MODULE NO.

9/10 MODULE TITLE Actions Taken on the Different

Ship’s Condition Affecting in Stability

COMPETENCE/S A-II/1 F3.C2: Maintain seaworthiness of the ship

CO1: Calculate ship stability in compliance with the IMO intact

COURSE OUTCOME
stability criteria under all conditions of Loading
A-II/1 F3.C2.KUP1.1: Ship stability: Working knowledge and
KNOWLEDGE application of stability, trim and stress tables, diagrams and
UNDERSTANDING stress calculating equipment
PROFICIENCY

Determine the different actions that should be taken when

the ship is on the following conditions:
LEARNING OUTCOME x state of having the condition of angle of Loll of an unstable ship
x partial loss of intact buoyancy caused by flooding.

Actions Taken on the Different

TOPIC
Ship’s Condition Affecting in Stability

LESSON PRESENTATION & ACTIVITIES

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LIST CONDITION CAUSED BY OFF-CENTER WEIGHT
Whenever the center of gravity of a ship is moved off centerline, the ship will create an
inclining moment. If no external forces are present, the ship will assume a list. This was
discussed in section 4.02 where the cosine correction was applied to the static stability curve.

3/18/2021 LESSON TOPIC:

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 6/18

Righting arms are significantly reduced when the center of gravity is offcenterline. This
reduction results in the ship having a negative righting arm (upsetting arm) at 0� inclination.
Where the corrected curve crosses the horizontal axis, positive righting arms are achieved.
This is the angle of list, the ship will roll about this point.

Possible Causes
1. Unequal distribution of weight on either side of center
line due to loading.
2. Shift of weight transversely.
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3. Addition or removal of weight unsymmetrically about
center line.

How to Recognize
1. Vessel assumes a permanent list to one side only.
2. Vessel has an average roll period about this permanent
list.
3. If known weight is in excess on one side.
4. If a positive GM is known to exist.

Corrective Measures

1. Determine Cause of list first.

2. Shift weight transversely to higher side
3. Add weight to high side or remove weight from low side.
*** NEVER attempt to correct list by the above
methods unless you are certain that GM is positive

***3/18/2021 LESSON TOPIC:

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 7/18
LIST CONDITION CAUSED BY NEGATIVE GM

When a ship's center of gravity moves vertically upwards and slightly above
the Metacenter, the ship will develop a list (or possibly capsize.) The
vessel may also "flop" over, developing the same list to the other side.

Possible Causes
1. Removal of low weight
2. Addition of high weight (ice, volcanic ash)
3. Moving weight upward
4. Free Surface Effect
5. Free Communication Effect

How to Recognize
1. Vessel will not remain upright and will assume a list to
either port or starboard.
2. Vessel "flops" to port or starboard.

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3. Vessel will have a very long, slow roll period about the
angle of list.
4. A small GM is known to exist plus any of the above.

Corrective Measures
1. Eliminate Free Surface and Free Communication Effects.3/18/2021 LESSON TOPIC:
https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 8/18
2. Add low weight symmetrically about centerline.
3. Remove high weight symmetrically.
4. Shift weight down symmetrically.

LIST CONDITON CAUSED BY OFF-CENTER WEIGHT AND NEGATIVE GM

The vessel's stability is reduced by both an increase in the height of the center of gravity and
movement from centerline. A negative GM condition exists, represented by the "uncorrected"
curve. An off-center weight, represented by the cosine curve, is added and a larger list
develops.

Possible Causes
1. A combination of the previous causes of list.

How to Recognize
1. Vessel will assume a permanent list either port or starboard (vessel will not flop).
2. Very slow roll period about this permanent list.
3. The known off-center weight isn�t proportional to the
ship�s list.

Corrective Measures

1. Correct Negative GM first.

a. Eliminate Free Surface and Free Communication

Effects.3/18/2021 LESSON TOPIC:

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 9/18
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b. Shift weight down, add weight low, or jettison weight high.
2. Correct for Gravity Off Centerline
a. Add weight to higher side
b. Remove weight from lower side
c. Shift weight to higher side

*** ALWAYS correct Negative GM prior to shifting weights transversely ***

FLOODABLE LENGTH

Bulkhead Deck - The uppermost deck to which the transverse watertight bulkheads extend
(usually the Damage Control deck.)

Margin Line - An imaginary waterline located three inches below the Bulkhead Deck.

Floodable Length - The maximum distance within the ship that can be flooded without
submerging the margin line

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FLOFFLOOD FLOODABLE LENGTH (continued) There are two different forms that Floodable Length
is discussed in the Damage Control Book. 1. The Curve of Floodable Length: To use this curve, draw a
line parallel to the forward line (60o to vertical) and place it at the baseline of the forward most bulkhead
where flooding exists. Project this line up to the Curve of Floodable Length. Draw a line parallel to the
aft line (60o to vertical) which intersects the Curve of Floodable Length at the same point. The baseline
distance between the forward and after lines is the Floodable Length

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2. A List of Floodable Length Compartment Groups: The Damage Control
Book will list all groups of consecutive compartments which
constitute Floodable Length. Example, for the FFG-7:
Stem - Frame 100
Frames - 32-140
Frames - 64-180
Frames - 100-212
Frames - 140-250
Frames - 180-292
Frames - 212-328
Frames - 250-368
Frames - 292-Stern

General rule of thumb for Floodable Length:

If the ship's LBP is > 300 FT : 15% of LBP (3 spaces **)

< 300 FT : 2 spaces **
< 100 FT : 1 space **
** A space is the area between two transverse watertight bulkheads from keel to waterline,
skin to skin.
Example: For a two compartment ship, flooding any two adjacent spaces will cause the ship to
reach its Floodable Length.

DYNAMIC STABILITY

Dynamic Stability is the ship's ability to resist external heeling forces.

Prior to launching any ship, its Dynamic Stability has been tested, and the3/18/2021 LESSON
TOPIC:

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 12/18

The results graphed in the Damage Control Book. As DCA, understanding these
Dynamic Stability curves is very important.

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The curve, called the Righting Moment curve, begins as the Righting Arm curve. A righting
moment is simply a force (WF) acting through some distance (GZ). Since displacement
remains constant through all angles of heel, the Righting Moment curve retains the same
shape as the Righting Arm curve. The vertical scale is changed by a factor of displacement.
The area under this curve is the Righting Energy the ship possesses, or the

The area under this curve is the Righting Energy the ship possesses, or the ship�s ability to
right itself. Heeling Moment curves can be projected onto this curve to determine the maximum
beam winds and seas the ship can withstand. A beam wind curve looks very similar to the
cosine correction curve because the ship�s surface area (or sail area) decreases when the

DYNAMIC STABILITY (continued)

SEMI-PERMANENT HEEL
Angle to which the ship will heel with a constant beam wind.

MAXIMUM ROLL The largest angle to which the ship will roll in the case of an instantaneous
beam wind.

AREA A Area where the Heeling Moment is greater than the Righting Moment. (Energy that
the wind has, but the ship does not)

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AREA B Area where the Righting Moment is greater than the Heeling Moment. (Energy that
the ship has, but the wind does not)

RESERVE
DYNAMIC
STABILITY

Righting Energy which enables the ship to withstand any additional heeling moments.
When a ship is exposed to heeling moments (100 kts beam wind) the ship inclines. If the
heeling energy is imparted instantaneously, the ship must be able to overcome that energy or
it will capsize. The ship will roll over to the angle of maximum roll, then right itself to where the
righting moment equals the heeling moment (angle of semi-permanent heel). If the wind stays
constant (100 kts), the ship remains at this angle of heel. Reserve Dynamic Stability is
"leftover" righting energy which allows the ship overcome any additional heeling forces (usually
there will be some sea state with 100 kts of wind.)

DESIGN CRITERIA
All Navy and Coast Guard surface ships are designed to withstand certain criteria. The "Law of
15's" is a rule of thumb that sets list/heel angle limits for various operating conditions. For the
following situations, a ship should not list or heel past

15o.3/18/2021 LESSON TOPIC:

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 14/18

1. Design Beam Winds and Seas

2. Hanging Heavy Objects over the Side
3. Crowding of Personnel to one Side
4. List after Damage
5. Full Speed, Full Rudder Turn
6. Flooding 15% LBP (Floodable Length)

LIMITATIONS TO DESIGN CRITERIA

Should any of the following four limitations be violated, the ship will not
meet its designed resistance to damage.

SURVIVABILITY OF THE SHIP

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If the ship lists to the Danger Angle ( the angle of maximum righting arm) within 10-15 minutes
after damage, the ship will probably capsize. If the ship lists past the angle of maximum
righting arm, it will capsize. Under normal conditions (average wind and sea state,) the ship
should survive damage which results in a list to the danger angle.

3/18/2021 LESSON

TOPIC:
https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 15/18

DCA DECISION FACTORS

Following damage, the DCA evaluates the overall situation and makes
recommendations to the Commanding Officer based on the survivability of the ship.
This includes:
1. Determine the corrective measures which will improve the situation.
2. If corrective measures will not improve the situation, inform the CO so he/she can decide
whether to abandon ship.

Four Major Considerations

1. Ship's ability to extinguish fires and control flooding.
2. Ship's ability to reach a safe haven.
3. Ship's ability to float and remain upright.
4. Ship's ability to stay in action and repel attack.

Corrective Measures

Once initial recommendations have been given to the Commanding Officer, the
DCA prioritizes the corrective measures.

Step One: Establish Flooding Boundaries.

Step Two: Dewater any space colored pink on the ships Flooding Effects Diagram.

Step Three: Size up the situation.

Determine if stability is critical before taking further action.

There are four instances where stability is considered critical:

1. GM is very small or slightly negative

Indications of Negative GM
- Feel of the Ship:Logy, Sluggish? List With No Off-Center Weight? Flopping From Side to
Side?
- Large Areas of FSE or FCE?
- Large Amounts of Added High Weight? If unsure, assume GM is negative!!

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3/18/2021 LESSON TOPIC:
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Corrective Measures for Negative GM

- Eliminate Free Surface and Free Communication Effects

- Jettison Top Side Weight
- Ballast Low Tanks
- Shift Solid Weights Down
- Restore Flooding Boundaries

2. Flooding exceeds Floodable Length

Indications that Floodable Length is exceeded
- Excessive Flooding (15% LBP)
- Minimal Freeboard

Corrective Measures for Floodable Length Exceeded

- Verify Flooding Boundaries

- Plug and Patch Damage
- De-water
3. List to the danger angle Indications of Danger Angle List
- Ships inclinometer

Corrective Measures for Danger Angle List

- Determine the cause of the list

- If due to -GM, move G down
- Otherwise shift G back to centerline

4. High winds or rough seas combined with flooding damage

Indications
- The ship is damaged and adverse weather conditions are
prevailing

Corrective Measures

- Maneuver the ship for favorable weather conditions

Repair Damage as possible3/18/2021 LESSON TOPIC:

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Step Four: Eliminate or Reduce List

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Methods to Correct List (With positive GM)Indications
- Vessel has a list with positive GM
- Vessel has known off-center weights which correspond to the angle of list.

Corrective Measures

- Pump Out Off-Center Flooding

- Shift Liquids Transversely
- Counterflooding
- Shift Solid Weights Transversely
- Deballast Wing Tanks

Methods to Correct Excessive Trim (Greater than 1% LBP)

Corrective Measures

- Shift weight towards higher end (bow or stern); Add weight to higher end; Remove weight
from lower end.

Methods to Relieve Hull Girder Stress

Indications of Excessive Hull Stresses
- Ship is in Hogging or Sagging condition as described in Section 4.5.
- Stress Fractures, Cracks, Panting Bulkheads, Sagging Decks and Stiffeners.

Corrective Measures for Hull Stresses

- For Sagging condition: Remove weight amidships and ballast fore and aft.
- For Hogging condition remove weight fore and aft, and ballast amidships.
- Shore up panting bulkheads and decks. Reinforce (wherepossible) cracks and areas where
structural failure has occurred.

CRITICAL STABILITY3/18/2021 LESSON TOPIC:

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html 18/18

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 [1] What new information have I learned from the activities?
[2] What surprised me about what I learned?

Read next topic Stress Table and Principles of Floatation

References (APA)

https://fas.org/man/dod-101/navy/docs/swos/dca/stg4-06.html

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