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The free surface effect is one of the mechanisms where a ship can become unstable and capsize. It refers to the tendency of fluids — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as fluids — to move in response to changes in the attitude of a ship's decks in reaction to sea states.

Consider a ship with a single large water tank, located on its centerline. If the tank is full, there is no change in the loading of the mass, or the ship's center of gravity as it rolls from side to side in heavy seas. If the tank is only half-full, however, the water in the tank will respond to the ship's rolls. As the ship rolls to port, for example, the water will move so that much of it is now on the port side of the ship, and will move the ship's center of gravity towards the port. This has the effect of slowing the ship's return to vertical.

What's worse, however, is that if the ship is rolling through the vertical towards the starboard the water in the tank will only begin to move towards the starboard side of the tank after the ship has passed vertical. The water moving in the ship's tank then slams into the starboard side of the ship, often with the effect of causing the ship to heel further over, as that mass hits the bulkheads of the tank. In heavy sea states, this can become a positive feedback loop, causing each rock to become more and more extreme, until the ship capsizes.

To avoid this hazard cargo vessels will use several tanks, not one single one, to minimize the effects of the free surface effect on the ship as a whole. Keeping tanks holding liquids full is another way to minimize the potential for problems.

This is the mechanism which caused the sinking of the RORO ferries M/S Estonia and Herald of Free Enterprise.^{[citation needed]}

• Discussion of the free surface effect on the Transport Canada website
• Possible causes of ferry disaster

• Metacentric height