What is Free Surface Effect?
Free surface effect is the destabilizing effect of liquids on ships.
When a partially filled tank also known as slack tank is tilted, the liquid inside the tank moves to the lower side of the tank. If not corrected or if the mass of the liquid is very high, then the ship can capsize. Hence, it is an important effect to consider.
Note–This effect only occurs on liquid cargo vessels is a common misconception.
Free Surface Effect Diagram
On board dry cargo ships such as containers, RORO or bulk, the slack tank are the ballast tanks. Since the vessel Is already loaded, the ballast tank may not be at full capacity.
 Represents diagram before FSE and  represents diagram after of FSE
M is center of mass
G is center of gravity
B is center of buoyancy
K is keel
WL is waterline, in (d) it’s the before waterline and W1L1 is the current waterline after tilting.
Theta is the angle at which the ship is tilted.
As you can see the liquid has shifted to right side, so the center of gravity and buoyancy will also shift to the right.
How to reduce Free Surface Effect?
We can counter this effect by:
- Avoiding slack tanks i.e.: Fully filling the tanks so the movement of liquid is restricted.
- Dividing the tank by its breadth so the effect is marginally reduced. Like introducing longitudinal divisions/bulkhead.
Here the breadth is b/3 instead of b. So, the FSE will be reduced by a factor of cube. This is why large tanks onboard has divisions such as port, starboard and central tank.
- Pocketing. It’s the effect when the liquid meets the top/bottom of the tank. This causes the liquid to move over a small length.