The free surface effect[1] is one of several mechanisms which can cause a craft to become unstable and roll over (capsize). It refers to the tendency of liquids — and of aggregates of small solid objects, like seeds, gravel, or crushed ore which can act as liquids — to slosh about: to move in response to changes in the attitude of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea states caused by waves & wind acting upon the craft). When referring to the free surface effect, the condition of a tank that is not completely full is described as a "slack tank", while a full tank is "pressed up". [2]
In a normally laden 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. This assumes the center of gravity of the vessel is relatively constant. If a moving mass inside the vessel moves in the direction of the roll, this counters the righting effect by moving the center of gravity towards the lowered side. The free surface effect can become a problem in a craft with large partially-full bulk cargo compartments, fuel or water tanks, (especially if they are spanning the full breadth of the ship), or from accidental flooding, such as has occurred in several accidents involving RORO type ferries. If a compartment or tank is either empty or full, there is no change in the craft's center of mass as it rolls from side to side (in strong winds, heavy seas, or on sharp motions or turns). However, if the compartment is only partially full, the liquid in the compartment will respond to the vessel's heave, pitch, roll, surge, sway or yaw. For example, as a vessel rolls to the left, liquid will displace to the left side of a compartment, and this will move the vessel's center of mass towards the left. This has the effect of slowing the vessel's return to vertical. Also, the momentum of large volumes of moving liquids cause significant dynamic forces, which act against the righting effect. When the vessel returns to vertical the roll continues and the effect is repeated on the opposite side. In heavy sea states, 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. While repeated oscillations of increasing magnitude are commonly associated with the free surface effect, they are not a necessary condition. 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.
To mitigate this hazard, cargo vessels use multiple smaller bulk compartments or liquid tanks, instead of fewer larger ones, and possibly baffles within bulk compartments or liquid tanks to minimize the free surface effects on the craft as a whole. Keeping individual bulk compartments or liquid tanks either relatively empty or full is another way to minimize the effect and its attendant problems. Tanks or compartments that do not straddle the vessel's centerline are somewhat less prone to destabilising oscillations. Similarly, narrow compartments (aligned bow to stern) and compartments at the extremes away from the centerline, are less prone to cause instability.
Flooding, liquid cargo leakage, or unintended water (from precipitation or waves) 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), the MS Herald of Free Enterprise (Zeebrugge, Belgium, March 1987), and MS Estonia (Baltic Sea, September 1994). In the case of the RORO ferry MS al-Salam Boccaccio 98 (Red Sea, February 2006) improper fire-fighting procedures caused flooding leading directly to instability and capsize, which resulted in over a thousand fatalities (388 rescued from 1312 passengers + 96 crew).
Free surface effect can affect any kind of craft: ranging from watercraft (where it is most common), to bulk cargo or liquid tanker semi-trailers and trucks (causing either jackknifing or roll-overs), or aircraft (especially fire-fighting water-drop and refueling tankers where baffles mitigate but do not eliminate the effects). The term "free surface effect" implies a liquid under the influence of gravity. Slosh dynamics is the overarching field which covers both free surface effects and situations such as space vehicles, where gravity is inconsequential, but inertia and momentum interact with complex fluid mechanics to cause vehicle instability.