VESSEL STABILITY

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VESSEL STABILITY CANADIAN COAST GUARD AUXILIARY - PACIFIC Principles of Stability Definitions Definitions Stability - the tendency of a vessel to rotate one way or the other (to right itself or overturn) Initial Stability - the stability of a vessel in the range from 0° to 7°/10°

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### VESSEL STABILITY

CANADIAN COAST GUARD AUXILIARY - PACIFIC

Definitions

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

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

Definitions

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

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

Six Motions Of A Ship

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

Definitions

ROLL -

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

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

HEEL -

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.

G

00

DISPLACEMENT

Capital W =

G

B

04

DISPLACEMENT

Capital W =

G

B

09

DISPLACEMENT

Capital W =

G

DISPLACEMENT

B

16

Capital W =

G

DISPLACEMENT

B

20

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.

Displacement
• The weight of the volume of water that the

ship\'s hull is displacing

• Units of weight

long ton = 2240 lbs short ton = 2000 lbs metric tonne = 2204.72 lbs

Force

A push or a pull.

Tends to produce motion or a change in motion.

Units: TONS, POUNDS, ETC.

5 T

15 T

5 T

5 T

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.

WEIGHT:Gravitational force. Direction toward center of earth

UNITS: LONG TONS, POUNDS, etc

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

UNITS: FT-LT, FT-POUNDS, etc

d

F

a

MOMENT = F x d

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

2 m

3 m

1 m

VOLUME - Number of cubic units in an object

UNITS: Cubic Feet, Cubic Inches, Cubic Metres

V = L x B x D

V = 3 m x 2 m x 1 m

V = 6 m3

2 m

3 m

V = 6 m3

1 m

SPECIFIC VOLUME - Volume per unit weight

UNITS: CUBIC METRES PER TONNE

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

wflooding = Volume Sp. Vol

wflooding = 6 m3 0.9756m3/tonne

wflooding = 6.15 tonnes

Stability Reference Points

M

etacenter

G

ravity

B

uoyancy

K

eel

CL

Stability Reference Points

M

other

G

oose

B

eats

K

ids

CL

B

The Center Of Buoyancy

RESERVE BUOYANCY

WATERLINE

B

B1

Reserve Buoyancy, Freeboard, Draft and Depth Of Hull

Remember this color!!

RESERVE BUOYANCY

WATERLINE

B

WL

WL

WL

WL

WL

B

B

B

B

Center Of Buoyancy

B

B

B

B

B

B

B

B

B

B

RULE OF THUMB = “B” FOLLOWS THE WATERLINE.

G1

G

G1

G

The Center Of Gravity

KG1

KGo

KGo

KG1

Center Of Gravity
• Point at which all weights could be concentrated.
• Center of gravity of a system of weights is found by taking moments about an assumed center of gravity, moments are summed and divided by the total weight of the system.
Movements In The Center Of Gravity
• G MOVES TOWARDS A WEIGHT ADDITION
Movements In The Center Of Gravity
• G MOVES TOWARDS A WEIGHT ADDITION
• G MOVES AWAY FROM A WEIGHT REMOVAL

G1

G

G

G

G

G

G

G

KG1

KGo

Movements In The Center Of Gravity
• G MOVES TOWARDS A WEIGHT ADDITION
• G MOVES AWAY FROM A WEIGHT REMOVAL
• G MOVES IN THE DIRECTION OF A WEIGHT SHIFT

B

B20

B45

METACENTER

The Metacenter

M

B

B2

B1

M20

M45

M

M70

B70

CL

B

B

B

B

B

B

B

Metacenter

B SHIFTS

Metacenter

M

M20

M

B

B20

CL

M20

M45

M

B

B20

B45

CL

M20

M45

M

M70

B

B20

B70

CL

B45

M20

M45

M70

B

CL

M

M90

B20

B90

B45

B70

Movements Of The Metacenter

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

OR

• WHEN B MOVES UP M MOVES DOWN.
• WHEN B MOVES DOWN M MOVES UP.

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

M

M

M

M1

M

M

M1

M

M1

M1

G

G

G

G

G

G

B

B

B1

B1

B1

B1

B

B

B

B

Linear Measurements In Stability

M

METACENTRIC HEIGHT

GM

BM

G

HEIGHT OF METACENTER

KM

B

HEIGHT OF GRAVITY

KG

CL

K

M

G

B

K

The Stability Triangle

M

G

Z

CL

M

M

M

M

M

Z

G

G

G

G

G

B

B

B

B

B

B1

B1

B1

B1

K

K

K

K

K

CL

CL

CL

CL

CL

M

G

Z

Sin q = opp / hyp

Where: opposite = GZ

hypoteneuse = GM

Sin q = GZ / GM

GZ = GM x Sin q

Growth of GZ with GM

M

G1

G

B

K

CL

M

G1

Z1

G

Z

AS METACENTRIC HEIGHT (GM) DECREASES,

RIGHTING ARM (GZ) ALSO DECREASES

INITIALSTABILITY

M

G

B

0 - 7°

CL

M

OVERALLSTABILITY

G

Z

B

B1

RM = GZ x Wf

CL

FINAL DISPLACEMENT

The Three Conditions of Stability

M

Z

G

B1

G

M

POSITIVE

G

B

B1

M

NEUTRAL

B

B1

NEGATIVE

M

G

B

K

Positive Stability

CL

M

Z

G

B

B1

K

Positive Stability

CL

G

M

B

K

Neutral Stability

CL

WHAT COULD CAUSE NEUTRAL STABILITY ?

G

M

B

B1

K

Neutral Stability

CL

G

M

B

K

Negative Stability

CL

G

M

B

B1

K

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 Effect
• Free Surface Effect can become a problem in a partially flooded vessel.
• The partially flooded vessel and the water in her, will respond to the craft\'s heave, , pitch, roll, surge, sway or yaw, e.g as the vessel rolls to port the water inside will move, so that much of it is now on the port side of the vessel, and this will move the craft\'s centre of gravity and center of moment towards port. This has the effect of slowing the craft\'s return to vertical.
Free Surface Effect
• The free surface effect then becomes worse if the vessel then rolls through the vertical to starboard. It takes time for the liquid in the vessel to respond and move to the starboard side.
• After the craft rolls through the vertical towards right (starboard), most of the liquid moving in the craft\'s tank then slams into the right (starboard) side of the tank.
Free Surface Effect
• This has the effect of causing the vessel to heel further over, as the water mass hits the sides of the vessel. In strong winds, and heavy sea, this can become a positive feedback loop, causing each roll to become more and more extreme, until the vessel capsizes.
Free Surface Effect
• The higher up this water movement occurs, especially above either the craft\'s center of buoyancy or center of gravity, the more pronounced the instabilities.
• If this vessel is being towed, it could capsize, and take the towing vessel with it.
Plimsoll 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.

The letters signify:

LTF

TF

Tropical Fresh Water Mark

Lumber, Tropical, Fresh

F

Fresh Water Mark

LF

Lumber, Fresh

T

LT

Lumber, Tropical

S

LS

Lumber, Summer

W

LW

Lumber, Winter

WNA

LWNA

Lumber, Winter, North Atlantic

LR

Lloyds Register of Shipping

Plimsoll Mark
Thumb Rules
• B follows the Waterline
• M moves opposite of B
• G moves