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DYNAMIC STABILITY





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 the results graphed in the Damage Control Book. As DCA, understanding these Dynamic Stability curves is very important.
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 ships 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 ships surface area (or sail area) decreases when the vessel is inclined.
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)
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.
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.
   
 1. Do Not Submerge Limiting Draft Marks 
 2. No Abnormal Topside Weights 
 3. Follow Liquid Loading Instructions 
 4. Watertight Integrity is Maintained 
   

SURVIVABILITY OF THE SHIP
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.
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 Considerations1. 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 negativeIndications 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!!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 LengthIndications 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 angleIndications 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 damageIndications
- The ship is damaged and adverse weather conditions are prevailingCorrective Measures
- Repair Damage as possible
- Maneuver the ship for favorable weather conditions
Step Four: Eliminate or Reduce List
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 StressIndications 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 (where possible) cracks and areas where structural failure has occurred.
CRITICAL STABILITYThe following table is an easy reference to the four thumbrules of critical stability and actions to be taken:

Critical Thumbrule
Actions to be Taken


Negative GM
FLB, Eliminate FSE/FCE, Shift Weight Down, Add Weight Low, Remove Weight High (Symmetrically)

Flooding Exceeds Floodable Length

FLB, De-Water, Shore Holes and Bulkheads


List to the Danger Angle
FLB, Determine Cause of List, Correct for -GM, Correct for Off-Center Weight


Damage with Bad Weather
FLB, Maneuver Ship Out of Weather, Repair Damage


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