Stability lrg

Grunt Productions 2007

A Brief By Lance GrindleyA Brief By Lance Grindley

VESSEL STABILITYVESSEL STABILITY



Stability - the tendency of a vessel to rotate one way or the other (to right itselfright itself or overturn)

Initial Stability - the stability of a vessel in the range from 0° to 7°/10°0° to 7°/10°

Definitions


Definitions

Overall Stability - a general measure of a vessel's ability to resist capsizingresist capsizing in a given condition of loading

Dynamic Stability - the work donework done in heeling a vessel to a given angle of heel


Roll – side to side or port to starboard. (Rotate about longitudinal axis) Pitch – up down or bow to stern. (Rotate about transverse axis)Yaw – twisting (rotate about vertical axis)

Sway – “sliding” laterally or side to side

Heave – up down as in lifted by waves.

Surge – “sliding” longitudinally or front back

Six Motions Of A Ship


Definitions

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

HEEL - Semi-permanent angle of inclination, caused by external forces.

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


Laws Of Buoyancy

• A floating object has the property of buoyancy• A floating body displaces a volume of water equal in weight to the weight of the body.


DISPLACEMENT

00

G

Capital W =


DISPLACEMENT

04

G

B

Capital W =


DISPLACEMENT

09

G

B

Capital W =


DISPLACEMENT

16

G

B

Capital W =


DISPLACEMENT

20

G

B

Capital W =


Laws Of Buoyancy

• A floating object has the property of buoyancy.

• 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.


DisplacementDisplacement

► The weight of the volume of water The weight of the volume of water that thethat the

ship's hull is displacingship's hull is displacing

► Units of weightUnits of weight

long ton = long ton = 2240 lbs2240 lbs short ton = short ton = 2000 lbs2000 lbs metric tonne = metric tonne = 2204.72 lbs2204.72 lbs


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

Units: TONS, POUNDS, ETC.

Force


Force

Parallel forces may be combined into one force equal to the sum of all forces acting in the same direction and so located to produce the same effect.

5 T

5 T

5 T

15 T


WEIGHT: Gravitational force. Direction toward center of earth

UNITS: TONNES, LONG TONS, POUNDS, etc

MOMENT: The tendency of a force to produce rotation about an axis

MOMENT = F x d

ad

F

UNITS: TONNE-M3, FT-POUNDS, etc

F is the force of your hand while d is the length of your “wrench” & Moment is the torque applied.


VOLUME - Number of cubic units in an objectUNITS: Cubic Feet, Cubic Inches, Cubic Meters

V = L x B x D

2 m3 m

1 m

V = 3 m x 2 m x 1 m

V = 6 m3


Salt water = 0.9756 m3/tFresh water = 1 m3/t

SPECIFIC VOLUME - Volume per unit weight

UNITS: CUBIC METRES PER TONNE

2 m3 m

1 m

V = 6 m3

wflooding = Volume Sp. Vol

wflooding = 6 m3

0.9756m3/tonne

wflooding = 6.15 tonnes


Stability Reference Points



CL

M

G

B

K

etacenter

ravity

uoyancy

eel



CL

M

G

B

K

other

oose

eats

ids


B

WATERLINERESERVE BUOYANCY

B1

B

The Center Of Buoyancy


B

WATERLINERESERVE BUOYANCY

Reserve Buoyancy, Freeboard, Draft and Depth Of Hull

Remember Remember this color!!this color!!


Center Of Buoyancy

B

WLWL

B

WL

B

WL

B

WL

B


BBBBBBBB

B

RULE OF THUMB RULE OF THUMB = “B” FOLLOWS = “B” FOLLOWS

THE WATERLINE.THE WATERLINE.


G

G1

KGo

KG1

GG1

KGo

KG1

The Center Of Gravity


Center Of GravityCenter Of Gravity

► Point at which all weights could be Point at which all weights could be concentrated.concentrated.

► Center of gravity of a system of weights is Center of gravity of a system of weights is found by taking moments about an found by taking moments about an assumedassumed center of gravity, moments are summed and center of gravity, moments are summed and divided by the total weight of the system.divided by the total weight of the system.


Movements In The Center Of Movements In The Center Of GravityGravity

► G MOVES G MOVES TOWARDSTOWARDS A WEIGHT A WEIGHT ADDITIONADDITION





► G MOVES G MOVES AWAYAWAY FROM A WEIGHT FROM A WEIGHT REMOVALREMOVAL


GGGGGGG1

KG1

KGo

G




► G MOVES G MOVES AWAYAWAY FROM A WEIGHT FROM A WEIGHT REMOVALREMOVAL

► G MOVES IN THE G MOVES IN THE DIRECTIONDIRECTION OF A WEIGHT OF A WEIGHT SHIFTSHIFT


G1


The Metacenter

CL

B

B20B45

M

M20

M45

M70

B70

METACENTER

M

BB1 B2


Metacenter

BBBBBBB


Metacenter

B SHIFTS

M


CL

B

M

0o-7/10o


CL

B B20

M

M20


CL

M

M20

M45

B

B20 B45


CL

B

B20

B45

M

M20

M45

M70

B70


CL

M20M45

M70

M90

B

B20

B45B70

B90

M


Movements Of The Movements Of The MetacenterMetacenter

THE METACENTER WILL CHANGE POSITIONS THE METACENTER WILL CHANGE POSITIONS IN THE VERTICAL PLANE WHEN THE SHIP'S IN THE VERTICAL PLANE WHEN THE SHIP'S DISPLACEMENTDISPLACEMENT CHANGES CHANGES

OROR

► WHEN B MOVES UP M MOVES DOWN.WHEN B MOVES UP M MOVES DOWN.► WHEN B MOVES DOWN M MOVES UP.WHEN B MOVES DOWN M MOVES UP.

RULE OF THUMB = “M” MOVES OPPOSITE OF “B”RULE OF THUMB = “M” MOVES OPPOSITE OF “B”


M

G

B

M

G

B

G

M

B

M1

B1

G

M

B

M1

B1

G

M

B

M1

B1

G

M

B

M1

B1


CL

M

G

B

K

GM

KG

BM

KM

Linear Measurements In Stability

METACENTRIC HEIGHTMETACENTRIC HEIGHT

HEIGHT OF METACENTERHEIGHT OF METACENTER

METACENTRIC RADIUSMETACENTRIC RADIUS

HEIGHT OF GRAVITYHEIGHT OF GRAVITY


The Stability TriangleThe Stability Triangle


M

GZ

CL

K

B

G

M

The Stability Triangle


CL

K

B

G

MM

CL

G

B

K

B1

CL

M

G

B

K

B1

CL

M

G

B

K

B1

CL

K

B

G

M

B1

Z


M

G Z

Where:Where:opposite = GZopposite = GZhypotenuse = GMhypotenuse = GM

Sin q = GZ / GMSin q = GZ / GMGZ = GM x Sin qGZ = GM x Sin q

Growth of GZ with GMGrowth of GZ with GM

Sin Sin = opp / hyp = opp / hyp


CL

K

B

G

M

G1


G

M

Z

G1 Z1

AS METACENTRIC HEIGHT (GM) DECREASES,

RIGHTING ARM (GZ) ALSO DECREASES


G

B

MINITIALSTABILITY

0 - 7°

CL


M

ZG

B B1

CL

OVERALLSTABILITY

RM = GZ x Wf

FINAL DISPLACEMENT


Conditions of StabilityConditions of Stability


GB1

M

Z

G

B1

M

B

G

B1

M

B

The Three Conditions of Stability

POSITIVE

NEUTRAL

NEGATIVE


CL

K

B

G

M

Positive Stability


CL

K

B

G

M

B1

Z

Positive Stability


CL

K

B

GM

Neutral Stability

WHAT COULD CAUSE NEUTRAL STABILITY ?


CL

K

BB1

Neutral Stability

GM


CL

K

B

GM

Negative Stability


K

B

GM

B1

Negative Stability

WHAT HAPPENS WITH NEGATIVE STABILITY ?











NEUTRAL STABILITY IS AS BAD AS NEGATIVE STABILITY, BECAUSE IF YOU GET TO NEUTRAL,

SOMETHING “OUTSIDE YOUR CONTROL” WILL PUSH YOU OVER THE EDGE!!


Free Surface EffectFree Surface Effect



► Free Surface Effect can become a problem Free Surface Effect can become a problem in a partially flooded vessel.in a partially flooded vessel.

► The partially flooded vessel and the water The partially flooded vessel and the water in her, will respond to the craft's heave, , in her, will respond to the craft's heave, , pitch, roll, surge, sway or yaw, e.g as the pitch, roll, surge, sway or yaw, e.g as the vessel rolls to port the water inside will vessel rolls to port the water inside will move, so that much of it is now on the port move, so that much of it is now on the port side of the vessel, and this will move the side of the vessel, and this will move the craft's centre of gravity and center of craft's centre of gravity and center of moment towards port. This has the effect moment towards port. This has the effect of slowing the craft's return to vertical.of slowing the craft's return to vertical.



► The free surface effect then becomes The free surface effect then becomes worse if the vessel then rolls through the worse if the vessel then rolls through the vertical to starboard. It takes time for the vertical to starboard. It takes time for the liquid in the vessel to respond and move liquid in the vessel to respond and move to the starboard side. to the starboard side.

► After the craft rolls through the vertical After the craft rolls through the vertical towards right (starboard), most of the towards right (starboard), most of the liquid moving in the craft's tank then liquid moving in the craft's tank then slams into the right (starboard) side of the slams into the right (starboard) side of the tank.tank.



► This has the effect of causing the This has the effect of causing the vessel to heel further over, as the vessel to heel further over, as the water mass hits the sides of the water mass hits the sides of the vessel. In strong winds, and heavy vessel. In strong winds, and heavy sea, this can become a positive sea, this can become a positive feedback loop, causing each roll to feedback loop, causing each roll to become more and more extreme, become more and more extreme, until the vessel capsizes.until the vessel capsizes.



► The higher up this water movement The higher up this water movement occurs, especially above either the occurs, especially above either the craft's center of buoyancy or center craft's center of buoyancy or center of gravity, the more pronounced the of gravity, the more pronounced the instabilities.instabilities.

► If this vessel is being towed, it could If this vessel is being towed, it could capsize, and take the towing vessel capsize, and take the towing vessel with it.with it.




Draft Marks and Plimsoll LinesDraft Marks and Plimsoll Lines


How to read draft marks…How to read draft marks…


Plimsoll MarkPlimsoll Mark

The Plimsoll Mark diagrammed above is for the starboard side of a vessel; on the port side, the markings are reversed. The center of the disk is placed at the middle of the loadline. The lines are one inch think.


Plimsoll MarkPlimsoll Mark

The letters signify:

LTF Lumber, Tropical, Fresh

LF Lumber, Fresh

LT Lumber, Tropical

LS Lumber, Summer

LW Lumber, Winter

LWNA Lumber, Winter, North Atlantic

LR Lloyds Register of Shipping

TF Tropical Fresh Water Mark

F Fresh Water Mark

T Tropical Load Line

S Summer Load Line

W Winter Load Line

WNA Winter Load Line, North Atlantic


Thumb RulesThumb Rules

► B follows the WaterlineB follows the Waterline► M moves opposite of BM moves opposite of B► G movesG moves

Towards additionTowards addition Away from removalAway from removal Direction of shiftDirection of shift

► ““G moves faster than M”G moves faster than M”► ““G is G is nearnear the waterline” the waterline”


These Guys Got it WrongThese Guys Got it Wrong





Health & Medicine

Stability lrg