Free surface effect

[2] In a normally loaded vessel any rolling from perpendicular is countered by a righting moment generated from the increased volume of water displaced by the hull on the lowered side.

In heavy seas, this can become a positive feedback loop, causing each roll to become more and more extreme, eventually overcoming the righting effect leading to a capsize.

For example, in the cases of both the SS Normandie and MS al-Salam Boccaccio 98, gradual buildup of water from fire-fighting caused capsizing in a single continuous roll.

Flooding, liquid cargo leakage, or unintended water (from precipitation, waves, or hull damage) in any compartments or on any decks of watercraft, and the resulting free surface effect are often a contributing cause of accidents, capsizes, and casualties e.g. the loss of TEV Wahine (Wellington, New Zealand, April 1968), MS Herald of Free Enterprise (Zeebrugge, Belgium, March 1987), and MS Estonia (Baltic Sea, September 1994).

In the case of the RORO ferry al-Salam Boccaccio 98 (Red Sea, February 2006), improper fire-fighting procedures caused flooding leading directly to instability and capsize.