analysis used documentation for six vessel classes from a passenger-transport fleet operating in Guanabara Bay and Ilha Grande Bay. The selected vessels included monohulls and catamarans, single- and double-deck arrangements, and capacities ranging from 500 to 2,000 passengers.
For each class, two broad conditions were compared: the 100% passenger condition, including seated and standing passengers, and a partial condition containing seated passengers only. Original hydrostatic data, centres of gravity and cross-curves were used.
What the comparison showed
In the seated-only conditions, representing approximately 40% to 60% of total passenger capacity, displacement reduced by about 5% to 15%. The lower displacement and reduced vertical centre of gravity increased initial and large-angle stability measures. At the same time, the passenger movement generated a greater heeling moment in the study cases.
Because the heeling moment acted on a smaller displacement, the resulting heeling arms and equilibrium angles increased. The comparison therefore showed that improved righting-arm characteristics at partial load do not, by themselves, guarantee that the passenger-crowding case is less severe.
Parameters that need careful definition
Available passenger area
The result depends on how occupied seating areas, retractable seats, spaces outside the passenger lounge, cargo areas and normally inaccessible zones are treated. A credible calculation must reflect the vessel’s practical arrangement and the spaces genuinely available to passengers during the scenario.
Passenger vertical centre of gravity
When initially seated passengers stand and move, both transverse and vertical passenger centres of gravity change. If the righting-arm curve is retained from the initial seated condition, the assessment may not fully represent the changed vertical centre of gravity.
Downflooding and deck-edge data
The paper also highlights the importance of clear downflooding-angle and deck-immersion information. Without those limits in the hydrostatic data or stability documentation, later verification of alternative loading conditions becomes difficult.
Passenger density
If the initial standing-passenger density is already close to the assumed grouping density, moving passengers within the same free area may generate only a limited additional moment. Comfort, trip duration and realistic crowd mobility should remain distinct from the regulatory density used for the safety calculation.
Concluding remarks
For vessels that carry seated and standing passengers simultaneously, the paper recommends checking at least one intermediate condition in which passengers are initially seated. The most severe practical combination of passenger number, distribution and available movement area should be identified and included in the stability documentation when it governs.
The wider lesson is that passenger crowding is not a single headcount check. It is a vessel-specific interaction between layout, operating condition, passenger mobility, displacement, righting capacity and the criteria applied by the relevant administration.
Read the complete analysis: Passenger Movement to the Same Side in the Assessment of Intact Stability (PDF).
About the author
João Henrique Volpini Mattos is a Naval Architect and Marine Engineer with a degree from UFRJ and more than 45 years of experience in naval architecture and structural analysis. He worked at DNV and served as technical manager for the CCR Barcas fleet for 13 years. He holds MBAs in Systems Analysis and C#, and is active in the development of engineering software. He currently teaches in the Shipbuilding MBA programs at IPETEC-UCP and remains active in naval engineering projects.
Connect with João on LinkedIn.
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This article is a technical discussion based on the author’s source paper. Vessel-specific decisions should be supported by the applicable rules, administration requirements and approved stability documentation.
