Why flooding changes everything — and fast
When a compartment floods, the vessel loses reserve buoyancy and gains free surface effect simultaneously. These two changes attack stability from opposite directions: buoyancy is reduced in the flooded volume, and if the flood water is not fully admitted (i.e., a partially filled space), free surface moment reduces the righting lever (GZ) across the entire range. A Master must understand this not to pass a stability exam, but because the decisions taken in the first minutes determine whether the vessel is recoverable.
The physics driving your decision
Lost buoyancy method — what actually happens The flooded compartment no longer contributes buoyancy. The vessel sinks bodily and trims toward the flooded space until the remaining intact waterplane and underwater volume re-establish equilibrium. The result is: increased draught, altered trim, and a reduced GM.
Free surface effect Any partially flooded space creates a free surface. The free surface moment (volume of flood water × density × moment of inertia of the water surface) is added to KG, raising the effective centre of gravity. On a yacht — with a relatively narrow, high-sided hull — this reduction in GM can be dramatic and rapid. A space that is either empty or completely full has no free surface; a space 50% flooded has the maximum effect.
Progressive flooding Floodwater does not respect watertight boundaries if openings, ventilation trunks, cable penetrations, or cross-connections allow passage. The Master must mentally map every route water can travel beyond the initially flooded compartment.
Trim effects and their consequences
Stern flooding trims the bow up: propeller may ventilate, rudder effectiveness reduces. Bow flooding trims the bow down: steerage improves initially but freeboard forward is lost quickly. Asymmetric (off-centreline) flooding immediately introduces a list which reduces the wall-sided righting lever and, critically, brings the deck edge closer to the waterline on the low side.
The Master's response — in command order
- Assess and contain — establish the rate of ingress, identify the source, close all watertight doors and openings to limit progressive flooding. Do not open adjacent spaces to investigate without first determining they are above the floodline.
- Counter-flood or ballast? — deliberately flooding an opposing tank to correct a list must only be done after confirming it will not worsen overall stability or accelerate sinking. This is a command decision requiring a stability calculation or reference to the vessel's damage stability information if available.
- Dewater — deploy pumps, but do not assume pumping is ahead of ingress without measuring both rates.
- Reassess survivability — consult the vessel's stability booklet for any damage stability data. If the vessel is approaching a critical angle of loll or residual GM is in doubt, prepare for abandonment before the situation is irreversible.
- Reduce free surface — either pump the space dry or flood it completely. A fully flooded but contained compartment has no free surface and may be the more stable outcome.
- Communicate — transmit a DSC Distress or Urgency call as appropriate; do not delay this while attempting repairs.