LESSON TOPIC: 4.3 TITLE: LOOSE WATER

Contact periods allotted this LESSON TOPIC:

Classroom: 1.5 Test: 0.0

Trainer: 0.5 Total: 2.0

TERMINAL OBJECTIVES:

ENABLING OBJECTIVES:

Loose Water is the shifting of liquid from side to side as a ship rolls. Water that partially fills a compartment, as a result of underwater damage, drainage, or fire fighting, is Loose Water.

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.

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:

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.

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.

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 allow opposing tanks to be cross-connected. When large, partially filled tanks are connected, Free Surface Effect increases, and the vessel becomes less stable. Ships like oilers and tenders use these valves to create long, slow roll periods during ammunition handling and refueling.

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

The firemain ruptures, flooding space 1-130-0-L with three feet of saltwater. Displacement prior to flooding was 4530 LT. The dimensions of the space are: L=30FT B=42FT H=8FT

1. Calculate the weight added by the flooding water:

2. Calculate the new displacement:

3. Calculate the virtual rise in G due to Free Surface Effect:

Free Communication Effect occurs when the ship’s hull is ruptured, allowing sea water to flow in and out as the ship rolls. This continuous weight addition and removal causes a horizontal shift in the center of gravity, which then equates to another virtual rise in the center gravity.

Three conditions must exist for Free Communication Effect:

When the vessel below is inclined, it experiences a horizontal weight shift due to the Free Surface Effect. The center of gravity shifts from G₀ to G₂. The center of gravity is shifted further from centerline due to the flooding weight addition/removal as the ship rolls. This reduces the righting arm from G₂Z₂ to G₄Z₄. By extending the line of gravitational force up to the centerline, position G₅ is found. This increase from G₃ to G₅ is the virtual rise of gravity due to the Free Communication Effect.

The virtual rise in the center of gravity due to the Free Communication Effect (G₃G₅) is found using the equation:

The factors which minimize Free Surface Effect (pocketing, surface permeability, swash bulkheads, etc) will also minimize Free Communication Effect. There is one additional factor associated with Free Communication: the size of the hole in the ship.

How the size of the hole affects Free Communication is not something that can be calculated. The FCE equation does not account for the hole. Basically, if the hole is small, less water will be added/removed to/from the ship. The larger the hole, the closer Free Communication Effect is to it’s calculated value.

The vessel below (viewed from stern) has a hole in the starboard side of compartment 3-82-0-L. Displacement prior to damage was 3700 LT. Flooding depth is 5 FT. Calculate the total virtual rise in the center of gravity (FSE + FCE). Compartment length is 30 FT.

1. Calculate the weight added due to flooding water:

2. Calculate the ship's final displacement:

3. Calculate the virtual rise in G due to Free Surface Effect:

6. Calculate the total virtual rise in the center of gravity: