Damage Stability Fundamentals

Intact stability assumes the yacht has not suffered the specified hull damage considered by damage-stability analysis. Damage stability begins after that assumption changes. One or more spaces are treated as opened to the sea under defined damage scenarios, and the naval architect determines the new displacement, heel, trim, waterline and remaining righting ability. The question is not whether the yacht behaves exactly as it did intact, but whether the damaged condition satisfies the applicable survival criteria.

Watertight bulkheads, decks and closures limit the amount of the hull that can flood after damage. Without subdivision, water entering one region could spread throughout a much larger volume and rapidly remove reserve buoyancy. Damage stability therefore relies on the physical subdivision system described in the preceding subject. The calculation tests how that system performs when particular compartments are assumed to be breached.

When water enters a damaged space, the hydrostatic problem changes substantially. The flooded region may be represented through lost-buoyancy or added-weight methods depending on the analysis. In either case, the yacht's effective buoyancy, displacement, centre of gravity and free-surface behaviour can change. The vessel then sinks, trims or heels until forces and moments reach a new equilibrium if a stable equilibrium remains.

A compartment is rarely an empty geometric box. Structure, machinery, furniture and equipment occupy some of its volume, leaving only part available to seawater. Damage-stability methods therefore use permeability assumptions appropriate to the space type and governing standard. Permeability influences the amount of water associated with flooding and consequently the final damaged displacement and equilibrium.

Damage on one side of a yacht can flood an off-centre volume and create a strong transverse moment. The yacht may heel until the shifted buoyancy and weight forces establish a new balance. That heel can immerse additional openings, reduce residual righting ability and affect escape or machinery operation. Damage analysis therefore evaluates transverse equilibrium rather than considering only whether enough total buoyancy remains to keep the deck above water.

Flooding near the bow or stern changes the longitudinal balance of buoyancy and weight. The resulting damaged equilibrium can include substantial bow-down or stern-down trim as well as overall sinkage. That change in waterline influences which openings become immersed and how much reserve buoyancy remains elsewhere. Damage stability is therefore fully three-dimensional even when the original hull damage is local.

After the damaged yacht reaches an equilibrium condition, its righting-arm behaviour can be recalculated over a range of heel. The resulting residual GZ curve describes the remaining ability to resist further heeling. Applicable criteria can consider the magnitude and range of positive righting arms, areas under the curve and limiting openings or angles. A yacht that merely remains afloat can still fail damage-stability requirements if its residual stability is inadequate.

Flooding does not always occur instantaneously to one final condition. Water can pass through openings or cross-flooding arrangements over time, creating intermediate stages with different heel and stability characteristics. Some governing methods therefore require survival to be checked during the progression of flooding as well as at final equilibrium. A final acceptable condition cannot automatically excuse a dangerous intermediate stage.

Deterministic damage rules analyse defined damage extents or specified combinations of flooded compartments. Probabilistic systems account for the likelihood of different damage locations and extents together with the probability of survival after each case. The mathematical approach differs, but both connect subdivision with the vessel's capacity to remain afloat and stable after damage.

The analysis depends on the real watertight arrangement, openings, tank configuration, loading conditions and lightship properties. A refit that changes a shell opening, bulkhead, tank, permanent weight or centre of gravity can therefore alter the damage-stability basis. Approved calculations and damage-control information should remain aligned with the physical yacht throughout its operating life.