Why are ferries with long vehicle decks especially affected by the free surface effect?

The free surface effect is a significant stability hazard for vessels that require large, open interior spaces. In the context of naval architecture, this effect occurs when unconfined liquid—such as floodwater—is able to shift freely across a deck or within a compartment. As the ship heels, the liquid flows toward the lower side, which shifts the vessel's center of gravity and exacerbates the angle of heel, potentially leading to capsizing.1

Ferries are especially susceptible to this phenomenon because their operational requirements necessitate long, expansive vehicle decks that must remain uninterrupted by internal bulkheads. Because these decks lack the structural subdivisions that would otherwise contain or limit the movement of water, any liquid entering the space can move unimpeded across the entire width of the ship. This lack of compartmentalization makes it difficult to mitigate the destabilizing shift of weight, creating a critical vulnerability in the ship's damage stability.1