DCASE MOD 4

The tendency of a ship to rotate one way or the other (to RIGHT ITSELF or overturn)

• Ship is more stable
• Higher CoG = Less Stable

The stability of a ship in the range from 0° TO 7°

• Add weight high OR
• remove weight from below

A general measure of a ship's ability to RESIST CAPSIZING in a given condition of loading

The WORK DONE in heeling a ship to a given angle of heel

“Sliding” longitudinally or front to back

Up, down as if lifted by waves

“Sliding” laterally or side to side

Twisting (rotate about vertical axis)

Up, down, or bow to stern.   (Rotate about transverse axis)

Side to side or port to stbd. (Rotate about longitudinal axis)

The action of a vessel involving a recurrent motion (Longitudinal Axis)

Semi-permanent angle of inclination, caused by external forces

Permanent angle of inclination caused by a shift in the center of gravity, -GM, or both

• Wind
• Rudder Angle
• Waves

A push or a pull. Tends to produce motion or a change in motion.

• Gravitational force. Directed toward the center of the earth
• Long Tons, lbs, etc.

• A force (weight or buoyant force)
• multiplied by a distance.
• FT-LT, FT-POUNDS, etc
• The effect of a moment is to cause rotation of about a point or axis.
• Think of a hinge

• NUMBER OF CUBIC UNITS IN AN OBJECT
• V = L  * B * D

Salt water = 35 FT3/LT

Fresh water = 36 FT3/LT

DFM = 43 FT3/LT

2,240 lbs

2204.72 LBS

2000 lbs

• Condition A (light)
• Condition B (minimum operating condition)
• Condition C (optimum battle)
• Condition D (full load)

• Metacenter (Mother)
• Gravity (Goose)
• Buoyancy (Beats)
• Keel (Kids)

Neutral Buoyancy

• Negative Buoyancy
• (Capsize)

Positive Buoyancy

Exists between Waterline and Main/Weather deck

B follows the waterline

DISPLACEMENT

M moves opposite of B

• POINT AT WHICH THE ENTIRE WEIGHT OF THE SHIP MAY BE CONSIDERED TO BE CONCENTRATED.
• aka average of all weights on board

• G moves toward a weight addition
• & away from a weight removal
• & in the direction of a weight shift

Hopefully on centerline and approximately “near” the waterline (see DC Book, Section IIa for specifics). If not on CL you will have a LIST

GM

Height of Gravity

KM

BM

GZ * Wf

• GM
• aka Metacentric Height

GM increases (G moves down) - Initial Stability increases

YES, even if the star is not on the hull, limiting drafts will be given in Section II(a).

• indicates how much space is open for water to flow within the compartment.
• Reducing Surface Permeability = REDUCED FREE SURFACE EFFECT!

• COMPARTMENT OPEN TO THE SEA
• COMPARTMENT PARTIALLY FLOODED
• COMPARTMENT OFF-CENTERLINE OR “UNEVEN” ABOUT THE CENTERLINE

• Horizontal weight shifts off centerline cause a “virtual rise” in G, reducing GM.
• Factors that reduce FSE?

Compartment size, pocketing, baffle plates, surface permeability, sluice valves, venting.

“Y” the distance from the ship’s centerline to the compartment’s center… it is squared

1) Off centerline or Uneven about centerline.

2) Open to the sea

3) Partially flooded.

KG

The angle of LIST!

• Verify from farthest and highest plug from riser!
• IAW NSTM 997

To understand where our metacentric height is

• Man All Spaces with Sea Valves (access to the sea);
• Space where work was done;
• Spaces in contact with blocks

1)Weight the ship down hard!!!

2)Conduct soundings on ship and surroundings.

3)Determine tons aground.

4)Calculate critical draft.

When the bow/stern crosses the plane of the dock.

• FLOODING OF PINK SPACES WILL DECREASE STABILITY BECAUSE OF:
• ADDED HIGH WEIGHT
• FREE SURFACE EFFECT
• OR BOTH.

The maximum distance within the ship that can be symmetrically flooded without submerging the margin line

• 1. Establish flooding boundaries
• 2. Dewater any space colored pink on the flooding effects diagram
• 3. Size up situation to determine whether stability is CRITICAL
• 4. Eliminate/reduce list

• Ship has negative GM
• Ship is listing to danger angle (1/2 of max GZ)
• Floodable length exceeded
• Damage with bad weather

Where weather deck is almost continuously awash

• Anything >1% LBP
• Actions: Shift center of gravity toward "high" end

Effects of flooding will be compounded

• FLB
• Eliminate FSE/FCE
• Shift weight down
• Add weight low
• Remove weight high

• FLB
• Dewater
• Shore holes and bulkheads

• FLB
• Determine cause of list
• Correct for negative GM
• Correct for off-center weight

• FLB
• Maneuver ship out of weather
• Repair damage

Righting Arms GZ will always be reduced due to G being off centerline.

1)-GM.

2)Listing to danger angle.

3)Floodable length exceeded.

4)Damage with bad weather.

• Limiting Draft Marks not Submerged
• No “Abnormal” Topside Weights
• Liquid Loading Instructions are Followed
• Watertight Integrity is Maintained

• G MOVED OFF CENTERLINE (99%)
• -GM (1%)
• COMBINATION OF -GM AND G OFF CL

• Determine cause of list
• 2. Shift weight transversely to high side
• 3. Add weight to high side or remove weight from low side

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

• Removal of low weights
• 2. Addition of high weights (ice)
• 3. Moving weights upward
• 4. Free Surface Effect
• 5. Free Communication Effect

• Eliminate FSE and FCE
• 2. Add low weight symmetrically
• 3. Remove high weight symmetrically
• 4. Move weight down symmetrically

• RID THE -GM
• ADD WT LOW SYMMETRICALLY
• 3. REMOVE THE OFF CENTER WT.

THE WORK DONE IN HEELING A SHIP TO A GIVEN ANGLE OF HEEL

• INCREASE WEIGHT LOW TO IMPROVE STABILITY
• ELIMINATE EXCESSIVE LIST / TRIM
• COUNTERFLOOD FOLLOWING DAMAGE TO OFF CENTER COMPARTMENT
• EXPLOSION ABSORPTION (CV & CVN)
• GROUNDING “Weigh the ship down hard”
• SUPPRESS FREE SURFACE EFFECT
• AMPHIB OPERATIONS

• Dirty (Fuel + Saltwater)
• Clean (Saltwater only / Separate tanks)
• Automatic (Fuel Compensating)
• Manual (Must turn valves energize pumps)

• GM
• aka metacentric height

A general measure of a ship's ability to RESIST CAPSIZING in a given condition of loading

• The WORK DONE in heeling a ship to a given angle of heel
• i.e. seas, lines, etc.

Rotation about longitudinal axis

• shift in center of gravity
• negative GM
• combination of both

• A FLOATING BODY DISPLACES A VOLUME OF WATER EQUAL IN WEIGHT TO THE WEIGHT OF THE BODY.
• A BODY IMMERSED (OR FLOATING) IN WATER WILL BE BUOYED UP BY A FORCE EQUAL TO THE WEIGHT OF THE WATER DISPLACED.

THE WEIGHT OF THE VOLUME OF WATER THAT THE SHIP'S HULL IS DISPLACING

• G MOVES TOWARDS A WEIGHT ADDITION
• G MOVES AWAY FROM A WEIGHT REMOVAL
• G MOVES IN THE DIRECTION OF A WEIGHT SHIFT

faster and further

Center of Gravity

Height of Metacenter

• Righting Arm
• Proportional to GM

• GZ x Wf
• Wf equal to Buoyancy force

• FSE/FCE
• Added weight high
• Grounding

• Weight removal low
• G goes up

• + if weight moves up
• - if weight moves down

• GG1 * sin theta
• Sin 0 = 0
• Sin 30 = .5
• Sin 90 = 1.0

• GG2 * Cos theta
• Cos 0 = 1.0
• Cos 60 = .5
• Cos 90 = 0

rectangle/square shape

• Pressing up or emptying out tanks
• (Removes FSE)

• Venting
• Baffles
• Sluice Valves

Distance from ship's centerline to compartment center of gravity

Distance Longitudinal Weight moves

• A design feature having draft aft greater than draft fwd.
• -Primarily done to increase plant effectiveness.

The difference between the fwd and aft drafts, in excess of Drag

DC Book

the distance that the drafts fore and aft increase due to a weight addition.

Use ft aft from MP (from diagram)

“IF THE PROPS ARE REVERSED AND THERE IS NO TENDENCY OF THE SHIP TO BACK AWAY FROM THE BEACH, NO FURTHER ATTEMPTS TO MOVE THE SHIP BY MEANS OF THE PROPELLERS SHOULD BE USED.”

• RIG GROUND TACKLE & KEDGE ANCHORS (IF POSSIBLE)
• COORDINATE LIGHTENING SHIP WITH HIGH TIDE
• TAKE A STRAIN ON GROUND TACKLE
• REQUEST SALVAGE ASSISTANCE (SERT, SUPSHIP SALVAGE)

• WEIGH THE SHIP DOWN HARD
• INVESTIGATE FOR DAMAGE (sound tanks/voids)
• DETERMINE TONS AGROUND
• Calculate Critical Draft

• SHIP IS HOGGING OR SAGGING
• -STRESS FRACTURES, CRACKS, "CRINKLING", OR
• PANTING OF BULKHEADS, DECKS AND STIFFENERS

• RELIEVE HOGGING OR SAGGING (Transfer weight around i.e. liquid loading)
• -SHORE UP BULKHEADS/DECKS.
• -REINFORCE WHERE POSSIBLE.

Hull Inspection will be conducted by divers

• NOTE Condition of Screws,
• Rudders,
• Sea Suctions & Discharges,
• Cathodic Protection,
• ANY DAMAGE

• Undocking
• You could have negative GM

• Breaking Up (loss of girder)
• Plunging (loss of longitudinal stability)
• Capsizing
• Bodily Sinkage (force of gravity exceeds force of buoyancy)

Large GM is STIFF and resists rolls

Small GM is TENDER and rolls easily and slowly

APT TO HANG AT THE END OF EACH ROLL BEFORE STARTING UPRIGHT

APT TO LOLL AND FLOP FROM SIDE TO SIDE

• SHIFT CENTER OF GRAVITY TOWARDS "HIGH" END
• Excessive Trim is >1% of LBP

• Follow Liquid Loading Instructions
• No Abnormal Topside Weights
• Don’t Submerge Limiting Draft Marks Prior to Damage
• Maintain Watertight Integrity

• It is done to verify the exact location of the ship's center of gravity (KG).
• Conducted during overhauls and major changes

• Maintain Awareness Of Ship's Liquid Loading Condition. (Full Load - Min Ops)
• Determine The Risks Associated With Violating LLI And Report To CHENG If Necessary.

the process of filling low compartments from the sea to improve ship stability, control list / trim, or increase/decrease draft

systems may be independent (clean ballast) or they may incorporate sections of the fuel and drainage systems (dirty ballast)

• AUTOMATIC
• (FUEL OIL COMPENSATION)
• MANUAL SYSTEMS

• INDEPENDENT (Clean)
• FUEL TANK SYSTEMS (Dirty)