Free surface effect

Free surface effect:

Liquid that only partially fills a compartment is said to have a free surface that tends to remain horizontal (parallel to the waterline). When the ship is inclined, the liquid flows to the lower side (in the direction of inclination), increasing the inclining moment.

Background:

If the tank contains a solid weight, and the ship is inclined, the center of buoyancy shifts in the direction of the inclination and righting arms (GZ) are formed.

Replacing the solid with a liquid of the same weight, when the ship is inclined, the surface of the liquid remains horizontal. This results in a transfer of “a wedge of water,” which is equivalent to a horizontal shift of weight, causing gravity to shift from G₀ to G₂.

The wedge of water transferred increases as the angle of inclination increases, therefore, the center of gravity shifts a different amount for each inclination.

Due to the horizontal shift of the center of gravity, the righting arm is now G₂Z₂. To determine the effect on stability, a vertical line is projected upward through G₂ (see below). Where this line crosses the ship’s centerline is labeled G₃. The righting arm G₃Z₃ is the same length as the righting arm G₂Z₂. Therefore, moving the ship’s center of gravity to position G₂ or G₃ yields the same effect on stability. Movement from G₀ to G₃ is referred to as a Virtual Rise of the center gravity.

To calculate the virtual rise in the center of gravity due to the Free Surface Effect, use the following equation:

B = The breadth (width) of the compartment

L = The length of the compartment

W_F = The ship’s final displacement (after flooding water added)

Factors Effecting Free Surface Effect

Pocketing

Free Surface Effect can be reduced, to some extent, by creating pocketing. Pocketing occurs when the surface of the liquid contacts the top or bottom of the tank, reducing the breadth (B) of the free surface area.

Since the effects of pocketing can not be calculated, it is an indeterminate safety factor. The Free Surface correction will therefore indicate less overall stability than actually exists.

Surface Permeability

Impermeable objects (engines, pumps, piping systems, etc) inside a flooded space project through and above the liquid surface. These objects inhibit the moving water and the “shifting of the wedge” may or may not be complete, thus reducing Free Surface Effect. The impermeable objects also occupy volume, reducing the amount of flooding water (movable weight) that can fill the space.

Swash Bulkheads (Baffle Plates)

In addition to some structural support, these bulkheads are designed to reduce Free Surface Effect. They are longitudinal bulkheads that hinder, but do not prevent, the flow of liquid from side to side as the ship rolls or heels. They are found in tanks, voids, double bottoms, bilges, etc.

Sluice Valves

Free flow (Sluice) valves on tankers allow opposing tanks to be cross-connected. When large, partially filled tanks are connected, Free Surface Effect increases, and the vessel becomes less stable.

Conditions of Free Surface Effect

1. FSE increases with increased length and width of compartment

2. FSE increases when displacement decreases (de-ballasting)

3. FSE is independent of the depth of the liquid