Free Surface Effect
All liquids in partially filled tanks have a free surface, which is free to slop backwards and forward with the motion of the ship. This free surface effect can cause a serious stability problem if the movement of the liquid is not contained. You might like to conduct a simple, practical experiment to demonstrate F.S.E. for yourself:
(a) Take a flat tray with raised sides and partially fill it with water. (A flat baking pan will work.)
(b) Now hold it level, supported by the palms of your hands, held horizontal at arms length and at shoulder height.
(c) Now gently raise your right hand a few centimetres.
As the water runs to the left of the tray/pan you will feel a marked increase in weight, tending to push your left hand down further and so aggravate the condition.
This is Free Surface Effect (F.S.E.). A ship reacts in the same way. It first rolls slightly to a small angle of heel as a result of the wave forces. The internal forces of the shifting water in slack tanks then increase the list further as the liquid flows to the low side. If this F.S.E. causes the vessel to list so that its deck edge is immersed below the waterline, it could well capsize. Fig 4.8 shows a vessel with a partially filled tank. Free surface effect reduces the size of GM. Therefore the size of GZ is reduced, and consequently the ability of the vessel to return to the upright position is reduced.
Free surface effect is at a maximum in tanks which extend right across the breadth of the vessel. By partitioning the tank longitudinally, the flow of liquids to the low side when the ship is heeled can be restricted. It is not removed completely, but the F.S.E. can be reduced to acceptable limits. Obviously, correct loading and ballasting of the ship is also important, but this is an operational consideration and not a design one. Practically all tanks, with the exception of the fore peak ballast tank, are longitudinally subdivided for this reason.
Tank subdivision is effected by a continuous watertight divider extending in a fore and aft direction to each end of the tank and vertically from the inner bottom of the tank to the underside of the tank top.
Fore peak tanks are usually narrow and do not present a very large free surface problem. For this reason, it is unusual to find any longitudinal subdivision in them.
Where tanks are not longitudinally divided by a watertight divider, there are usually longitudinal wash bulkheads which act as baffle plates. While these do not stop the sideways motion of fluids in the tank, they are designed to retard the flow so that the heeling force created by the free surface effect is out of phase with the rolling of the vessel. This tends to damp the vessel's rolling instead of aggravating it, which can be quite beneficial.
The depth or quality of the liquid in the tank does not affect the free surface to any great degree. Free surface area is the main factor. Only a completely empty or completely full tank will have zero free surface.