Damage Control TrainingStability and Buoyancy Lessons

LESSONTOPIC: 4.6TITLE:HULLDAMAGEASSESSMENT/LIST INSTABILITY

Contact periods allotted this LESSON TOPIC:Classroom: 2.0 Test: 0.0Trainer: 0.0 Total: 2.0MEDIA: Classroom lecture with visual mediaTERMINAL OBJECTIVES:

6.0 EVALUATE shipboard stability by analyzing weight andmoment considerations. (JTI 3.2.1, 6.0, 6.1, 6.2)

ENABLING OBJECTIVES:6.33 DESCRIBE the relationship between metacentric heightand roll period.6.34 DEFINE floodable length, angle of maximum roll, angleof semi-permanent heel, and reserve dynamic stability.6.35 DESCRIBE the three possible causes for list afterdamage and the indicators for each.6.36 Given a DC book section II(a), CALCULATE danger angle.6.37 DESCRIBE the survivability design considerationsoutlined in NAVSEA DDS-079-1 and NAVSEAINST 9096.3(series)which enable the ship to withstand external, dynamic forcesand damage.6.38 Given a written summary of damage, a description of theships behavior and list, EXPLAIN the cause of the list andthe procedures necessary to eliminate or reduce the list.6.39 DESCRIBE the intended method of disabling a ship as itapplies to: homing influence torpedoes, impact (WWII)torpedoes, anti-ship missiles, influence and contact mines.

6.40 Given a ship's beam and average roll period in calmwater, use the Roll period equation to CALCULATE the ship'smetacentric height.6.41 DRAW a righting moment curve with a heeling momentcurve that identifies angle of maximum roll, angle of semi-permanent heel, and reserve dynamic stability remaining.

SITUATION ESTIMATE AFTER DAMAGETwo possible situations exist following the infliction of damage:

Damage is so extensive that the ship never stops listing,trimming, or settling, and goes down within minutes

orThe ship stops heeling, changing trim, and settling shortlyafter initial damage.

Experience has shown that the loss of ships lasting several hours afterdamage and then sinking is directly traceable to progressive flooding.

CAUSES OF LOSS1. Flooding

a. Bodily Sinkage - The addition of weight (flooding water) hascaused the force of gravity to exceed the force of buoyancy.

b. Capsizing - Loss of transverse stability: the incliningmoment exceeds the righting moment (IM > RM) and the ship rollsover.

c. Plunging - Loss of longitudinal stability: the trimmingmoment exceeds the longitudinal righting moment (TM > RM) andthe ship sinks by the bow or stern.

2. Breaking Up - Caused by strength member failure

a. Excessive stresses from hull damage or improper distributionof liquid (failure to follow the ships Liquid LoadingInstructions (LLI).

b. Violent longitudinal whipping (ex: grounding at high speed,underwater detonation.)

c. Fire: Extreme thermal effects on structural members.

METACENTRIC HEIGHT AND STABILITYIn lesson 4.01 the relationship between Metacentric Height (GM) and rightingarms (GZ) was discussed. For small angles of heel (0-7/10) GZ isproportional to GM. Therefore, GM can be used as a representation of initialrighting arms. These basic rules apply:

If GM is large, the ship has large righting arms and will havestiff, fast rolls.If GM is small, the ship has small righting arms and will havetender, slow rolls.If GM is very small, the ship is apt to hang at the end of each rollbefore starting upright.If GM is slightly negative, the ship will loll (stay heeled at theangle of inclination where righting and upsetting forces are equal)and flop from side to side.If GM is negative, the ship will capsize when inclined.

SHIPS ROLL PERIODThe ship's roll period is directly related to the Metacentric Height of yourvessel. Should you desire an estimate of your Metacentric Height, thefollowing method works well when underway.The ship must be making way, approximately 8-10 knots on a relatively calmday. Have the OOD cycle the rudder, from right full to left full or viceversa. Using a stop watch, determine how long it takes to go from 0inclination to full inclination to starboard, back to 0 inclination, to fullinclination to port and back to 0 inclination. This is the ships rollperiod. Repeat if necessary to ensure that the time is as accurate aspossible.

Where:B = Maximum Beam of the Ship (FT)T = Period of Roll (Seconds)GM = Metacentric Height (FT)

LIST IN STABILITY

HEEL - A heel is a "temporary" inclination of a ship, causedby outside forces such as winds, waves, or during a ship'sturn. LIST - A list is a "permanent" inclination of a ship, causedby one of the following conditions:

1. Off-Center Weight (99% of the time)2. Negative GM (1% of the time)3. Combination of Off-Center Weight and -GM

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

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

Possible Causes1. Unequal distribution of weight on either side of centerline due to loading.

2. Shift of weight transversely.3. Addition or removal of weight unsymmetrically aboutcenter line.

How to Recognize1. Vessel assumes a permanent list to one side only.

2. Vessel has an average roll period about this permanentlist.

3. If known weight is in excess on one side.4. If a positive GM is known to exist.

Corrective Measures1. Determine Cause of list first.2. Shift weight transversely to higher side3. Add weight to high side or remove weight from low side.

*** NEVER attempt to correct list by the abovemethods unless you are certain that GM is positive***LIST CAUSED BY NEGATIVE GMWhen a ship's center of gravity moves vertically upwards and slightly abovethe Metacenter, the ship will develop a list (or possibly capsize.) Thevessel may also "flop" over, developing the same list to the other side.

Possible Causes1. Removal of low weight2. Addition of high weight (ice, volcanic ash)3. Moving weight upward4. Free Surface Effect5. Free Communication Effect

How to Recognize1. Vessel will not remain upright and will assume a list toeither port or starboard.

2. Vessel "flops" to port or starboard.3. Vessel will have a very long, slow roll period about theangle of list.

4. A small GM is known to exist plus any of the above.

Corrective Measures1. Eliminate Free Surface and Free Communication Effects.2. Add low weight symmetrically about centerline.3. Remove high weight symmetrically.4. Shift weight down symmetrically.

LIST CAUSED BY OFF-CENTER WEIGHT AND NEGATIVE GM

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

Possible Causes1. A combination of the previous causes of list.

How to Recognize1. Vessel will assume a permanent list either port orstarboard (vessel will not flop).

2. Very slow roll period about this permanent list.3. The known off-center weight isnt proportional to theships list.

Corrective Measures1. Correct Negative GM first.

a. Eliminate Free Surface and Free CommunicationEffects.b. Shift weight down, add weight low, or jettison weighthigh.

2. Correct for Gravity Off Centerlinea. Add weight to higher sideb. Remove weight from lower sidec. Shift weight to higher side

*** ALWAYS correct Negative GM prior to shifting

weights transversely ***FLOODABLE LENGTH

Bulkhead Deck - The uppermost deck to whichthe transverse watertight bulkheads extend(usually the Damage Control deck.) Margin Line - An imaginary waterlinelocated three inches below the BulkheadDeck. Floodable Length - The maximum distancewithin the ship that can be flooded withoutsubmerging the margin line.

1

FLOFFLOOD FLOODABLE LENGTH (continued)

There are two different forms that Floodable Length is discussed in theDamage Control Book.

1. The Curve of Floodable Length: To use this curve, draw a lineparallel to the forward line (60o to vertical) and place it at thebaseline of the forward most bulkhead where flooding exists. Projectthis line up to the Curve of Floodable Length. Draw a line parallelto the aft line (60o to vertical) which intersects the Curve ofFloodable Length at the same point. The baseline distance betweenthe forward and after lines is the Floodable Length.

2. A List of Floodable Length Compartment Groups: The Damage ControlBook will list all groups of consecutive compartments whichconstitute Floodable Length. Example, for the FFG-7:

Stem - Frame 100Frames - 32-140Frames - 64-180Frames - 100-212Frames - 140-250Frames - 180-292Frames - 212-328Frames - 250-368Frames - 292-SternGeneral 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 bulkheadsfrom keel to waterline, skin to skin.

Example: For a two compartment ship, flooding any two adjacent spaces willcause the ship to reach its Floodable Length.

DYNAMIC STABILITYDynamic Stability is the ship's ability to resist external heeling forces.Prior to launching any ship, its Dynamic Stability has been tested, and theresults graphed in the Damage Control Book. As DCA, understanding theseDynamic Stability curves is very important.The curve, called the Righting Moment curve, begins as the Righting Armcurve. A righting moment is simply a force (WF) acting through some distance(GZ). Since displacement remains constant through all angles of heel, theRighting Moment curve retains the same shape as the Righting Arm curve. Thevertical scale is changed by a factor of displacement.

The area under this curve is the Righting Energy the ship possesses, or theships ability to right itself. Heeling Moment curves can be projected ontothis curve to determine the maximum beam winds and seas the ship canwithstand. A beam wind curve looks very similar to the cosine correctioncurve because the ships surface area (or sail area) decreases when thevessel is inclined.

DYNAMIC STABILITY (continued)

SEMI-PERMANENTHEEL

Angle to which the ship will heel with aconstant beam wind.

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

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

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

RESERVEDYNAMICSTABILITY

Righting Energy which enables the ship towithstand any additional heeling moments.

When a ship is exposed to heeling moments (100 kts beam wind) the shipinclines. If the heeling energy is imparted instantaneously, the ship must beable to overcome that energy or it will capsize. The ship will roll over tothe angle of maximum roll, then right itself to where the righting momentequals the heeling moment (angle of semi-permanent heel). If the wind staysconstant (100 kts), the ship remains at this angle of heel. Reserve DynamicStability is "leftover" righting energy which allows the ship overcome anyadditional heeling forces (usually there will be some sea state with 100 ktsof wind.)

DESIGN CRITERIAAll Navy and Coast Guard surface ships are designed to withstand certaincriteria. The "Law of 15's" is a rule of thumb that sets list/heel anglelimits for various operating conditions. For the following situations, a shipshould not list or heel past 15o.

1. Design Beam Winds and Seas2. Hanging Heavy Objects over the Side3. Crowding of Personnel to one Side4. List after Damage5. Full Speed, Full Rudder Turn6. Flooding 15% LBP (Floodable Length)

LIMITATIONS TO DESIGN CRITERIAShould any of the following four limitations be violated, the ship will notmeet its designed resistance to damage.

1. Do Not Submerge Limiting DraftMarks

2. No Abnormal Topside Weights

3. Follow Liquid Loading Instructions

4. Watertight Integrity is Maintained

SURVIVABILITY OF THE SHIPIf 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 theship lists past the angle of maximum righting arm, it will capsize. Undernormal conditions (average wind and sea state,) the ship should survivedamage which results in a list to the danger angle.

DCA DECISION FACTORSFollowing damage, the DCA evaluates the overall situation and makesrecommendations to the Commanding Officer based on the survivability of theship. This includes:

1. Determine the corrective measures which will improve thesituation.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 MeasuresOnce initial recommendations have been given to the Commanding Officer, theDCA prioritizes the corrective measures.

Step One: Establish Flooding Boundaries.Step Two: Dewater any space colored pink on theships Flooding Effects Diagram.

Step Three: Size up the situation.Determine if stability is critical beforetaking 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 FreeboardCorrective Measures for Floodable Length Exceeded- Verify Flooding Boundaries- Plug and Patch Damage- De-water3. 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 areprevailing

Corrective Measures- Repair Damage as possible- Maneuver the ship for favorable weather conditions

Step Four: Eliminate or Reduce ListMethods to Correct List (With positive GM)Indications- Vessel has a list with positive GM

- Vessel has known off-center weights which correspondto the angle of list.

Corrective Measures- Pump Out Off-Center Flooding- Shift Liquids Transversely- Counterflooding- Shift Solid Weights Transversely- Deballast Wing TanksMethods to Correct Excessive Trim (Greater than 1% LBP)Corrective Measures

- Shift weight towards higher end (bow or stern); Addweight 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 describedin Section 4.5.- Stress Fractures, Cracks, Panting Bulkheads, SaggingDecks and Stiffeners.

Corrective Measures for Hull Stresses- For Sagging condition: Remove weight amidships andballast fore and aft.- For Hogging condition remove weight fore and aft, andballast amidships.

- Shore up panting bulkheads and decks. Reinforce (wherepossible) cracks and areas where structural failure hasoccurred.

CRITICAL STABILITYThe following table is an easy reference to the four thumbrules of criticalstability and actions to be taken:

CriticalThumbrule

Actions to be Taken

Negative GM

FLB, Eliminate FSE/FCE, ShiftWeight Down, Add Weight Low,Remove Weight High(Symmetrically)

FloodingExceeds

FloodableLength

FLB, De-Water, Shore Holesand Bulkheads

List to theDangerAngle

FLB, Determine Cause of List,Correct for -GM, Correct forOff-Center Weight

Damage withBad Weather

FLB, Maneuver Ship Out ofWeather, Repair Damage