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Chapter II-1 of SOLAScovers a ship's construction as it relates to survival: how it is subdivided into watertight compartments, how much flooding it must withstand, and the machinery and electrical systems that keep it under control. It is the chapter that turns "don't sink" into a set of measurable design outcomes, and it underpins the Cargo Ship Safety Construction Certificate (and, for passenger ships, the Passenger Ship Safety Certificate) that Port State Control checks.
Chapter II-1 is organised in parts: A (general), A-1 (structure of ships), B and B-1 (subdivision and damage stability), C (machinery installations), D (electrical installations), E (additional requirements for periodically unattended machinery spaces), F (alternative design and arrangements), and G (ships using low-flashpoint fuels, via the IGF Code). Most of it is design-and-survey territory, but several parts translate directly into the drills, checks, and equipment a serving crew handles.
A ship is divided by transverse and longitudinal watertight bulkheads into compartments so that, if one or more are bilged, the ship still floats and stays upright enough to be evacuated or recovered. Since 1 January 2009 the standard is probabilistic: the ship's Attained Subdivision Index A must be at least the Required Index R. A is calculated from the probability of each damage location and extent, multiplied by the probability the ship survives that damage. Intact stability is separately governed by the 2008 Intact Stability (IS) Code, made mandatory under II-1. Onboard, this chapter is why the ship carries approved damage control plans and booklets, a stability instrument or approved loading conditions, and why watertight doors, cross-flooding arrangements, and their indicators must be maintained and logged.
Watertight doors below the bulkhead deck must be capable of being closed from the bridge and locally; their position is indicated on the bridge, and drills in their operation are required. Openings in the shell and watertight bulkheads (valves, scuppers, sanitary discharges) have specific closure and non-return requirements. Bilge pumping and drainage arrangements must be able to control flooding in any compartment. These are the systems a duty engineer and deck watch actually operate — hence the emphasis on keeping indicators, alarms, and remote controls working.
Part C sets requirements for main and auxiliary machinery, the means of going astern, boiler and pressure-vessel protection, communications between the bridge and the engine room, and the means of stopping machinery from outside the space. Two features matter most day to day: steering gear (a main and an auxiliary system, with stricter redundancy for large tankers and gas / chemical carriers, plus routine testing before departure) and the requirement that essential machinery can be controlled and, in emergency, stopped safely.
Every ship needs a main source of electrical power sufficient for all services, and an independent emergency source (a generator or, for limited duration, batteries) able to power emergency lighting, navigation lights, communications, fire pumps, watertight-door indication, and other survival-critical services for a defined period after a main-power blackout. Part D also covers protection against electric shock, fire, and other hazards. This is the regulatory basis for the emergency generator test and the black-out recovery drill.
Part E allows periodically unattended machinery spaces (UMS)where alarm, control, and safety systems meet defined standards — the reason many ships run "E0" at night with the engineer on call. For passenger ships built from 2010 that are 120 m or longer or have three or more main vertical zones, Safe Return to Port requires that, after a casualty within a defined threshold, the ship can proceed to port under its own power with essential systems and designated safe areas intact — a major driver of modern passenger-ship redundancy.
Older cargo ships were assessed deterministically — the ship had to survive a defined extent of side damage in specified loading conditions. From 1 January 2009 SOLAS II-1 uses a probabilistic method for most cargo and passenger ships: an Attained Subdivision Index (A), computed from the probability and survivability of every conceivable damage, must be at least the Required Subdivision Index (R). R rises with ship length and, for passenger ships, with the number of persons carried.
Safe Return to Port (SRtP), in force for passenger ships built from 2010 that are 120 m or longer or have three or more main vertical zones, treats the ship as its own best lifeboat. After a casualty (fire or flooding) below a defined threshold, essential systems — propulsion, steering, navigation, communications, and life-support 'safe areas' — must remain available so the ship can return to port under its own power.
SOLAS II-1 requires a main and an auxiliary steering gear so that a single failure does not leave the ship unable to steer. On most cargo ships the auxiliary can be a second power unit sharing the rudder stock; tankers, gas and chemical carriers of 10,000 GT and above have stricter requirements, including the ability to regain steering quickly after a single failure in the power actuating system.